Some pathogenic microorganisms (bacilli of anthrax, Clostridium perfringens, S. pneumoniae, causative agents of plague and tularaemia) are capable of producing a capsule in animal and human bodies. Certain microorganisms produce capsules in the organism as well as in nutrient media (causative agents of rhinoscleroma, ozaena, and pneumonia).

Capsule production makes the microbes resistant to phagocytosis and antibodies, and increases their invasive properties. Thus, for example, capsular anthrax bacilli are not subject to phagocytosis, while non-capsular variants are easily phagocytized.

The high virulence of capsular microbes is associated with the toxic substances contained in the capsule.

In chemical composition, the capsular material in some microbes consists of complex polysaccharides, and in others, it consists of proteins. It can be different in separate strains of the same species, and on the other hand, may be similar in different bacteria. There are nitrogenous and nitrogen-free compounds in the capsular polysaccharides. They give the micro-organisms type specificity. In types II and III S. pneumoniae, the capsule is a glucoside of cellobiuronic acid in a highly polymerized state. In types I and IV the capsule contains highly polymerized compounds of aminosaccharides and organic substances. In some bacilli, the capsule consists of polypeptides of d-glutamic acid, while in bacteria of Friedlander’s pneumonia, it is a polymer carbohydrate, and in anthrax bacilli, it consists of glycoprotein.

Besides toxigenicity, invasiveness, and capsule production, pathogenic microbes are capable of producing substances that inhibit the defence mechanisms of the organism and increase the pathogenic action of many causative agents of infectious diseases. O. Bail named them aggressins. They were found in peritoneal and pleural exudates of laboratory animals infected with anthrax bacilli, S. pneumoniae, and other microbes. Aggressins alone, separated from bacteria and exudate cells by filtration, upon injection into the animal are harmless, but upon their addition to a non-lethal dose of the microbes, a severe infectious process develops, often ending in the death of the animal.

Aggressins were found in causative agents of enteric fever, paratyphoid, cholera, anthrax, diphtheria, plague, tuberculosis and pyogenic diseases.

Aggressins are probably not one substance but several different substances occurring in the processes of vital activity of pathogenic microorganisms (some compounds of the surface structures of the microbial cells, products of DNA and RNA splitting).

V. Brown and colleagues established that virulent bacteria produce a special substance in the host body which stimulates their growth and destroys the non-virulent types of these bacteria. It is produced by bacteria from the breakdown products of desoxyribonucleic acid.

It follows from these, far from complete, data that virulence is a multiple biological property stipulated by nucleoid and cytoplasmic genes (plasmids) determining the production of active toxic substances (exotoxins, pathogenicity enzymes, etc.), by changes in the cell membrane structures, and by certain peculiarities of metabolism of pathogenic microorganisms owing to which they survive under conditions of the continuously changing environment of the macro-organism.

Virulence is a dynamic property which is controlled by the mutation process occurring constantly both in the causative agent and in the host and which provides the continuity of the selection of changes advantageous for both. The ranges of virulence vary from absolute parasitism to the level of saprophytism. Pathogenic species may coexist with the macro-organism as latent forms for long periods of time.

The author is an associate professor (retd.) and former head of the department of botany at Ananda Mohan College.