Study Notes on Pathogens and Pathogenicity

The below mentioned article provides a study note on Pathogens and Pathogenicity.

Introduction to Microbial Pathogenicity:

Sources and Spread of Infection in the Community:

Pathology is the ability of the microbes to initiate the infection.

It requires three tributes:

(1) Transmissibility or communicability — from one host or reservoir to a fresh host;

(2) Infectivity — the ability to breach the new host’s defences;

(3) Virulence a variable factor that may enhance or reduce the capacity of the pathogen to cause overt infection.

Pathogens (Pathogenic microorganisms) cause diseases and they may be:

(a) Opportunistic or

(b) True pathogens.

Opportunistic Pathogens:

Many commensal or non-pathogenic may be transmissible from person to person or derived from the environment and are present, in large numbers, on the skin, in the upper respiratory tract, in the intestine and lower urinogenital tract, hence they are normal micro flora of the body and sometimes they may act against invading pathogenic microorganisms and are unable to invade the tissues as they cannot overcome the healthy body defences.

Sometimes, when the body defence mechanism is lowered and when these commensals leave their natural habitat and reach other parts of the body e.g. coliform bacilli (Escherichia coli) are mostly harmless commensals in the intestine, but they may cause infection in the urinary tract; similarly Clostridium welchii, an intestinal commensal, can cause gangrene in locally damaged tissues; streptococcus viridans is the commensal of the mouth, after tooth extraction they may invade the blood stream and settle on previously damaged heart values as opportunistic pathogens.

True Pathogens:

They are those microorganisms which are able to overcome the normal body defence mechanism and initiate the infection.

Several properties are essential for pathogenicity of microorganisms:

(a) Transmissibility:

The ability of a pathogen to grow profusely in the body and to be shed in large numbers in body fluids or secretions which are capable of dissemination and reach new host after surviving in the adverse conditions, e.g., dessication in the dry dust.

(b) Infectivity:

Pathogenic microbes are able to initiate the infection by penetrating the healthy body’s first line of defence, that is skin, mucous membranes to which they readily gain access. To infect a person, only a few of the pathogens can cross the protective barriers in the respiratory and alimentary tracts.

The pathogen may initiate a localised lesion at the site of infection, e.g., staphylococcal boil on the skin or streptococcal pharyngitis in the throat. The capacity of the microbes to initiate the infection is mostly related to the dosage of the pathogen, its phase of growth and its virulence factors.

In salmonella family, the infecting dose of Salmonella typhi is very small, whereas large number of S. typhimurium (food poisoning salmonellae) must be ingested to produce acute vomiting and diarrhoea.

Microorganisms which are in logarithmic stage of growth are more likely to overcome host resistance than those in the latent phase:

Streptococcus pyogenes is more infective when transferred directly from a person with a sore throat than when it is inhaled after drying in dust particles, because Strept. pyogenes has the capsular M protein (anti-phagocytic component) during the active phase of sore throat infection.

(c) Virulence:

The virulence of a pathogen is the ability to kill susceptible animal (mouse, guinea pig etc.). The tubercle bacillus isolated from Indian patients with tuberculosis are often less virulent to the guinea pigs than strains isolated in Britain. The assessment of virulence for animal is not necessarily applicable to virulence of man.

Shigella dysenteriae causes much more severe infection than Sh. sonnei. Similarly, the gravis strain of Corynebacterium diphtheriae causes more deaths than the mitis strain. Type 1 polio virus is more likely to initiate epidemics of paralytic poliomyelitis than is Type 2.

Pathogenesis:

Pathogenic bacteria produce diseases by virtue of one or both of the main attributes:

Toxigenicity, and Invasiveness.

Toxigenicity:

Toxins may be:

(a) Exotoxins, or

(b) Endotoxins.

(a) Exotoxins:

German and French workers were first to prove that the products of the diphtheria bacilli, diffused from the local infection or injected as bacteria free filtrates of cultured diphtheria bacilli could produce widespread systemic damage in guinea pigs. The toxin produced by diphtheria bacilli in the throat is carried by the blood stream (toxaemia) throughout the body.

When the bacteria grow actively in broth culture, apparently they secrete the poison which is called “exotoxin.” Other bacteria which secrete highly potent toxin are tetanus bacilli and Clostridium botulinum. 1.0 mg of tetanus and botulinum toxin can kill more than one million guinea pigs and it is estimated that 3 kg of botulinum toxin can kill the world population (6.9 billion = 690 crores).

Exotoxins are mainly produced by Gram-positive bacilli (except Shigella bacillus neurotoxin and Cholera enterotoxin) and have special affinity for specific tissues; for example, tetanus, botulinum and diphtheria toxins all affect different parts of the nervous system, tetanus toxin affects control mechanisms that govern motor cells in the anterior columns of the spinal cord, botulinal toxin paralyses cranial nerves by blocking the transmission of effectors messages from their endings, and diphtheria toxin has affinity for peripheral nerve ending as well as for specialised tissues like heart muscle.

Exotoxins behave like Enzymes:

Alpha toxin of CI. welchii is a phospholipase (lecithinase C) which acts on phospholipids of cell membrane; diphtheria toxins depress the formation and or release of acetylcholine in different parts of the nervous system.

Endotoxins:

They are complex phospholipid polysaccharide protein macromolecules. Most of the endotoxins are lipopolysaccharides and additional endotoxin released by few Gram-negative pathogenic bacteria (Yersinia pestis and Bordetella pertussis ) is protein in nature and is present in the bacterial cytoplasm.

They are released only after natural autolysis or artificial disruption of bacterial cells and, therefore, they are called endotoxins. Typical endotoxins are particularly associated with Gram-negative bacteria (Salmonella, Shigella, Escherichia, Neisseria) and are distinguishable from exotoxins by the following properties:

(1) They are present in the outer layer of the bacterial cell wall;

(2) They are heat stable;

(3) They are much less toxic and specific in their cytotoxic effects than exotoxins;

(4) They cannot be converted into toxoids.

(5) Homologous antibodies cannot render them non-toxic, if combined.

The complex antibodies cannot render them non-toxic, if combined;

The complex phospholipid-polysaccharide-protein molecule can be separated by phenol extraction into:

a. Lipopolysaccharide moiety;

b. Protein fraction;

c. Phospholipid fraction.

The lipopolysaccharide moiety can be split further into different sugars including those that determine the antigenic specificity of the endotoxin and lipid which is mainly responsible for the toxicity.

Pyogenic effect (fever) is the toxic effect produced by the smallest amount of endotoxin 0.002 Hg endotoxin per kg body weight is injected intravenously into rabbit or man, it causes within 15 minutes an elevation of body temperature which lasts for several hours.

Invasiveness:

It is the other main attribute of pathogenic bacteria. It is its capacity to invade and multiply in the healthy tissues e.g. pneumococcus produces the disease depending entirely upon the quality of invasiveness just as botulinus bacillus depends entirely on its toxigenicity.

Thus, the diphtheria bacillus must be initially invasive in order to establish itself in the tissues of the oropharynx, where it manufactures its toxin; the gravis strain of diphtheria bacillus has the greater capacity to invade and multiply in the tissues with a consequent greater production of toxin than mitis strain.

Strepto pyogenes is mainly an invasive pathogen. It also produces an erythrogenic toxin which is responsible for the rash of the scarlet fever. The invasiveness of Staph, aureus, Strepto. pyogenes and CI. welchii is due to their production of keratolytic and loculicidal toxins which enable them to breach tissue barriers and protect themselves against phagocytosis.

Pathogenic bacteria which are predominantly invasive are:

(1) The First Category:

The pathogenic Gram-positive cocci initially attract phagocytes by chemotactic mechanisms, resist phagocytosis, but ultimately they may be engulfed and destroyed by phagocytes.

(2) The Second Category:

Tubercle bacilli, typhoid, brucella bacilli, though they are readily phagocytosed, are more resistant to destruction when within phagocytes and become intracellular parasites which are disseminated by phagocytes throughout the body.

There is fight between phagocytes and anti-phagocytic bacteria, when specific antibody acting as opsonins come to the aid of the phagocytes in destroying the anti-phagocytic bacteria, then these bacteria are destroyed by phagocytes and there is a dramatic fall in temperature-the crisis-as observed in pneumococcal pneumonia.

In infection with intracellular parasites there is clinical illness that persists for some week with low fever.

Capsules and Pathogenicity:

The bacterial capsule plays an important role in conferring the virulence on bacteria by enabling them to resist phagocytosis and bactericidal, substances in body fluids, therefore the capsulation is important for the virulence of pneumococci, streptococci of Group A and Group C, the anthrax bacillus, the plague bacillus, Klebsiella pneumoniae and Haemophilus influenza.

It has been observed that the non-capsulate bacteria are rapidly phagocytosed and, within a few hours, are mostly destroyed, whereas the capsulate bacteria remain free and soon multiply to large numbers.

The mechanism of the anti-phagocytic property of capsulate bacteria is not known, but it may be that the lipid containing cell membrane of the phagocyte’s pseudopodia is inhibited from making contact with the hydrated capsule gel because of the surface charge.

However, heavily capsulate harmless saprophytic bacteria and some non- virulent strains of bacillus anthracis and plague bacilli are fairly susceptible to phagocytosis.

Aggressin:

Other non-toxic protective or aggressive factors that may contribute to the ability of capsulate and non-capsulate pathogens to invade and multiply in the host tissues are:

(1) Hyaluronidase or spreading factor, is an enzyme that dissolves the hyaluronic acid or cement-like substances that binds cells together and so allows pathogens (strepto-pyogenes, staph.aureus) to permeate through the tissues.

(2) Coagulase a thrombin-like enzyme produced by all pathogenic staphylococci may help to protect the pathogen from phagocytosis into two ways:

(a) By forming fibrin barriers around staphylococci and staphylococcal lesions, and

(b) By inactivating the bactericidal substance present in the normal blood serum.

(3) Streptokinase secreted by Strepto.pyogenes may promote the spreading of streptococcal lesions.

(4) Collagenase, produced by CI. welchii, may play some part in the pathogenesis of gas gangrene.

(5) Neuraminidase produced by some bacteria and viruses, acts on mucoproteins of cell surface and may facilitate attacks on the cell.

Organotropism:

The affinity of many pathogenic microbes for specific tissues or organs is known as organotropism. Pneumococcus and meningococcus both have the natural habitat in the nasopharynx, but the virulent pneumococcus has a predilection for lung tissues and the meningococcus for the meninges of the brain, whereas gonococcus mainly affects the mucosa of urethra.

Brucella abortus localize in the uterus of cows because of the presence of erythritol, a growth factor for Br. abortus, in the bovine placenta. However, Coxiella burnetti and chlamydiae localize in the uterus but they do not use erythritol.

Comparative Characteristics of Toxins:

Exotoxins:

1. Composed of proteins, have the properties of enzymes, some have been obtained in crystalline state.

2. Easily diffuse from the cell into the surrounding medium.

3. Highly toxic, characterised by the selective affection of certain organs and tissues.

4. Thermolabile.

5. During parenteral injection, they produce highly active antibodies (anti toxins).

6. Under influence of 0.3-0.4 per cent formalin and a temperature of 38-40°C change to antitoxin (Toxoid)

Endotoxins:

1. Composed of phospholipid. Polysaccharide protein complexes.

2. Firmly bound within the bacterial cell.

3. Less toxic, selective action poorly expressed.

4. Thermo-stable.

5. During parenteral injection, they produce precipitins, lysins, opsonins, agglutinins and complement fixing antibodies.

6. Under the influence of formalin and temperature are rendered partially harmless.

Host Factors Contributing To Infection:

Various host factors, affecting an individual or a community, contributing to the occurrence of the infection are:

1. Very young children and very old individuals are susceptible to infection, because of inadequate body defence mechanism.

2. Malnutrition favours the incidence and severity of infection in poor community.

3. Metabolic diseases like diabetes and hormonal upsets due to corticosteroid therapy predispose to the infection.

4. Haematological disorders like leukaemia, neoplasm and renal diseases may reduce host resistance to infection.

5. Factors such as age, obesity, size of wound, wound drainage, duration of operation and length of stay before or after operation may affect the nosocomial infection (hospital acquired infection) caused by staphylococci and Gram-negative bacilli.

6. Acquired immunodeficiency syndrome (AIDS) patients are highly susceptible to various infections.

7. Community factors affecting host resistance are the complex components of poverty (overcrowding, inadequate food, clothing and housing, hypothermia and poor personal hygiene).

8. Drug addiction and alcoholism may predispose to infection.

Sources of Infection for Man:

The term infection (Latin inficiere-infectum-to infect) signifies the sum of biological processes which take place in the human or animal body upon the penetration of pathogenic microorganisms in the host’s body by injuring body tissues and producing a reaction on the part of the host, Source of infection is defined as the normal growth habitat of the microbes, e.g., a site in the body of the human or animal host.

Objects contaminated with live or temporarily inactive microbes may be called as vehicles or reservoirs of infection, not source of infection.

Man Acquires the Infection:

(a) From outside sources, which is known as exogenous infection, e.g., from human patients with clinical infections, healthy human carriers of the pathogenic microorganism.

(b) Within the patient’s own body, the infection is said to be endogenous. Exogenous infection

1. Patients:

Some microbial infections are acquired from sick patients with active infections (pulmonary tuberculosis, leprosy, whooping cough, syphilis, gonorrhoea, measles, smallpox, mumps and influenza).

2. Healthy Carriers:

Healthy persons, though they carry many species of pathogenic microbes, sometimes show a subclinical infection and are commonly capable of disseminating these pathogenic microbes to other persons who will manifest the signs and symptoms of illness; they are then termed as carriers, thus they are potential sources of infection; some infectious diseases are contracted from carriers much more frequently than from patient’s, e.g., streptococcal, staphylococcal, pneumococcal and meningococcal infection, diphtheria, typhoid fever, bacillary dysentery, and poliomyelitis.

3. Convalescent Carriers:

Are persons in whom a limited, localised infection continues for a period of weeks or months after clinical recovery from a manifest infection.

4. Contact Carriers:

Are those persons who acquire the pathogen from a patient.

5. Paradoxical Carriers:

Are those who acquire the infection asymptomatically from another carrier. If the carriage persists for more than an arbitrary period of time, e.g. one year in the case of typhoid infection, the person is called a chronic carrier.

(A) Endogenous Infection:

Endogenous infection may occur in carriers of potentially pathogenic organisms, when these previously harmless bacteria invade.

Other surfaces or tissues in the carrier, e.g. Escherichia coli derived from the bowel, where it is harmless, may cause acute suppurative infection in the urinary tract, Staph, aureus from the nostrils may cause boil in the skin infection in a wound; and pneumococci from the nasopharynx may cause bronchitis and bronchopneumonia.

The source of endogenous infection is thus the site in a patient body (e.g. colon, skin or nasopharynx) where the organisms grows harmlessly as a commensal.

Staphylococcal sepsis of the skin and wounds, which is mainly endogenous, may—under certain circumstances—become transmissible, as in hospital, where conditions may favour cross- infection between patients. Thus, the cross-infected patients suffer from exogenous infection.

However, patients with endogenous infections caused by organism of low virulence are not likely to infect other persons, e.g., patients with bronchopneumonia due to pneumococci of less virulence are not a danger to relatives, nurses or other patients and there is no need to isolate them from other patients in hospital.

Modes of Spread of Infection:

A variety of mechanism can spread respiratory and alimentary infections from host to host, while single mechanism, for which the parasite is specially adapted, can spread venereal and arthropod-borne blood infection:

(1) Respiratory Infection:

The causative microbes are disseminated into the environment in masses of infected secretion, e.g., secretion transferred from the nose or mouth on fingers, handkerchiefs, cups, spoons or secretions expelled in spitting or blowing the nose; they are also discharged to a less extent in the droplet spray produced by sneezing, coughing and speaking, but in normal breathing, they are very rarely disseminated.

The secretion contaminates handkerchief, clothing, bedding, floors, furniture’s, and household articles (fomites) which may act as vehicles or reservoirs of infection.

It dries in dust, most kind of respiratory microbes may remain alive for several days, even for several months, if protected from direct sunlight, e.g., tubercle bacilli, diphtheria bacilli, streptococci, staphylococci and smallpox virus.

(a) Infection may be passed to the recipients by contact, either:

(i) Direct contact i.e. touching of bodies as in hand-shaking, kissing and contact of cloth, or

(ii) Indirect contact e.g. contaminated eating utensils, door handles, towels; the recipients may transfer the microbes from contaminated finger to his sore or mouth.

(b) Infection may be dust borne by inhalation of air borne infected dust particles. The infected numerous dust particles are disseminated into the air from the skin and clothing during normal body movements; from dried contaminated handkerchiefs during use, from bed sheets while bed making, from floor during sweeping and walking, and from furniture while dusting.

The larger infected dust particles may settle within a few minutes on the floor and other exposed surface, e.g. skin, clothing, wounds and surgical supplies; whereas the smaller dust particles remain air-borne for up to 1 to 2 hours and may be inhaled into the recipient’s nose, throat, bronchi, lung alveoli.

Within the room of its origin, air-borne infection is very common and dangerous; but its spreading to other rooms in the same building is rare,

(c) The third least important means of respiratory infections is droplet spray, except in the case of the pathogens that are rapidly killed by drying, e.g., meningococcus, whooping cough bacillus, measles and common cold viruses, sneezing, coughing, speaking and other forceful expiratory activities expel a spray of droplets derived from the saliva of the anterior mouth which is infected with small numbers of pathogenic microbes from the nose, throat, lungs.

Numerous droplets are expelled, but only a few are infected.

The large droplets (over 0.1 mm in diameter) fly forwards and downwards from the mouth to the distance of a few feet, they reach the floor or body sources (eye, face, mouth and clothing) or the persons standing in front of the other person producing the spray, within a few seconds. These particles cannot be inhaled. Measles, chicken pox and dog distemper are common viral infections spread by the larger droplets.

The small droplets (below 0.1 mm in diameter) evaporate immediately to become minute solid residue or “droplet nuclei” (mainly 1 to 10 jam in diameter), which remain air­borne and may be inhaled into the nose, throat, lung. Few of the droplet nuclei are infected with pathogenic microbes. Measles, chicken pox, and dog distemper are common viral infections spread by the droplet nuclei.

(2) Mucous membrane, skin wound, discharges and burn infection:

Gonococcal ophthalmia caused by gonococci and inclusion blennorrhea caused by sub-group A of chlamydia may be acquired from the infected genital tract of the mother, when the mucous membrane of eyes of newborn infants gets infected as the infant passes down the infected birth canal.

Leptospira excreted in the urine of the infected animals enter the human body through the skin, similarly larvae of Schistosoma and Ankylostoma penetrate the skin.

The superficial infections may be acquired by:

(a) contact with infected hands, clothing or other articles,

(b) by exposure to the disposition of contaminated droplet spray.

Pathogenic streptococci and staphylococci derived from respiratory tract are important. Causes of wound and burn infection; if pathogenic staphylococci are disseminated from pus discharge, they may cause infection of broken skin and burn wound.

(3) Venereal Infection:

The diseases transmitted exclusively by sexual intercourse are called as venereal diseases, since the causative agents e.g. Treponema pallidum and Neisseria gonorrhoeae are highly susceptible to lethal effects of drying, they are to be transmitted only by sexual contact. Gonococcus—which causes vulvovaginitis in young girls— may be acquired, under unhygienic conditions, from the common use of towels and bathing facilities.

This is non-venereal spread of infection in young girls. Sub-group A of chlamydia (Lymphogranuloma venereum-LGW) infection is transmitted by sexual contact. Acquired immuno deficiency syndrome (AIDS) caused by Human immuno deficiency virus (HIV) can be transmitted through sex commonly by homosexuality.

(4) Alimentary Tract Infection:

Persons with intestinal infection may disseminate the pathogenic microbes in excreta. These microbes are transmitted in various ways, the so-called faecal oral routes, leading to their ingestion by the recipient, e.g., Salmonella typhi.

Most of the intestinal pathogens are not much resistant to drying, they may die within a few hours, they may survive on cloth, or in dust for several days. They are more likely to be spread by moist vehicles (water or food) in which they may survive for several weeks.

(a) Water-borne infection may occur when faeces contaminate a water supply, e.g., river or well, which is used without purification for drinking purpose. In typhoid and cholera, water is the common vehicle of infection and the infecting doses may be small.

Water is supplied to the citizen by the municipality after purification. Purification, which renders the water non-infective, is carried out on a large scale by storage, filtration and chlorination. Small amounts of water required for drinking purpose may be treated by boiling or by the addition of hypochlorite tablets.

(b) Hand Infection:

The hands of the nurses can get infected while attending the patients and touching the bedpans. Similarly, the hands of the carrier can be contaminated with bacteria contained in the faeces. Such persons may contaminate the foodstuffs, eating utensils, wash hand basins, towels, door handles and other fomites (inanimate objects).

A recipient may eat contaminated food or put in his mouth the contaminated utensils or may pick up the microbes on his fingers and then transfer them into his mouth.

(c) Food-borne infection may occur through:

(i) A carrier handling the food (food as a vector of disease):

(ii) Preparation of the food in utensils contaminated by handling or washing in contaminated water;

(iii) Flies alighting on the food after feeding on infected faeces. Enterotoxins may be produced on the food contaminated by bacteria during their growth in favourable conditions e.g. Staph, aureus and Clostridium botulinum.

These preformed enterotoxins are prerequisites for bacterial food poisoning, whereas food-borne infection may result from the ingestion of only a few pathogenic microorganisms, e.g., Sh. dysenteriae which liberate enterotoxin.

(5) Urine:

Typhoid bacilli are excreted in the urine and faeces, similarly leptospirae are excreted by animals through the urine and enter the mucous membrane and skin.

(6) Arthropod-Borne Infections:

In systemic infection, the causative microbes are present in large number in blood, so they may be transmitted to other individuals by blood­sucking arthropods such as mosquito (malaria, filaria, yellow fever), flea (plague), louse (epidemic typhus fever); mite (scrub typhus) and tsetse fly (trypanosomiasis).

(7) Laboratory Infection:

Laboratory workers may occasionally become infected from artificial cultures or infected diagnostic or necropsy materials collected from patients or experimental animals.

Brucella, rickettsia and Pasteurella tularensis especially cause laboratory infection; while other organisms (tubercle bacilli, anthrax bacilli, pathogenic leptospiral, Borreliae, freshly isolated typhoid, dysentery bacilli, psittacosis organisms and serum hepatitis virus) should be handled very carefully.

The pipetting of infected liquids by mouth leading to their accidental ingestion is certainly a danger of laboratory infections. Accidental self-inoculation with a syringe may take place or the conjunctiva may be sprayed when the needle become loosened from a syringe during an injection.

Many laboratory procedures (the expulsion of liquid from a pipette or the use of the mechanical blenders, the centrifugation of tubes bearing traces of liquids on their rim) or drop separated from an inoculation loop may cause laboratory infections, when working particularly with pathogens which may cause air borne infection such as tubercle bacilli, thus it is necessary that all these procedures should be carried out with in an especially ventilated protective cabinet or inoculation hood.

(8) Congenital Transmission:

Serum hepatitis virus can be transmitted from the infected pregnant mother through placenta to the foetus. Similarly, human immune deficiency (HIV) can be transmitted through placenta to the newborn. Congenital malaria is comparatively rare.

The intrauterine transmission of malaria is well-established. The congenital malaria is very common in non-immune infected mothers. It is also believed the passage of the parasite to the foetus occurs only when the placental barrier has been injured.

Congenital toxoplasmosis results from the congenital infection in infants and young children. It usually appears as a form of encephalitis, accompanied by chorioretinitis, hydrocephalus or microcephaly, mental retardation and convulsions.

The infection passes from the mother through the placenta late in pregnancy when neutralisation of antibodies cannot take place. The child is usually born jaundiced with purpuric or maculopapular rash; enlarged liver and spleen.

Congenital trypanosomiasis in man is possible and is rare; a case was reported in a child born of an infected mother, in Germany, who died less than three months after birth, because of proven trypanosomiasis. Later, two more proven congenital infections in infants were reported.

Congenital Filariasis:

The microfilariae of Wuchereria bancrofti may pass from the infected pregnant mother to the newborn through the placenta filter.

Basic Principles of Disease Transmission:

Portals of entry and portals of exit are two basic principles of disease transmission:

1. Portals of Entry:

Normally, microorganisms enter the body through certain routes and cause the infection; but they cannot gain entrance through other abnormal portals of entry.

(a) Cuts or abrasions of the skin (e.g. bites of arthropods or animals);

(b) Mucosa of respiratory tract (nose, throat, lungs), eyes and mouth, gastrointestinal, genitourinary tracts are the most important common portals of entry.

Abnormal Portals of Entry:

If harmless saprophytic microorganisms are introduced parenterally into the brain or into the peritoneal cavity by a hypodermic needle, they set up the rapid fatal infection. Many organisms can cause the infection if they enter only through their obligate portals of entry.

Thus, dysentery bacilli can produce severe bacillary dysentery, if they are consumed through water or food; but if they are rubbed into the skin wounds, they will not initiate any infection. Severe, even fatal infections (abscess, boil, carbuncle) can be produced if staphylococci are rubbed in the skin; but if swallowed, they are ineffective. Some organisms can enter through any portal.

Path of Organism in the Body:

From the sites (portals of entry) of infection, organisms may pass into the blood stream, initiate a secondary (metastatic) infection in internal organs (i.e. in the meninges of the brain and spinal cord-meningitis).

When the enteric pathogens (typhoid bacilli, dysentery bacilli, vibrio cholera) are swallowed, they enter the stomach unaffected temporarily by gastric juice (especially in the presence of food), they set up ultimately gastrointestinal infection.

The respiratory pathogens locate in or on tonsils, throat (Streptococci, diphtheria bacilli, whooping cough bacilli) may cause the infection, if they pass down into the lungs, they may produce pneumonia (pneumococci) and tuberculosis (tubercle bacilli). The organisms (Treponema palladium, gonococci) find their portals of entry through genital mucosa.

(2) Portal of Exit:

The pathway by which organisms leave the body of one patient to another patient or healthy individual to initiate the infection is known as portals of exit (Fig 84.1).

Vectors:

Vectors may be Animate of Inanimate:

(a) Inanimate vectors may be the food, discharges (faeces, saliva, pus), water, bandages, dressing, instruments, bedding, eating utensils and other inanimate objects (fomites) contaminated with infectious discharges.

(b) Animate vectors are generally arthropods or mammals: female anopheles mosquito is the vector for malaria; vectors of rabies are dogs or other mammals. Other animate vectors are the hands (very common) of the patients, nurses, doctors and patient’s relatives coming in contact with an infectious patient or his fomites.

Since man cannot be eliminated with Dichloro Diphenyl Trichloroethane (DDT) or muzzled and put on a leash, he himself may be regarded as one of the most dangerous vector of the diseases (e.g. man is the only significant living vector of AIDS, poliomyelitis, measles and syphilis and he is the most frequent live vector of human tuberculosis).

Application:

An intelligent well trained professional nurse should know how to deal with the pathogenic microorganisms leaving the patient body to the environment. While attending the patient she should protect herself and the environment from these pathogens.

The transmission of most infectious agents should be stopped by her at the portal of exit by immediate collection of and proper disposal of all body excretions, secretions, discharges, tissue likely to contain the pathogens (faeces, urine, saliva, mucus, pus, tissue drainage, used bandage etc.).

In case of malaria, infected blood sucking female anopheles mosquito must be destroyed by Dichloro Diphenyl Trichloroethane (DDT) spray.

If the most probable portal of entry is intelligently understood by the nurse, she can adopt the appropriate methods of protection. For example, the respiratory portals of entry can be covered by the masks; cut or abrasions on the hand can be protected by bandage or surgical gloves.

The alimentary canal is protected by appropriate selection and disinfection (as by pasteurization of milk, chlorination of water and cooking) and sanitary preparation of food. The skin must be protected from the biting arthropods by clothing, fly repellants. Tissues and blood can be protected from the infection by vaccination or by prophylactic use of antibiotics and chemotherapeutic.

The Influence of Environmental Factors on the growth of Microorganisms:

Chemical, physical, environmental and biological factors influencing the growth of microorganisms are:

Nutrition, moisture, temperature, presence or absence of oxygen, osmotic pressure, pH etc.

Chemical Factors:

1. Nutrition,

2. Oxygen,

3. pH,

4. Surface,

5. Phenols, Cresols and derivatives,

6. Dyes,

7. Salts of heavy metals,

8. Oxidising agents,

9. Formaldehyde,

10. Antiseptics.

Physical Factors:

1. Moisture,

2. Temperature,

3. Dessication,

4. Light,

5. High pressure,

6. Osmotic pressure.

Biological Factors:

1. Symbiosis;

2. Metabiosis;

3. Satellism;

4. Synergism;

5. Antagonism.

(A) Chemical Factors:

1. Nutrition of Microorganism:

(a) Metabolism means the digestion and utilisation of food to

(i) synthesize the proteins, facts, carbohydrates and other substances of which living cells are made up, and to

(ii) furnish the energy necessary for living and reproduction. The protoplasm (cell substance) of all living cells is therefore identical. All living cells utilise and metabolize all foods by means of enzymic mechanism. There are slight differences among different species of bacteria.

Foods of Microorganism:

Since Carbon (C), Hydrogen (H) , Oxygen (O), Nitrogen (N), Phosphorus (P) and Sulfur (S) with smaller amounts of Magnesium (Mg), Iron (Fe), Copper (Cu), Sodium (Na), Potassium (K) and other elements are the main components of protoplasm and other structures of all living cells;

the nutrients of all cells must contain all these elements. In addition, the growth factors, such as nicotinic acid (niacin), thiamine, riboflavine are utilised by bacteria in much the same manner as those cells of human body do. The composition and metabolism of human cells are similar to those of many microorganisms.

Organic compounds:

Proteins, carbohydrates, fats.

Inorganic compounds: Sodium chloride, water, carbon dioxide, sulphur, iron, hydrogen.

Carbon Source for Growth:

Like plants, some non-parasitic bacteria are able to utilise carbon dioxide (CO₂) as the main source of carbon and are called autotrophs (autos-self, trophe-nutrition) or lithographs . Energy is obtained in these organisms by the oxidation of inorganic compounds (Chemosynthetic autotrophs) or from the sunlight (Photosynthetic autotrophs).

The majority of bacteria require organic nutrients, such as carbohydrates, amino acids, peptides or lipids to serve as the source of carbon and energy. These organisms are called heterotrophs (Gr. hetero-another, trophe-nutrition) or organotropics and they obtain their energy by breakdown of the organic carbon source.

They differ in the utilisation of organic compound. Thus some species of the genus Pseudomonas can utilise any one of a hundred organic compounds (sugars, acids, alcohol etc.) as the sole source of carbon and energy. On the other hand, many bacteria are much more specific in their requirements.

Nitrogen Source for Growth:

Bacteria differ widely in their ability to synthesise the main nitrogenous structural units-amino acids and nucleotides. Some are able to grow on ammonium salts as the sole source of nitrogen, while others require a variety of amino acids and nucleotides preformed in the medium.

2. Influence of Oxygen and Redox Potential:

Majority of bacteria are able to grow either aerobically i.e. in presence of air and free oxygen or anaerobically, in the absence of oxygen and they are called as facultative anaerobes (e.g. Salmonella typhi, Staphylococcus aureus and Streptococcus pyogenes).

Certain other bacteria will grow only in the presence of air or free oxygen and are described as obligate aerobes ( e.g. tubercle bacilli, Bacillus cereus; common saprophytes of the soil).

Still others will grow only in absence of free oxygen and are usually killed in its presence, they are known as strict anaerobes (i.e. Clostridium tetani). In the latter case, the ultimate determining factor is the state of oxidation of the environment, this being described in terms of oxidation-reduction or “redox” potential.

A sufficiently low redox potential for the growth of strict anaerobe is usually provided by placing the culture media in an atmosphere of hydrogen with the complete exclusion of oxygen (e.g. Mcintosh and Fieldes anaerobic jar).

It has been suggested that, in the presence of oxygen, a strict anaerobe is liable to produce toxic peroxides which it cannot destroy owing to lack of catalase, an enzyme present in most aerobes and facultative anaerobes.

Finally, there is a group of organisms which grow best in the presence of a trace of free oxygen and often prefer an increased concentration of carbon dioxide (CO₂), these are called microaerophilic.

3. Influence of Hydrogen Ion Concentration:

Hydrogen Ion Concentration:

Hydrogen atoms, when dissolved in water, immediately assume a positive electrical charge they are called hydrogen ions and the acidity of any solution is due to them. Acids such as hydrochloric or acetic acid, possess the property of acidity because they give off hydrogen ions (i.e. they ionize) in aqueous solutions.

If a solution contains a large number of these ions, it is very acidic and it is said to have a high hydrogen ion concentration. If it is only slightly acidic or alkaline, it is said to have a low hydrogen ion concentration.

pH hydrogen ion concentration is expressed by numbers used in the symbol of pH , which stands for hydrogen ion concentration. pH is an essential factor in bacterial metabolism and growth. The majority of commensal and pathogenic bacteria grow best at a neutral or very slightly alkaline reaction (pH 7.2 to 7.6).

Some bacteria flourish in the presence of considerable degree of acidity and are termed acidophilic, e.g., Lactobacillus. Others are very sensitive to acid, but tolerant to alkali, e.g., Vibrio cholera.

Most of the bacteria are rapidly killed by strong acid or alkali solutions e.g 5 per cent hydrochloric acid or sodium hydroxide but the mycobacteria (e.g tubercle bacilli) are resistant to them.

According to their effect on bacteria, bactericidal chemical substances can be divided into surface active substances, phenols and their derivatives, salts of heavy metals, oxidising agents and the formaldehyde group.

4. Surface Active Substances:

Change the electrical charges. Bacterial cells lose their negative charge and acquire positive charge which impairs the normal function of the cytoplasmic membrane.

Bactericidal substances with surface active action include fatty acids and soaps which harm only the cell wall and do not penetrate into the cell.

5. Phenol, Cresol and Related Derivatives:

First of all injure the cell wall and then cell proteins. Some substances of this group inhibit the function of coenzyme (diphosphopyridine nucleotide) which participates in the dehydrogenation of glucose and lactic acid.

6. Dyes:

Dyes are able to inhibit the growth of bacteria. The basis of this action is the marked affinity for the phosphoric acid groups of nucleoproteins. Dyes with bactericidal properties include brilliant green, acriflavine etc.

7. Salts of Heavy Metals:

(Lead, copper, zinc, silver, and mercury) cause coagulation of the cell proteins, silver, gold, copper, zinc, tin, lead etc. have an oligodynamic action (bactericidal activity). Thus, for example, silver plated objects, silverware in contact with water may render the metal bactericidal to many species of bacteria.

The mechanism is that the positively charged metallic ions are absorbed on to the negatively charged bacterial surface and alter the permeability of the cytoplasmic membrane. Thereby, the nutrition and reproduction are, possibly, disturbed. Under the influence of the salts of heavy metals, viruses may become irreversibly inactivated as they are very sensitive to these salts.

8. Oxidising Agents:

Chlorine is commonly used in decontaminating water, because chlorine acts on dehydrogenase, hydrolase, amylase, proteinase of bacteria; chloride of lime and chloroform are used as disinfectants.

In medicine, iodine is used successfully as antimicrobial substance in the form of tincture iodine which oxidises proteins of bacterial cytoplasm and causes their denaturation. Potassium permanganate and hydrogen peroxide are other oxidising agents.

Many species of viruses are resistant to heat, chloroform, ethyl and methyl alcohol and volatile oils. All viruses are resistant to 50 per cent glycerine, except Rinderpest virus, sodium hydroxide, chloride of lime and chlorine can destroy viruses as they are oxidising agents.

9. Formalin:

(40 per cent formaldehyde) has bacteriocidal effect, because it possibly unites with the amino groups of proteins and brings out denaturation of proteins. Formaldehyde destroys both the vegetative forms and the spores of bacteria.

It is also used to decontaminate diphtheria and tetanus toxins and, at the same time, it detoxifies the toxins transforming them into anatoxins (toxoids). Phage and tobacco mosaic virus inactivated by formalin can sometimes renew their infectivity.

(B) Physical Factors:

1. Moisture:

Since the bacterial cell consists of water, moisture is essential for its growth. Drying in air is injurious to many microorganisms, it may prevent the growth of microorganisms and may not kill them. One method adopted by microbiologist to keep certain microorganisms alive for long period (20 years or more) to dry them and store them in a refrigerated vacuum.

Even delicate, non-sporing organisms may survive drying for many years if they are desiccated a high vacuum (0.01mm Hg) or less in sealed glass ampoule which is stored at room temperature in the dark.

This is the basis of lypholisation or freeze drying process of preserving bacterial cultures or vaccines and viral vaccines in the laboratory or also during long distance transport. Quick freezing of bacterial and viral suspension at a very low temperature provokes conditions at which crystals do not form and subsequent disruption of the microorganisms does not occur.

Certain Fragile Microorganisms:

Neisseria gonorrhoeae (gonococci) causing gonorrhoea, Treponema pallidum causing syphilis, Entamoeba histolytica causing amoebic dysentery — die almost at once when subjected to drying, while tubercle bacilli, Staphylococcus aureus and smallpox virus may survive for several months in sputum, pus, crust, respectively.

2. Temperature:

Low temperatures halt putrefying and fermentative process. The process of metabolism is inhibited, as a result bacteria die and the cells are destroyed under the influence of the formation of ice crystals during freezing. Vibrio cholera looses its viability at a temperature of -32°C. Some species of bacteria remain viable at a temperature of liquid air (-190°C) and of liquid hydrogen (-253°C).

Diphtheria bacilli are able to withstand freezing for 3 months and enteric fever bacteria (Salmonella typhi) are able to live long in ice. Bacillus spores withstand a temperature of -253°C for 3 days. Many microorganisms remain viable at low temperature and viruses are especially resistant to low temperature.

Thus for example Japanese encephalitis virus in 10% Brian suspension does not loose its pathogenicity at -70°C for a year; the causative agents of influenza and Trachoma at -70°C for 6 months and Coxsackie virus at -40°C for 1 Vi years.

Only certain species of pathogenic bacteria are very sensitive to low temperature (e.g. meningococci, gonococci.) During short periods of cooling, these species die quite rapidly. This is taken into account in laboratory diagnosis and the specimens for the presence of meningococci or gonococci should be sent to the laboratory protected from cold.

Some microorganisms can grow best at a low temperature (4° to 10°C) are said to be psychrophilic, whereas those bacteria growing best at temperature (from 20°C to 37°C) are called mesophylls.

Most of the pathogenic non-sporing bacteria are killed at 58°C to 60°C for 30-60 minutes. Bacillus spores can withstand boiling (100°C) from a few minutes to 3 hours, but under the effect or dry heat at 160°-170°C in 60-90 minutes. Heating at 121°C (15 lbs per sq. in gauge pressure) kills the bacteria and their spores within 20-30 minutes.

Some microorganisms will grow only at high temperature ranging from 45° to 75°C and are called as thermophilic e.g. Bacillus stearothermophilus.

The bactericidal action of high temperature is due to the inhibition of the catalase, oxidase, dehydrogenase activity, protein denaturation and an interruption of the osmotic pressure. High temperature causes rapid destruction of viruses, but some of the viruses (viruses of serum hepatitis, poliomyelitis) are resistant to environmental factors.

They remain viable long in water, in the faeces of patients or carriers and are resistant to heat at 60°C and to small concentration of chlorine in water.

3. Dessication:

Dessication is accompanied by dehydration of the cytoplasm and denaturation of bacterial proteins. Gonococci, meningococci, treponema, leptospira and phages are sensitive to dessication.

Vibrio cholera persist for 2 days on exposure to dessication, dysentery bacteria for 7; plague bacilli for 8, diphtheria bacilli for 30; bacilli for enteric fever for 70; staphylococci and tubercle bacilli for 90 days. The dry sputum of tubercular patients remain infectious for 10 months.

Dessication in a vacuum at a low temperature does not kill bacteria, rickettsiae, viruses. This method of preserving cultures is employed in the manufacture of stable long storage or live vaccines against tuberculosis, plague, brucellosis, small pox, influenza.

4. Light:

Direct sunlight has the greatest bactericidal action. Different kinds of light have a bactericidal or sterilizing effects. They are ultraviolet rays (electromagnetic rays with a wave length of 200- 300µ, X-rays (electromagnetic rays with a wavelength of 0.005-mu); gama rays (short wave X-rays); beta particles or cathode rays (high speed electrons ); alpha particles (high speed helium nuclei) and neutrons.

Rays of short waves used for the disinfection of wards, infectious material, for the conservation of products the preparation of vaccine, treating operation and maternity wards, have a high bactericidal effect. Viruses are very quickly inactivated under the influence of ultraviolet rays with a wavelength of 20-300 mu.

These waves are absorbed by the nuclei acid of viruses. Longer waves are weaker and do not render viruses harmless. Viruses in comparison to bacteria are less resistant to X-rays and gamma-rays. Beta rays are more markedly viricidal.

5. High Pressure and Mechanical Injury to Microbes:

Bacteria withstand easily atmospheric pressure. Some bacteria, yeasts and moulds withstand a pressure of 3,000 atmospheric pressure. The movement of liquid media has a harmful effect on microbes. The movement of water in rivers and streams, undulations in stagnant water are important factors in self-purification of reservoirs from microbes.

Ultrasonic oscillation (Waves with a frequency of about 20,000 hertz per second) has a bactericidal activity and it is used for the preparation of vaccines and disinfection of various objects.

6. Osmotic Pressure:

Like other living cells, bacteria have a semi-permeable cytoplasmic membrane which is subjected to osmotic pressure. Bacteria are very tolerant of changes in the osmotic pressure of their environment and can grow in media with widely varying contents of salts, sugar and other such solutes because of the thickness and mechanical strength of their walls.

For most species, the upper limit of sodium chloride concentration permitting growth lies between 5 to 15 per cent.

Though halophilic (or osmophilic) species occur which can grow at higher concentration up to saturation. Sudden exposure of bacteria to solution of high concentration (e.g. 2-25 per cent sodium chloride) may cause plasmolysis (i.e. temporary shrinkage of the protoplast and its retraction from cell wall due to osmotic withdrawal of water), this occurs much more rapidly in Gram-negative than in Gram-positive bacteria.

Sudden transfer from a concentrated to a weak solution or to distilled water, may cause plasmolysis i.e. swelling and bursting of cell through excessive imbibition of water.

C. Biological Factor:

1. Symbiosis is an intimate mutually beneficial relation of organisms of different species.

They develop together better than separately. Sometimes the adaptation of two organisms become so close that they lose their ability to exist separately (symbiosis of the fungus and blue green algae).

2. Metabiosis is one type of relationship in which an organism continues the process caused by another organism, liberating it from all its life activities and thus creating conditions for its own further development (nitrifying bacteria).

3. Satellism a symbiont known as the favourable microbe, accelerates the growth of the other (some yeasts producing amino acids, vitamins etc.), enhance the growth of microbes which are very specific regarding their nutrient requirements.

4. In synergism there is an increase in the physiological function of microbial association (Borrelia and fusobacterium).

5. In antagonism there is struggle for oxygen, nutrients and a habitat (antibiotics).

Application to Nursing:

The bacteriologist, nurse will use the methods of the control of the microorganisms by the knowledge of the relation of microorganisms to their environmental factors.

For example, high temperature affect the living cell structure causing death; pathogenic bacteria are capable to produce infection or toxin according to the presence or absence of oxygen. The inhibition or destruction of microorganisms depends upon the removal of food, water and oxygen.

In nursing practice, the microbial growth can be controlled by altering the temperature and subjecting the microorganisms to disinfectants.

In nursing situation (operating rooms, nurseries, communicable disease units, isolation wards, intensive care units), microorganisms can be destroyed by physical environmental factors (X-rays, gamma rays etc.).

Introduction of antiseptics in surgical practice prevented the access of microbes into the wounds. Aseptics are attained by disinfection of the air and equipment’s of the operating room, by sterilisation of surgical instruments and materials and by disinfecting the hands of surgeon and the skin of the operating field.

Modern methods of aseptics have been well perfected, by which almost all operations are accompanied by primary healing of wounds without suppuration and the post-operative septicaemia has been completely eliminated.

Dental microbiologist would be able to adopt the suitable methods control the growth of microorganisms according to their relationship with the environment. The growth of microorganisms can be slowed down or completely stopped by altering the environmental growth factors. The high temperature can affect the structure of the living bacterial cells, so that they may be completely destroyed.

Anaerobic bacteria can grow very well in absence of oxygen and elaborate the toxin which is responsible for the pathogenic effect. In the presence of oxygen, the same bacteria cannot produce such toxin and, hence, they are not pathogenic.

It may not be always possible to inhibit or destroy microorganisms by simple removal of food, oxygen, water. In nursing practice, the microbial growth can be controlled primarily by subjecting the microorganisms to the sterilization and disinfection. Ultraviolet rays can be sometimes used to kill the microorganisms in nursing situations (operation rooms, nurseries, communicable diseases).