Pathogenicity is the potential capacity of certain species of microbes to cause an infectious process. It is characterised by a complex of pathogenic properties in the microbe formed in the process of the historical development of the struggle for existence and adaptation to parasitic life in plant, animal and human organisms. Pathogenicity is a specific characteristic of pathogenic microbes.

Pathogenic microbes, for the most part, are characterised by a specific action. Each species is capable of giving rise to a definite infectious process.

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The specificity of the infectious process is quite an important feature which becomes evident in the localisation of the causative agent, selectivity of tissue and organ affection, the clinical picture of the disease, mechanisms of isolating microbes from the organism and in the production of immunity. The peculiarities of each causative agent as an extreme stimulant are taken into account when devising methods of clinical and laboratory diagnosis, therapy and prevention of infectious diseases.

Historically developed ecological factors play an essential role in the development of the specificity of pathogenic microorganisms, and their ability to cause diseases in certain species of hosts. These factors ensure a definite and regular nature of the transmission of the causative agent from one individual to another.

Virulence signifies the degree of pathogenicity of the given culture (strain). Virulence, therefore, is an index of the qualitative individual nature of the pathogenic micro-organism.

Virulence in pathogenic microbes changes under the influence of natural conditions.

It can be increased by a sequence of passages through susceptible laboratory animals, and also by transformation, transduction and lyso-genic conversion.

Virulence can be weakened by the action of different factors on the micro-organism, e. g. the defence forces of the organism, antimicrobial preparations, high temperatures, immune sera, disinfectants, seeding from one nutrient medium to another, etc. Artificial reduction of the virulence of pathogenic microbes is widely used in the preparation of live vaccines, applied for the specific prophylaxis of a number of infectious diseases.

The virulence of microorganisms is closely linked with the genetic function, the auxotrophic property in particular; in the presence of a deficit in two growth factors in mutant strains virulence is lost and cannot be restored while the immunogenic property is maintained.

In characterising pathogenic microbes, a unit of virulence has been established — Dim (Dosis letalis minima), representing the minimum amount of live microbes which, in a certain period of time, bring about the death of the corresponding laboratory animals. Since animals have an individual sensitivity to a pathogenic microbe, then the absolute lethal dose Del (Dosis certa letalis), which will kill 100 per cent of the experimental animals has been established. This provides for a more accurate characteristic. At present, LD50 (the dose which is lethal to one-half of the infected animals) is considered to be the most suitable, the use of which allows for a minimal correction in evaluating the virulence in pathogenic bacteria, and may serve as an objective criterion for comparison with other units of virulence.

That number of pathogenic bacteria which is capable of giving rise to an infectious disease is known as the infectious dose of a pathogenic micro-organism.

In tests on volunteers, it was established, for instance, that the infectious dose is 108 cells for enteropathogenic 0124 E.coli, 105-1010 for Salmonella organisms, 105 for Salmonella typhi (a dose of 107 caused the disease in 50 per cent of infected volunteers, a dose of 108-109 in 89 to 95 per cent), 106-1011 for the El Tor vibrio cholerae, and 10 to 100 bacterial cells for Shigella dysenteriae.

The action of small and large doses of microbes is of great significance in the development of the infectious process, in the length of the incubation (latent) period, and in the severity and outcome of the disease.

Under favourable conditions, one microbial cell with a cell division rate of 20 minutes can give a progeny of 250000 individuals in six hours, and in several hours the amount of microbes may attain many thousand million, which creates a large physiological burden on the tissues and organs of the infected organism.

The virulence of pathogenic microorganisms is associated with toxin production, invasiveness, capsule production, aggressiveness and other factors.

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