Studies in microbic dissociation and variation with special reference to the acid-fast and the diphtheroid bacilli

Abstract

A study has been made of dissociation in strains of a virulent bovine, attenuated bovine (B.C.G.) and a human tubercle bacillus, and of the more rapidly growing fish tubercle and "leprosy" bacillus. A less detailed examination has been made of the tortoise tubercle bacillus and Mycobacterium rubrum. A number of strains of diphtheroids have also been examined. The most outstanding colony types have been described and an examination made of their stability and of some related characters such as virulence (in certain strains), emulsibility in saline and changes in cell morphology. The variants have been numbered in preference to classifying them as S, R and I forms; it seemed desirable to investigate the intermediate types in some detail, as early in the work it became evident that among acid -fast organisms such forms were more numerous than one was led to expect from the general literature on microbic dissociation among either rapidly or slowly growing organisms. Each variant described was definite in its characteristics and was clearly differentiated from other colonies of the same species.From the results obtained in this study of dissociation among certain strains of Mycobacteria and Corynebacteria it is felt that while the recent literature gives one aspect of the subject in considerable detail (that of the S and R types and their associated characters) it does not present a true picture of the changes taking place when an organism undergoes dissociation. Although the colonies of the two genera examined probably tend to show a greater complexity of form than those of other commonly occurring organisms and consequently allow greater scope for variability, it seems justifiable to assume that similar changes are occurring in other species, although they may be of a less obvious nature. Since, in the majority of cases, the "intermediate types" have received but scant attention from recent investigators (who have come under the influence of one or two systematists) the importance of the reactions among such forms is still unknown. This belief that the changes do occur is strengthened by the reports of early workers who described a considerable number of variants in most of the species examined. The results recorded here are not in agreement with those of many investigators, but they indicate that until a study is undertaken to determine in detail not only the reactions of a few colonies but also those of other forms, there is small probability of understanding the true cause of dissociation.In the foregoing results it has been shown that to overlook the so- called "intermediate form" as an unstable and unsatisfactory type with which to work, would be to disregard an important aspect of the phenomenon. Reports suggesting hard and fast classifications on the basis of stability of colony structure and certain related characters must be accepted, at present at least, with reservations. The phenomenon, it is believed, is not purely haphazard. It is a definite biological problem of practical importance and as such demands a thorough and detailed investigation in many different species.