Microbial Dissociation

Microbial Dissociation, a process causing partial or complete transformation of a pure bacterial culture of normal type into one or several subtypes that differ from the original microbe in their morphological structure, colony appearance, serological and biochemical properties. The process of D. can proceed not only in the direction from the primary culture to variant types but also in reverse. D. occurs either spontaneously or under the influence of certain factors, such as: cultivation at unfavorable temperatures, use of unsuitable nutrient media or starvation, physical state of the medium (humidity, dryness, volume), oxygen pressure, antiseptic substances, foreign proteins, specific sera, products of microbial growth (filtrates of homologous and even heterologous cultures), etc. At present, the designation of primary colonies with the letter S (English smooth-soft, smooth) and secondary with R (English rough-rough, coarse) is generally accepted. Between these two types there exists a whole series of intermediate variant colony types; they are designated by the letter O. The characteristic features of S colonies on solid media are as follows: small, round, convex, regular in shape, delicate, with a shiny surface; R colonies are irregular in shape with eroded, fringed edges, wrinkled, rough, often glassy, usually firmly attached to the medium [see separate table (pp. 55-56), figs. 2 and 3]; O colonies have a slimy mucoid structure. When growing in broth, S causes uniform cloudiness, while R forms a flaky precipitate. In physiological salt solution (0.8% NaCl), S gives a uniform suspension, while R forms flakes. The morphological differences of microorganisms forming S, O, and R colonies are as follows: type S consists of normal microorganisms; the intermediate type O is characterized by the presence of long threads, coccal forms, and giant cocci; in type R, the individuals are significantly shortened compared to S (intestinal bacillus, typhoid, dysentery, etc.), in motile bacterial species they are either completely devoid of motility or possess it to a weak degree, in encapsulated species there are no capsules. Microorganisms of R colonies are usually less active (decomposition of sugars, proteins, liquefaction of gelatin) and have reduced virulence and toxigenicity compared to type S (diphtheria, typhoid, dysentery); exceptions are the intestinal bacillus and anthrax, in which R is more virulent than S. Type S, being a carrier of two or more antigens, is most suitable for immunization. As for the stability in maintaining the characteristic features of newly obtained variant types, R possesses it to a greater degree than O and more difficultly reverts to the basic S. To achieve this, multiple passages through a normal animal organism are used. Secondary colonies form either in the center or at the edges of the primary culture, and may be isolated in the form of papillae or merge into a homogeneous secondary culture. Primary colonies, upon reaching their highest development, undergo degenerative changes, become glassy, and on their surface appear secondary daughter colonies. Dissociative processes also occur in liquid media, where they proceed more energetically than on solid ones. D. m. is observed in representatives of all bacterial groups, both aerobic and anaerobic, both pathogenic and saprophytic, among all morphological species, perhaps with the exception of spirochetes. There exists a certain parallelism in the course of the D. process in different species, but how far it goes cannot yet be said. At present, the view is increasingly taking root that the basis of D. of microbes lies in the general biological law of variations and mutations, widely distributed among unicellular organisms, and since D. is stimulated by the presence of unfavorable factors and type R is more resistant than S, some scientists base this process on the principle of preservation of the species. (See also Variability of microbes.)