1. The H37 strain of human tubercle bacillus has been dissociated into two variants,—the R, or avirulent, and the S, or virulent, variants. 2. When their dissociation is complete, colony morphology for each is distinctive and typical. 3. The morphology of the individual bacilli of the two variants is different and characteristic. 4. The R variant develops best in an acid medium; the S develops best in an alkaline medium. 5. A method of assortment to facilitate the procedure of forced dissociation has been described. 6. The difference in virulence has been proven by observations on the pathogenesis of the variants.

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The recent advances in the study of the other bacteria with application to the dissociation phenomenon, have been applied in the study of acid-fast organisms. For some time, we have realized that the term "dissociation" as employed at present, is not adequate to explain the instability and subsequent variation which occur in cultures. But for uniformity of bacteriological nomenclature, we have adopted the term until a better one is coined. In describing the "R" and "S" colonies, we have had to depart somewhat from the general usage of these terms, that is the "R" meaning rough, and "S" smooth. The colonies of acid-fast organisms are relatively varied and complex. It seems better to employ the letter "R" to indicate greater resistance to environment and relative avirulence; and "S" to indicate colonies which are more sensitive to environment while possessing for certain species relatively great virulence. The terms "rough" and "smooth" apply directly only to avian tubercle bacillus, when cultivated on plain gentian-violet-egg medium. The avirulent colony isolated from this culture is flat and somewhat rough in appearance. The virulent is perfectly smooth, round and resembling a moth-ball. The physical properties are different. They have been fully described elsewhere. When the bovine "R" and "S" are cultivated on plain gentian-violet-egg medium, differentiation is very difficult. At times they are almost indistinguishable, but the addition of 0.25 per cent sodium taurocholate to the medium, alters completely the topography of the colony. The "S" appears in perfectly round smooth moth-balls, and the "R" in larger, spreading and somewhat rough colonies. Lacking suitable media, the human tubercle bacillus H37 has been more difficult to dissociate. After 2 years' study, using various media, we have been able to dissociate two types of colonies; but as the animal experiments are not yet completed, very little more than that can be said at present. We have dissociated two extreme types of colonies from four BCG cultures obtained from various sources. Each of these four cultures has revealed the same types of colonies. For details the reader is referred to a recent paper (10). In this publication we have included photographs taken from time to time in order to keep a record of our observations. When studying the photographs, the reader will notice considerable variation in some of the colonies. Unquestionably, there are more than two types of colonies developing during the life cycle of the organism, but at present we have considered and confined ourselves to only the two extreme types, one which can produce progressive disease, leading to the death of the animal, and the other which is but slightly virulent, and sometimes not at all so for susceptible animals. Full details of the technique employed by us have been described in the test. Anyone attempting to duplicate the work must strictly adhere to the technique described. Departures from it may lead to failure. The underlying factors favoring dissociation are not yet...