Quick Notes on Microbial Pathogenicity

The following points highlight the six main factors that contribute for microbial pathogenicity. The factors are: 1. Adhesion 2. Invasiveness 3. Toxigenicity 4. Communicability 5. Infectivity Dose 6. Route of Infection.

Microbial Pathogenicity: Factor # 1. Adhesion:

The initial event of microbial pathogenicity is the adherence of the pathogen to the body surface of the victim. This attachment is not a chance event but a specific reaction between surface receptors on host cells and adhesive structures (ligands) on the surface of the bacterial pathogen. These adhesive structures are called adhesins.

Adhesins are structures or specialized molecules that bind to complementary receptor sites on the host cell surface. They may occur as organized structures such as fimbriae, pili, or colonization factors. Adhesins serve as virulance factors, and loss of adhesive often renders the strain avirulent. Adhesins are generally made up of protein and are antigenic in nature.

Neisseria gonorrhoeae, the cause of gonorrhea, adheres specifically to the epithelial cell layer of the human cervix, urethra, and conjuctiva by means of pili and thus avoids being washed away by the flow of mucus or tears. Escherichia coli strains that cause “scours”, a diarrheal disease of newborn pigs, also possess pili that allow the bacteria to attach firmly to the living of mucosa of the small intestine.

Certain proteins, however, located on the outer surface of the bacterial cell wall have been demonstrated to be essential for the initiation of infection. For instance, Streptococcus pyogenes that cause streptococcal sore throat attach superficially to the epithelial cells of the throat by means of cell wall proteins referred to as M proteins.

A number of viral pathogens also adhere on the host cell surface. The surface of influenza virus is studded with haemgglutinin spikes that attach the virus to specific micro-protein receptors on the surface of host cells.

Neuraminidase spikes on the virus surface also may possibly aid attachment by degrading the protective mucous layers of mucous membranes and allowing viral attachment to lipid and glycoprotein containing receptors on host cells.

Major adhesins, the concerned pathogen, and the functions are given in Table 44.2.

Microbial Pathogenicity: Factor # 2. Invasiveness:

Invasiveness refers to the ability of a microbial pathogen to grow in host tissues in such large numbers that the pathogen inhibits host function. A microorganism may still be able to produce disease through invasiveness even if it produces no toxin.

For instance, the major invasiveness factor for Streptococcus pneumoniae is the polysaccharide capsule that prevents the phagocytosis of pathogenic strains defeating a major defence mechanism used by the host to prevent invasion.

Encapsulated strains of S. pneumoniae are able to cause extensive host damage because they are highly invasive. They flourish in lung tissues in enormous numbers and lead to pneumonia. Non encapsulated strains are quickly and efficiently taken up and destroyed by phagocytes.

Invasiveness may be localized or generalized (systemic). Invasive pathogens cause localized lesions and the latter may be superficial or deep-seated (e.g., staphylococcus abscess), while highly invasive pathogens charateristically produce generalized (systemic) infections. Generalized or systemic infection involves the spread of the pathogen from the site of entry by contiguity, through tissue spaces or channels, along the lymphatics or through the bloodstream.

Circulation of bacteria in the blood is known as bacteremia. Transient bacteremia is a frequent event even in healthy individuals and may take place during chewing, brushing of teath, or straining at stool. Bacteremia of greater severity and longer duration is seen during generalized infections as in typhoid fever. Septicemia is the condition of generlized infection where bacteria circulate and multiply in the blood and cause high swinging type of fever.

Invasiveness generally enhances the production of extracellular enzymes by the microbial pathogen. These enzymes can destroy the defence mechanisms of the host and are referred to as aggresins. Aggresins are diffusible substances and either protect the pathogen from phagocytosis or enable the pathogen to resist intracellular digestion within a phagocyte.

However, some important aggresins are described as under:

1. Hyaluronidase:

Hyaluronidase is an enzyme produced by microorganisms such as Clostridium perfringens, Pneumococci and Haemolytic streptococci. This enzyme destroys haluronic acid, which is an important intracellular cementing substance responsible to bind tissues into their normal structure and opposes the invasion of microorganisms through the tissues. Hyaluronidase is, therefore, known as spreading factor.

2. Collagenase:

Collagenase is another patent proteolytic enzyme and is produced by Clostridium perfringens. It breaks down collagen which constitutes an integral part of connective tissues and tendons.

3. Coagulase:

Coagulase is produced by some virulent strains of Staphylococci coagulase. This enzyme acts with an activator in plasma to coagulate fibrinogen in the serum of certain animals. As a part of this action fibrin coats the cell walls of the bacteria thus protecting them against the action of phagocytes.

4. Streptokinase:

Streptokinase or Fibrinolysins are produced by β-haemolytic streptococci and dissolves human fibrin but not animal fibrin. Streptokinase appears to be an enzyme that is more active in virulent strains of the microorganisms. It is antigenic.

5. Lecithinase:

Lecithinase or α-toxin is the enzyme that destroys various tissue cells and is especially active in lysis of red blood corpuscles. Lecithinase is also found in snake venom. It is produced by Clostridium perfringens.

6. Leucocidin:

Leucocidin is a substance produced by some staphylococci and streptococci that can kill leucocytes (WBC) in vitro. It is produced by Staphylococcus aureus and is neutralized by staphylococcal antitoxin. The latter does not affect leucocidin produced by streptococci.

Table 44.3 shows the names, pathogens involved, and the mechanism of action of various aggresins.

Microbial Pathogenicity: Factor # 3. Toxigenicity:

Toxigenicity refers to the ability of a microbial pathogen to cause disease by means of a preformed toxin that inhibits host cell function or kills host cells. Toxin (L. toxicum = poison) is a substance, such as a metabolic product of the pathogen, that affects the normal metabolism of host cells to such an extent that a deleterious effect manifests on the host.

Bacterial toxins can be divided into two main categories, namely, exotoxins and endotoxins. Exotoxins are toxic proteins excreted by bacteria into their surrounding medium, while endotoxins are potentially toxic lipopolysaccharides located in the outer membrane of many gram-negative bacteria and released only when the bacterial cells disintegrate. The primary characteristics of these two toxin groups are compared in Table 44.4.

(i) Exotoxins:

Exotoxins are toxic proteins excreted by bacteria into their surrounding medium. They are soluble, heat-labile, and may travel from the limited site of infection to distantly located other body tissues or target cells in which they exert their effects.

The primary or general characteristics of exotoxins are that they are:

(i) Heat-labile proteins inactivated at 60-80°C,

(ii) Most lethal substances toxic even in very small quantity,

(iii) Synthesized by genes present in plasmids or pro-phages,

(iv) Very specific in their mechanism of action,

(v) Highly immunogenic and stimulate the production of neutralizing antibodies called antitoxins,

(vi) Easily inactivated by formaldehyde, iodine, and other chemicals to result in immunogenic toxoids and

(vii) Usually unable, to produce fever in the host body.

Exotoxins are often given the name of the disease they produce (e.g., tetanus toxin, diphtheria toxin, botulinum toxin) and can be classified into four types based on their structure and physiologjcal activities.

These types are:

(i) AB exotoxins affecting general tissues,

(ii) AB exotoxins affecting specific host sites,

(iii) Membrane-disrupting exotoxins, and

(iv) Superantigen exotoxins.

1. AB Exotoxins Affecting General Tissues:

As exotoxins that affect general tissues of the host are composed of two covalently bonded subunits, A and B. The B subunit binds to a cell surface receptor, allowing the transfer of the A subunit across the targeted cell memberane, where it functions to damage the cell.

Isolated A subunits are enzymatically active but lack binding and cell entry capability, whereas isolated B subunits possess binding capability but are nontoxic and biologically inactive. Diphtheria toxin and Shiga toxin are good examples of AB exotoxins that affect the general tissues.

Diphtheria toxin:

Diphtheria toxin, produced by Corynebacterium diphtheriae was isolated by Roux and Yersin in 1888 and is an important virulence factor in the pathogenesis of diphtheria. Rats and mice are relatively resistant to diphtheria toxin, but human, rabbit, guinea pig, and bird cells are very susceptible, with only a single toxin molecule required to kill each cell.

Diphtheria toxin is an AB toxin secreted by cells of C. diphtheriae as a single polypeptide. Its subunit B promotes specific binding of the toxin to a host cell receptor (Fig. 44.5).

After binding, proteolytic cleavage between subunit A and B allows entry of subunit A into the host cytoplasm. Subunit A then disrupts protein synthesis by blocking transfer of an amino acid from a transfer ribonucleic acid (tRNA) to the elongating polypeptide chain.

The toxin specifically inactivates elongation factor-2 (EF-2), a protein in eukaryotic cells involved in elongation of the polypeptide chain, by catalysing the attachment of adenosine diphosphate (ADP) ribose from NAD⁺. Following ADP-riboxylation. the activity of the modified elongation factor-2 decreases dramatically and protein synthesis is shutdown resulting in, finally, the death of the host cell.

Diphtheria toxin is produced only by strains of C. diphtheriae that are lysogenized by a bacteriophage called phage β, which carries the toxin-encoding tox gene. Non-toxigenic, non-pathogenic strains of C. diphtheriae can also be converted to pathogenic strains by infection with phage β.

2. AB Exotoxins Affecting Specific Host Sites:

There are various exotoxins, mostly AB exotoxins, that affect the specific sites in the body of the host by acting extracellularly or intracellularly.

These exotoxins can be categorized as:

(i) Neurotoxins (e.g., tetanus toxin, botulinum toxin),

(ii) Enterotoxins (e.g., cholera toxin, E. coli heat-labile toxins), and

(iii) Cytotoxins (e.g., nephrotoxin. hepatotoxin, cardiotoxin).

(i) Neurotoxins:

Neurotoxins (e.g., tetanus toxin, botulinum toxin) target the cells of central nervous system and usually are ingested as performed toxins.

Tetanus toxin:

Tetanus toxin is produced by Clostridium tetani, an obligate anaerobic bacterium normally occurring in soil and occasionally causing disease in animals (including humans). C. tetani inhabits deep wound punctures in the host body that become anoxic. Although C. tetani does not invades the body tissue from the initial site of infection, its toxin spreads via the central nervous system and causes spastic paralyses, the hallmark of tetanus that often leads to death.

Tetanus toxin is a AB exotoxin and on contact with the central nervous system, this toxin is transported through the motor neurons back to the spinal cord, where it binds specifically to ganglioside lipids at the termini of the inhibitory interneurons. The inhibitory interneurons normally work by releasing an inhibitory neurotransmitter, usually glycine, that binds to receptors on the motor neurons.

Normally, glycine from the inhibitory interneurons stops the release of acetylcholine by the motor neurons and inhibits muscle contraction, allowing relaxation of the muscle fibers. However, if tetanus toxin blocks glycine release, the motor neurons cannot be inhibited, resulting in tetanus, continual release of acetylcholine and uncontrolled contraction of the poisoned muscles (Fig. 44.6).

This results in a spastic, twitching paralysis, and affected muscles are constantly contracted. If the muscles of the mouth arc involved, the prolonged contractions restrict the mouth’s movement, resulting in the condition known as trismus or lockjaw. If respiratory muscles are involved, prolonged contraction may result in death due to asphyxiation.

Botulinum toxin:

Botulinum toxin is produced by Clostridium botulinum, an obligate anaerobic bacterium normally occurring in soil and occasionally causing disease in animals (including humans). C. botulinum is not invasive and virtually its effects are due to toxicity. C. botulinum sometimes grows directly in the body causing infant or wound botulism, and also grows and produces toxin in improperly preserved food. Botulism usually becomes fatal due to respiratory failure from flaccid muscle paralysis.

Botulinum toxin consists of seven related AB exotoxins, which are considered to be the most potent biological toxins known to man. It is demonstrated that one milligram of botulinum toxin is sufficient enough to kill more than 1 million guinea pigs.

Out of the seven distinct botulinum toxins known, at least two are encoded on lysogenic bacteriophages specific for Clostridium botulinum. The major toxin is a protein that forms complexes with nontoxic botulinum proteins to give a heteromeric biologically active macromolecule.

The biologically active toxin complex binds to presynaptic membranes on the termini of the stimulatory motor neurons at the neuromuscular junction, blocking the release of acetylcholine.

Because transmission of the nerve impulse to the muscle is through acetylcholine interaction with a muscle receptor, the botulinum-poisoned muscle cannot receive an excitatory signal and contraction is prevented. This results in flaccid paralysis and death by suffocation, which are characteristic of botulism. The mechanism of action of botulinum toxin is given in Fig. 44.7.

(ii) Enterotoxins:

Enterotoxins (G. enter = intestine) are the exotoxins whose activity directly affects the mucosa of small intestine generally causing profuse secretion of fluid into the intestinal luman.

Variety of bacteria, including food-poisoning ones (e.g., Staphylococcus aureus, Bacillus cereus, Clostridium perfringens) and the intestinal pathogenes (e.g.. Vibrio cholerae, Escherichia coli, and Salmonella enteritidis), produce enterotoxins. Out of these, cholera toxin (V. cholerae) is the classic entertoxin and has been studied extensively.

Cholera toxin:

Cholera toxin (choleragen) is an AB exotoxin and the genes of it are located on the bacterial chromosome. It is the best studied enterotoxin. Its B subunit is made up of five parts arranged in a donut-shaped ring and contains the site by which the cholera toxin binds specially with the ganglioside GM1 (a complex of glycolipid) in the cytoplasmic membrane of epithelial cells of intestine, but the B subunit itself does not cause an alteration in the permeability of the cytoplasmic membrane.

The B subunit ring anchors itself to the cell membrane of epithelial cells and then inserts the smaller A subunit into the cell. The A subunit activates enzyme adenylate cyclase to convert adenosine triphosphate (ATP) to intestinal cyclic adenosine monophosphate (CAMP). High concentrations of CAMP produces the movement of chloride and bicarbonate ions from the mucosal cells into the intestinal lumen.

This change in ion concentrations causes the movement of massive quantities of water into the lumen (Fig. 44.8). In the acute phase of cholera, the rate of movement of water into the intestine lumen is more as compared to its reabsorption by the intestine and this results in massive net fluid loss. Cholera patients, therefore usually die from dehydration, and the best treatment is oral supply of fluid containing electrolytes and other solutes.

(iii) Cytotoxins:

Cytotoxins are the AB toxins that act upon cells/tissues of specific organs in victim’s body and are designated as per the cell/tissue or organ for which they are specific. Examples of cytotoxins are nephrotoxin (kidney), hepatotoxin (liver), and cardiotoxin (heart).

3. Membrane-disrupting Exotoxins:

Some exotoxins lyze host cells by disrupting the integrity of the plasma membrane.

There are two subtypes of membrane-disrupting exotoxins:

(i) Protein exotoxins and

(ii) Enzyme exotoxins.

Normal Microflora of Human Body; Infection; Microbial Pathogenicity

Protein exotoxins:

Various pathogenic bacteria produce proteins that disrupt the host plasma membrane causing cell lysis and death. Some such proteins are leucocidins and haemolysins. Leucocidins kill phagocytic leucocytes and most of them are produced by pneumonococci, streptococci, and staphylococci bacteria.

Since the phagocytic leucocytes destroyed by the leucocidins, the number of leucocytes drastically decreases resulting fall in host resistance. Haemolysins disrupt the plasma membrane of red blood cells (erythrocytes) and some of them also work on cells other than erythrocytes.

These proteins bind to cholesterol portion of the host cell plasma membrane, insert itself into the membrane forming a pore or channel through which the cytoplasmic contents leak out. Since the osmolanty of the cytoplasm is higher than that of extracellular fluid, this causes quick movement of water into the cell resulting in the swelling and rupture of the cell (Fig. 44.9).

Enzyme exotoxins:

Some membrane-disrupting exotoxins are the phospholipasc enzymes. These enzymes attack the phospholipid of the cell plasma membrane by removing the charged head group from the lipid portion of the phospholipids (Fig. 44.10).

This distabilizes the plasma membrane and, as a result, the cell undergoes lysis and dies. An example of phospholipase enzyme is the α-toxin secreted by Clostridium perfringens to cause gas gangrene disease. In this disease the toxin dissolves membrane lipids resulting in almost complete destroy of the local population of WBCs that are drawn in by inflammation to fight the infection.

4. Superantigen Exotoxins:

Superantigens are the antigens that provoke drastic immune response. Certain exotoxins function as superantigen and hence called superantigen exotoxins. Superantigen exotoxins act indirectly on host cells, using a novel immune mechanism to cause extensive host tissue damage. They directly stimulate large number of immune response cells resulting in extensive inflammatory reactions.

Several diseases can be attributed to superantigen exotoxins. Staphylococcal enterotoxins are good examples of superantigen exotoxin that cause Staphylococcal food poisoning (disease by Staphylococcus aureus) characterized by fever, vomiting, and diarrhoea. Staphyloccus aureus also produces superantigen exotoxin responsible for toxic shock syndrome.

5. Role of Exotoxins in Disease Pathogenesis:

Bacterial exotoxins cause diseases in humans by following three ways:

(i) Ingestion of performed exotoxin (intoxication),

(ii) Colonization of the surface of mucosa followed by exotoxin production, and

(iii) Colonization of wound or abscess followed by local exotoxin production.

Ingestion of performed exotoxin (intoxication):

Variety of bacteria grow in food and produce exotoxin in it. When such food is consumed, the performed exotoxin is also consumed and causes disease symptoms in the body of the human. Staphylococcal food poisoning is a classical example in which diseased is caused by ingestion of performed exotoxin.

This poisoning is caused by Staphylococcus aureus, which is a gram-positive bacterium and is very resistant to heat, drying, and radiation. It occurs in the nausal passages and on the skins of humans worldwide. It readily enters the food from these sources and produces heat-stable enterotoxins that make food dangerous. Typical symptoms of this poisoning include severe abdominal pain, diarrhoea vomiting and nausea.

Colonization of the surface of mucosa followed by exotoxin production:

Bacteria colonize the mucosal surface of intestine and produce exotoxin that causes disease locally or enters the bloodstream and is distributed systemically causing disease at distant sites in the body.

The classical example of this type is cholera (by Vibrio cholerae) in which once the bacteria enter the body, they adhere to the intestinal mucosa and secrete cholera toxin. The toxin catalyses ADP-ribosylation and, as a result, it stimulates hyper-secretion of water and chloride (CI^–) and bicarbonate (HCO₃^–) ions and the patient becomes the victim of dehydration.

Colonization of wound or abscess followed by local exotoxin production:

Bacteria grow in a wound or abscess where they produce exotoxin. The latter causes local tissue damage or destroys phagocytes that enter the infected area. Gas gangrene or clostridial myonecrosis (Gk. myo = muscle, necrosis = death) caused by Clostridium perfringens is an example of this type. In this disease the exotoxin, β-toxin, produced by C. peifringens causes the muscle tissue destruction in the wound or abscess.

A summarized account of the three main ways of the roles of exotoxins is given in Fig. 44.11.

(ii) Endotoxins:

Endotoxins, as stated in the beginning, are potentially toxic lipopolysaccharides located in the outer membrane of many gram-negative bacteria and released only when the bacterial cells disintegrate. Some endotoxins are also released during bacterial multiplication.

Bacterial endotoxins are heat-stable, toxic only at high dose, weekly immunogenic, and generally capable of producing systematic effects such as fever, sock, blood coagulation, diarrhoea, weakness, inflammation, intestinal haemorrhage, and fibrinolysis (enzymatic breakdown of fibrin, the most significant contributor in blood clotting). Endotoxins have been studied primarily in Escherichia, Shigella, and especially Salmonella.

1. Structure and Function:

Lipopolysaccharide (LPS) is made up of three covalently linked subunits, namely, lipid A, core polysaccharide, and O-polysaccharide (Fig. 44.12). It has been studied that the lipid A portion of LPS is responsible for toxicity while the polysaccharide portion makes the complex water soluble and immunogenic. Animal studies indicate that both the lipid and polysaccharide portions are necessary for an in vivo toxic effect.

A variety of physiological effects are caused in the host-body by endotoxins. Fever is an almost universal symptom because endotoxin stimulates host cells to release proteins called endogenous pyrogens, which affect the temperature-controlling center of the brain.

In addition, endotoxins can cause diarrhoea, rapid decrease in lymphocyte, leukocyte, and platelet numbers, release of cytokines, and generalized inflammation, Large doses of endotoxin can be fatal due to haemorrhagic shock and tissue necrosis.

The toxicity of endotoxins is, however, much lower than that of exotoxins. For example, in mice the, LD₅₀ for endotoxin is 200-400 μg per animal, whereas the LD₅₀ for botulinum toxin (an exotoxin) is about 25 picograms (pg), about 10 million times less.

Recent evidence suggests that lipopolysaccharide (LPS) affects macrophages and monocytes by binding to LPS-binding proteins, the special plasma proteins. The LPS-LPS-binding protein complex then binds to receptors on monocytes, macrophages, and other cells and initiates several events including the production of cytokines IL-1, IL-6, and tumor necrosis factor. IL-1 and tumor necrosis factor induce fever.

2. Endotoxins and Pharmaceutical Industry:

Bacterial endotoxins usually contaminate various pharmaceutical products such as antibiotics and intravenous solutions, and administration of such contaminated drugs may cause complications, even fatality, to the patients. Recently, bacterial endotoxins have proven to be problematic for individuals and companies working with cell cultures and genetic engineering.

In the light of this, it has become necessary that the medicinal products must be endotoxin-free. Sensitive tests should be done to identify endotoxins and appropriate methods should be applied to remove them.

(i) Endotoxin identification:

Limulus amoebocyte lysate (LAL) assay is one of the most accurate tests to identify endotoxins even though the toxins are present in trace amounts (Fig. 44.13). The assay is based on the fact that when an endotoxin contacts the clot protein from circulating Limulus amoebocytes, a gel-clot forms.

The assay kits available contain proclotting enzyme, calcium, and procoagulogen. The bacterial endotoxin, lipopolysaccharide and calcium activate the procloting enzyme to form active clotting enzyme, which then catalyzes the cleavage of procoagulogen into coagulogen (polypeptide subunits).

These subunits of coagulogen join by disulphide bonds to form a gel-clot. This reaction can be measures quantitatively with a spectrophotometer with as little as 10 picogram/ml of lipopolysaccharide. Limulus assay is used to detect endotoxins in fluids used to prepare intravenous solutions for injections, drug formulations, scrum, cerebrospinal fluid, and drinking water.

(ii) Removal of endotoxins:

Endotoxins can be removed in two ways:

(i) Endotoxins present on glassware of medical devices can be inactivated by heating the equipment at 250°C for half an hour, and

(ii) Soluble endotoxins can be removed by conventional filtration techniques because they range in size from 20 kDa to large aggregates with diameters up to 01 μm.

Microbial Pathogenicity: Factor # 4. Communicability:

The ability of a microbial pathogen to spread from one host to another is referred to as communicability. This property does not influence the production of disease in an individual host but determines the survival and distribution of a pathogen in a community. A correlation need not exist between virulence and communicability.

In fact, a highly virulent pathogenic strain may not enjoy a high degree of communicability due to its rapidly lethal effect on the host. Usually, infections in which the pathogen is shed in secretions (e.g., respiratory or intestinal disease) are found to be highly communicable.

For examples, human infections represents a dead end in hydrophobia in one host but there being an interruption in the spread of the pathogen to the other hosts. Development of epidemic and pandemic diseases needs such a pathogenic strain that possesses high degree of virulence and communicability.

Microbial Pathogenicity: Factor # 5. Infectivity Dose:

It is required for successful infection that adequate number of microbial pathogen enters into a host. The doses may be estimated as the minimum infecting dose (MID) or minimal lethal dose (MLD) which are, respectively, the minimum number of microbial pathogen required to produce clinical evidence of infection or death, respectively, in a susceptible host under standard conditions.

As the hosts exhibit considerable individual variation in susceptibility, these doses are more correctly estimated as statistical expression. MID 50 and MLD 50, are the dose required to infect or kill 50% of the hosts tested under standard conditions.

Microbial Pathogenicity: Factor # 6. Route of Infection:

Certain bacterial pathogens (e.g., streptococci) may initiate infection what ever may be the mode of entry into the host, but others can survive and reproduce only when they enter inside the host by the optimal route. Vibrio cholerae, the cholera vibrios, cause infection only when they are introduced orally; they fail to establish infection when introduced subcutaneously.

This difference is probably related to modes by which different bacterial pathogens are able to initiate tissue damage and establish themselves in the body of the host. Bacterial pathogens also differ in their sites of election in the host body after they are introduced into tissues.

They also differ in the ability to produce damage of different organs in different species of their hosts. Mycobacterium tuberculosis, the tubercle bacillus, injected into rabbits cause lesions mainly in the kidneys and infrequently the liver and spleen, but in guinea pigs the lesions mainly become apparent in the liver and spleen, the kidneys being spared.

The reasons for such relative multiplication in host tissues are largely obscure, through they may be related to the presence of such substances in the tissues that may selectively prevent or favour their multiplication.