NON‐PHOTOCHROMOGENIC MYCOBACTERIA SEROTYPE DAVIS

Abstract

Non‐photochromogenic mycobacteria belonging in serotype Davis deviate from the typical pattern of M. avium. The divergencies in the in vitro experiments occurred especially in connection with drug resistance, effect of temperature, and catalase test. In the in vivo examinations, there were divergencies in all the animal studies. The Davis strains are unstable in subcultures on Löwenstein‐Jensen and 7H‐10 agar media and pure cultures of smooth, transparent colonies (“light”) were dominated by opaque variants (“dark”) after 7–8 passages. It is probable that the variants are the chief reason for the inhomogeneity within the Davis group. On the basis of virulence experiments, the strains were divided into three groups. The most virulent (Group A, virulent for hens, rabbits, guinea pigs) gave growth of “light” colonies and the attenuated (Group C) “dark” colonies. However, the correlation between the two characteristics is not absolute, since both types of colonies were represented in pure culture in Group B (virulent for hens and guinea pigs, attenuated for rabbits), i.e. also the strains which give growth of “dark” colonies may possess virulence for the classical experimental animal, the hen. In the in vitro experiments, the attenuated strains (Group C) were more sensitive to streptomycin, viomycin, rifampicin, and penicillin than the strains in the other groups. This greater sensitivity of the variants must be taken into consideration when selecting test strains for assay of the effect of new drugs or drug combinations.