Essay on Disease Causing Bacteria in Humans | Microbiology

Here is a list of some essays on disease causing bacteria in humans. Also learn about about its prevention and control.

Essay # 1. Diplococcus:

The pneumococcus (Diplococcus pneumoniae) is the causative agent of lobar pneumonia. The disease is nearly always caused by this organism, although other bacteria are occasionally involved.

Diplococcus Pneumonia:

Cells oval or spherical, measuring 0.5 to 1.25 μ, typically in pairs, occasionally singly or in short chains. The distal ends of each pair of cells tend to be pointed or lancet-shaped. Encapsulated. Non-motile. Young cells Gram-positive. Usually grow poorly or not at all on artificial media.

Whole bile or 10 per cent solutions of sodium taurocholate or sodium glycocholate added to actively growing broth cultures will dissolve the organisms. The pneumococci are generally classified according to types.

The cells contain two types of antigens:

(i) The so-called “somatic antigen” and

(ii) The “polysaccharide hapten” or soluble specific substances (SSS).

The somatic antigen is probably a nucleoprotein and is found to react with all pneumococci regardless of types. The carbohydrate hapten is type specific and serves to differentiate the various types.

In other words, by means of immunological reactions, the various types of pneumococci can be distinguished from one another by the composition of the polysaccharide comprising the capsular material. These polysaccharides belong to a group of substances called haptens, or partial antigens. They are not antigenic in themselves but may become so when combined with protein.

At least 32 types of D. pneumoniae are recognized on the basis of serological reactions, chiefly the Quellung phenomenon as induced by type-specific immune rabbit serums.

Typing of Pneumococci:

A number of methods are employed for the determination of pneumococcal types:

i. Precipitin Test:

A sample of sputum (about 1 ml.) from a suspected case of pneumonia is injected into the peritoneal cavity of a mouse. The animal will appear sick in about 8 hr. The mouse is usually killed after this period of time, the peritoneal cavity opened, and the exudate washed into a Petri dish with the aid of about 2 ml. of saline solution. The washings are centrifugated, and the supernatant liquid is used for the precipitin test.

ii. Microscopic Agglutination Test:

The peritoneal exudate is prepared as given under (1), and small drops of the washings are placed on a slide. The various type serums are added to the drops and spread out in thin films. The films are allowed to dry, then stained and examined under the microscope for the presence of clumps. The type of serum producing clumps indicates the type of organism present in the peritoneal exudate.

iii. Quellung Test:

This test is based on the observation that the capsules of pneumococci become swollen when placed in contact with specific immune serum. A loopful of undiluted immune serum is placed on a slide and mixed with a fleck of sputum. A loopful of dilute methylene blue solution is then added and the mixture examined under the microscope.

If the mixture is homologous, the capsules will appear swollen with very distinct outlines. The test is made with many types of antiserums and as many flecks of sputum.

Pneumococci, regardless of serological type, manifest three chief culture phases or stages – mucoid, smooth, and rough. The mucoid form represents the typical phase of the species. The most frequently observed dissociative trend is M → S → R. Serological types are recognizable only in the mucoid form, owing to the presence of type specific polysaccharides in the capsular material.

Both smooth and rough forms are devoid of capsular material, but possess species-specific antigens common to all members of the species. Smooth and rough forms are nonpathogenic, possess distinctive growth characteristics, and require special technique for accurate observations.

Disease Produced – The commonest cause of lobar pneumonia, the incidence being as high as 95 per cent. The organism is present in the alveoli and bronchioles of the lung, in the lymph channels, and sometimes in the blood. The organism may also produce pericarditis, arthritis, meningitis, otitis media, mastoiditis, endocarditis, rhinitis, tonsillitis, conjunctivitis, septicemia, osteomyelitis, and peritonitis.

Diagnosis – Organisms may be detected by appropriate laboratory examinations of sputum, discharges of the respiratory tract.

Source of Infection – Sputum, blood, and exudates in pneumonia; cerebrospinal fluid in meningitis; saliva from respiratory tract of normal individuals.

Mode of Transmission – Direct contact with infected person or carrier; inhalation of droplets expelled from the throat during coughing and sneezing.

Incubation Period – Generally believed to be 1 to 3 days.

Susceptibility and Immunity – Resistance generally high but may be lowered by exposure to wet and cold, by physical and mental fatigue, and by alcoholism. Acquired immunity may follow an attack of pneumonia. Immunity relatively slight and of short duration.

Prevalence – Affects a large proportion of the population. No race, colour, or sex exempt. Occurs in all climates but most often in winter and spring and in colder regions. Epidemics occur under conditions of crowding.

Prevention and Control – Crowding in living and sleeping quarters should be avoided. General resistance should be maintained by adequate nourishing food, sufficient sleep, fresh air, and personal hygiene. Isolation of suspected cases of pneumonia. Disinfection of discharges from nose and throat, and articles soiled by such discharges.

Treatment consists of intramuscular or oral administration of the appropriate form of penicillin. Tetracycline antibiotics equally effective. Sulfonamides and erythromycin are usually effective. Mortality rate has been reduced to an insignificant figure since the employment of antibiotics.

Essay # 2. Escherichia:

The escherichiae are short rods, motile or non-motile. Glucose and lactose fermented with production of acid and gas. Acetylmethylcarbinol not produced. Methyl red test positive. Carbon dioxide arid hydrogen produced in approximately equal volumes from glucose. Generally not able to utilize uric acid as sole source of nitrogen. Found in feces; occasionally pathogenic to man, producing enteritis, peritonitis, cystitis, etc. Gram-negative. Widely distributed in nature.

Disease Produced – E. coli is a normal inhabitant of the intestinal tract of man and other vertebrate animals. It is generally nonpathogenic; in certain instances it has been found to overcome the defense mechanisms of the body to produce septicemia, peritonitis, inflammation of the liver and gall bladder, cystitis, meningitis, and other infections. Since the organism is found in the intestinal contents, its presence in water and foods generally means contamination with fecal material.

Essay # 3. Gaffkya:

The members of this genus occur in the animal body and in special media as tetrads, whereas in ordinary culture media they occur in pairs irregular masses. Aerobic to anaerobic. The most important species is G. tetragena.

Gaffkya Tetragena:

Cells spherical, measuring 0.6 to 0.8 μ. in diameter, with pseudocapsule (in body fluids) surrounding four of the cells showing typical tetrads. Gram-positive.

Disease Produced – G. tetragena is generally believed to be a normal inhabitant of sputum or saliva. It is found in tuberculous sputum, in the blood in cases of septicemia, in the pus of abscesses, and in the spinal fluid in meningitis. It appears to be a secondary invader of low virulence, invading the tissues only when weakened by some other infectious organism. Pathogenic for Japanese mice.

Essay # 4. Haemophilus:

This genus contains several important disease-producing species. Cells are minute, rod-shaped, sometimes thread-forming, and pleomorphic. Non-motile. Gram-negative. Strict parasites, growing only in presence of certain growth accessory substances. May or may not be pathogenic. Found in various lesions and secretions, as well as in normal respiratory tracts of vertebrates.

Haemophilus Influenza:

Cells are minute rods, measuring 0.2 to 0.3 by 0.5 to 2.0 μ, occurring singly, in pairs, occasionally in short chains, at times as long thread forms. Frequently show a marked tendency to bipolar staining. Some strains are encapsulated. Non-motile. Gram-negative.

H. influenzae is an obligate parasite and grows only in the presence of hemoglobin and other body fluids. It will not grow in the absence of the X and V factors present in blood. On the basis of the precipitin reaction, six types (A to F) are recognized. The majority of the strains from the respiratory tract are not type specific.

Disease Produced – H. influenzae was at one time believed to be the etiological agent of influenza, but evidence now points to the fact that the disease is caused by a filterable virus. The organism is commonly present in the normal nose and throat and has been found to be a secondary invader in a number of bacterial and virus infections, including scarlet fever, measles, chicken pox, and whooping cough.

It is also believed to be responsible for cases of endocarditis, sinusitis, meningitis, bronchopneumonia, and acute infectious conjunctivitis, or “pink-eye”. Type B organisms have been found to be the cause of severe throat infections in children.

Source of Infection – Discharges from nose and throat or articles soiled by such discharges.

Mode of Transmission – Contact with an infected person; use of towels or other freshly contaminated articles; inhalation of droplets expelled during coughing, sneezing, and talking.

Incubation Period – Usually 24 to 72 hr.

Susceptibility and Immunity – Children under five years most often affected; incidence decreases with age. Immunity after attack low-grade and variable.

Prevalence – May be isolated from pharynges of almost all normal individuals, and virulent encapsulated strains are found in chronic infections of the nasal sinuses and in the pharynges following viral colds.

Prevention and Control – Isolation of infected individuals; avoidance of overcrowding; disinfection of articles soiled by discharges from nose and throat. The antibiotics Aureomycin, Terramycin, and streptomycin have been found useful in controlling infections.

Haemophilus Ducreyi:

Small rods, measuring 0.5 by 1.5 to 2 μ, with rounded ends, occurring singly and in short chains. Non-motile. Gram-negative. Requires the X factor for growth.

Disease Produced – The cause of soft chancre or chancroid, an acute inflammatory lesion that occurs upon the genitals or, less frequently, the skin surrounding the genitals. Lesion starts as a small pustule which eventually ruptures to form an open ulcer. Infection easily spreads to other areas. Genital lesions frequently accompanied by painful inflammatory swelling and suppuration of regional lymph nodes.

Source of Infection – Discharges from ulcerated lesions.

Mode of Transmission – Chiefly by sexual intercourse or by articles soiled with discharges from ulcerated lesions. Organism quickly loses its viability outside of body and soon dies.

Incubation Period – From 3 to 5 days, or longer.

Susceptibility and Immunity – Susceptibility to disease is general; probably no natural immunity. One attack does not confer protection against subsequent infection.

Prevalence – Widespread in distribution. Particularly common where sexual promiscuity occurs.

Prevention and Control – Disease spread largely by sexual contact. Prophylactic measures before, during, and following exposure to disease.

Avoid sexual contact until lesions are healed. Sulfonamides appear to be specific for disease. Aureomycin and chloramphenicol may be used if organisms have developed resistance to sulfonamides. It is not a serious disease and yields readily to local treatment.

Haemophilus Aegyptius:

Small rods, measuring 0.25 to 0.5 by 1 to 2.5 μ, occurring singly, occasionally in short chains, and at times in the form of threads. Show bipolar staining. Non-motile. Gram-negative. Requires both V and X factors for growth.

Disease Produced – The cause of sore eyes or pinkeye. Lacrimation, irritation, and vascular injection of conjunctivae of one or both eyes, followed by edema of the lids, photophobia, pain, and mucopurulent discharge. A nonfatal disease lasting 2 to 3 weeks. In most cases disease runs a mild course.

Source of Infection – Discharges from conjunctiva or upper respiratory tract of infected persons.

Mode of Transmission – Contact with infected persons through contaminated fingers, or articles soiled with such discharges. Incubation Period. Generally 24 to 72 hr.

Susceptibility and Immunity – Children under five most often affected; incidence decreases with age. Immunity of low grade and variable.

Prevalence – Disease widespread throughout world, particularly in warmer climates. Largely confined to southern states and California in summer and early autumn.

Prevention and Control – Personal cleanliness, and treatment of affected eyes. Isolation of affected individuals. Disinfection of discharges and articles soiled by such discharges. Specific treatment consists of local application of Aureomycin, Terramycin, and streptomycin.

Essay # 5. Leptospira:

The leptospirae are finely coiled organisms 6 to 20 μ in length. Spirals 0.3 μ in depth and 0.4 to 0.5 μ in amplitude. In liquid medium one or both ends are bent into a semicircular hook, each involving one-tenth to one-eighth of the organism. Spinning movements in liquid and vermiform in semisolid agar, forward or backward.

Stain with difficulty except with Giemsa stain and silver impregnation. Require oxygen for growth. Chiefly saprophytic organisms, being found in water, and sometimes in normal mouth. Most important member producing disease is L. icterohaemorrhagiae, the cause of infectious, jaundice or Weil’s disease. Most satisfactory method of differentiation based on antigenic structure of leptospires, which can be determined by agglutination-lysis and cross-absorption tests.

Leptospira Icterohaemorrhagiae:

Cells measure 0.25 to 0.3 by 6 to 9 μ; occasionally 20 to 25 μ in length. Spiral amplitude 0.4 to 0.5 μ, regular, rigid; spiral depth 0.3 μ regular. One or more gentle waves occur throughout the entire length. In liquid media, one or both ends may be semi-circularly hooked; in semisolid media, organism appears serpentine, waved or bent. Very active flexibility. Axial filament does not extend beyond cell body. Flagella absent.

Disease Produced –  The cause of infectious jaundice or Weil’s disease in man. Found also in the blood of dogs and wild rats. An acute systemic infection characterized by headache, malaise, muscular pains, gastrointestinal symptoms, vomiting, and fever at the start, followed by jaundice. About half of patients do not develop jaundice.

Relapses occurring in about 20 per cent of all cases. About same percentage of cases are fatal. In severe cases, Leptospira icterohaemor hemorrhages may occur at various sites, and rhaiae, the cause of Weil’s disease or kidney damage may be marked. Organisms found in blood and urine of patients.

Diagnosis – Disease diagnosed by inoculating guinea pigs or hamsters with blood taken early in course of disease.

Source of Infection – Urine and feces of rats, dogs, cats, mice, and other animals. Water and soil become contaminated with discharges of infected animals.

Mode of Transmission – Infection in man probably occurs through rubbing contaminated soil into the skin, eyes, and nose, or from swallowing contaminated water. Disease shows selection for such trades as fish dealers, abattoir workers, sewer workers, miners, veterinarians, and agriculturists. Occasionally infections result from handling dogs and other animals. Organism can penetrate uninjured skin.

Incubation Period – From 4 to 19 days, usually 9 to 10 days.

Susceptibility and Immunity – Susceptibility is general. A refractory state develops following recovery. Immune bodies may be demonstrated for a considerable period after recovery. Urine may show the presence of the organism for months after convalescence.

Prevalence – Disease of world-wide distribution in rats. Sporadic cases in humans reported throughout the United States.

Prevention and Control –  Avoidance of swimming in potentially contaminated waters. Rodent control in human habitations.

Disinfection of urine and feces of patients.

Penicillin, streptomycin, and the tetracyclines are leptospirocidal in vitro but of undemonstrated value in vivo.

Vaccine has been employed with promising results. Horse antiserum has been used to advantage.

Essay # 6. Moraxella:

These organisms are small, short rods which occur as diplobacilli and which are sometimes described as diplococci; occasionally occur singly. Non-motile. Gram-negative. Do not require V or X- factors for growth. Aerobic. Found as parasites and pathogens in warm-blooded animals, being especially found in association with diseases of the eye.

Moraxella Lacunata:

Short rods, 0.4 to 0.5 by 2 μ, occurring singly, in pairs, and in short chains. Ends rounded or square in the chains.

Disease Produced –  The cause of subacute infectious conjunctivitis, or angular conjunctivitis which usually attacks both eyes. There is rarely much swelling or ulceration of the conjunctiva. Condition runs a subacute or chronic course.

Diagnosis – Made by smear preparations of the pus, which is especially abundant during the night.

Source of Infection – Discharges from conjunctivas.

Mode of Transmission – Contact with an infected person or with articles freshly soiled with discharges from such a person.

Susceptibility and Immunity – Disease may be produced in human beings by inoculation. No acquired immunity.

Prevention and Control – Isolation of patient; disinfection of conjunctival discharges and articles soiled by such discharges.