Bacterial dissociation is the complex set of alternative changes affecting microbial structure and metabolism. This type of variation is specific only for the bacteria. It is based upon some modifications as well as on different genotypic alterations. During dissociation bacterial population is splitted into S (smooth) or R (rough) microbial forms. They show almost opposite morphologic and cultural properties.
S forms are characterized by smooth and convex colonies. Motile S form species possess flagella. Capsulated strains have well-defined capsules. S forms usually render intensive biochemical activity. They are considered to be more virulent because of their enhanced resistance against phagocytosis. S forms are often isolated in the acute stage of the disease.
R forms produce rough, irregular and in most cases flattened colonies. They are less biochemically active. In course of infection the host phagocytes efficiently engulf and digest them as these bacteria are devoid of capsules.
Most R forms are regarded as less or non-virulent. Nevertheless, certain pathogens of outstanding virulence are registered. Plague, anthrax, diphtheria and tuberculosis causative agents are extremely virulent in R forms.
Dissociation is provoked mainly by the number of related mutations, which appear after the integration of temperate phages, episomes, IS-elements and transposons into nucleoid sequence. These mutations can impair the process of LPS synthesis within the cell wall, thus leading to the creation of microbial R forms. Nonetheless, this forms are regarded as more resistant to the environmental harmful effects.
For instance, Corynebacterium diphtheriae comes into toxic R form after lysogenization with temperate bacteriophage, carrying tox-genes. Therefore, lysogenic conversion of corynebacteria is responsible for the virulence of corynebacterial population.