All the strains of human tubercle bacilli described in this report as recently isolated from clinical cases proved to be virulent and showed but slight differences in pathogenic properties. Neither did they show extreme variation in virulence when compared with Strain H-37, isolated many years ago. Between twelve bovine strains cultivated in vitro for varying periods, there was a much wider range in virulence, some being so attenuated as to give regressive lesions in both rabbits and guinea pigs, while other strains were highly pathogenic. In general, the more recently isolated strains were the more virulent. Attenuation occurs with prolonged in vitro cultivation, but the rate of attenuation is apparently not the same for all strains.

Studies of cultures recovered from inoculated animals demonstrated that in the case of bovine tubercle bacilli virulence is correlated with three phenomena: the number of bacilli which can be stained in tissues of inoculated animals, the number of organisms recoverable in cultures from the tissues, and the proportion of smooth colonies in these cultures. All cultures of either human or bovine type recovered from animals showed either two or three types of colonies.

In general the percentage of the smooth form varied directly with virulence. However, some smooth colonies were present in strains having little or no pathogenicity. One example was cited of an avian strain which had lost its virulence while retaining the smooth colony form—indicating that, although pathogenicity is usually correlated with smooth colony form, it is not necessarily so. And smooth variants devoid of virulence do occur.

In the comparison of the effect of the same human strain on rabbits and guinea pigs, it was shown that native resistance (of the rabbit) was associated with a power to dissociate the inoculated bacilli into a greater proportion of rough forms, and then to destroy them. This power is not possessed by the naturally susceptible animal (guinea pig).

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