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It has been known for more than half a century that virulent tubercle bacilli can undergo hereditary modifications lessening their pathogenicity. Quantitatively, the decrease in virulence can be measured in terms of the severity and duration of the lesions produced in experimental animals by the injection of known doses of living bacilli. The results obtained by this technique suggest that each of the variant forms of tubercle bacilli is endowed with a characteristic type of behavior in vivo.Many attempts--not to be reviewed here--have been made to correlate the levels of virulence with the morphological and biochemical characteristics of the culture variants, but such studies have not yet thrown any light on the properties which determine the degree to which tubercle bacilli are capable of causing disease. It is still unknown whether the difference in virulence of the culture variants depends upon the ability of the bacterial cells to multiply, to survive, or to exert toxic effects in vivo. The experiments to be described in the present paper were designed to determine the comparative rate and extent of multiplication in vivo of strains of tubercle bacilli known to differ in their pathogenicity for a variety of experimental animals. The level of virulence of these strains had been evaluated previously in this and other laboratories in terms of the extent of lesions and of the mortality caused by the injection of known amounts of culture into rabbits, guinea pigs, and mice. It will be shown that for each strain, the degree of virulence revealed by these techniques bears a close relation to the numbers of living bacilli found to be present in the tissues of mice at various intervals of time after inoculation under defined conditions with known amounts of culture. Materials and MethodsCultures.--The following strains of tubercle bacilli were used:--

A new scheme is proposed for Schaefer's seroagglutination types of mycobacterial strains. In place of numbers and proper names and segregation into three species, Mycobacterium avium, M. intracellulare, and M. scro fulaceum, arabic numbers are assigned in order under the heading "M. avium Complex." For those species with but one or two recognized serotypes, the suggested designation is the species name followed by the term "serotype" followed by an arabic number if more than one type can be identified. It is proposed that a "clearing registry" be established within the framework of the International Working Group on Mycobacterial Taxonomy to prevent different authors from proposing the same serotype numbers for different serotypes.The scheme proposed by Schaefer (4) for the recognition and classification of certain mycobacterial strains by agglutination with specific rabbit antisera has been a great help in identification and in taxonomic and epidemiologic studies, especially of those strains included in the species Mycobacterium avium, M.intracellulare and M. scrofulaceum (2-6). Recently, however, concern has been expressed by some investigators regarding three aspects of this system. First, the scheme now consists of a mixture of Arabic numerals, Roman numerals, and proper names and is therefore awkward. Second, the addition of new serotypes would be difficult with the present jumble of numbers and names. Third, there is doubt and uncertainty about the clear-cut separation of the species M. avium, M. intracellulare, and M. scro fulaceum. Some M. intracellulare strains, especially those belonging to serotypes Davis, Watson, 111, IV, V, and VI, possess virulence for chickens. In addition, strains of these six serotypes were sufficiently distinctive by the differential sensitin test so that Meissner and her colleagues (1) termed them an "intermediate group" between M. avium and M. in tracellulare. Biochemically, these two species are similar. Also, many mycobacterial strains that can be classified as M. scrofulaceum on the basis of pigment characteristics and biochemical reactions agglutinate as one of the "intracellulare" serotypes. An example is the Watson strain, which is one of the reference strains of the Watson serotype.The proposed new scheme is a compromise reached after extensive correspondence and meetings principally among the members of the International Working Group on Mycobacterial Taxonomy (IWGMT) and the Tuberculosis Panel of the Unites States-Japan Cooperative Medical Science Program. The grouping of the types into three separate species is abandoned in favor of the designation "M. avium complex," and 20 numbers in the scheme are reserved after the former M. intracellulare group for possible new types related to this group or to M. avium. The designations for M. scrofulaceum strains start with 41 , and those for M. gordonae strains will start with 51. A few types for the rather heterogeneous M. gordonae strains already have been recognized (E. Wolinsky and T. K. Rynearson, Unpublished data), and...

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