Effect of Temperature on the Rate of the Transparent to Opaque Colony Type Transition in Mycobacterium avium

The results of drug susceptibility tests were found to be affected by changes that occur spontaneously in populations of Mycobacterium avium maintained in the laboratory. Because the transparent colony type variant was resistant to antituberculosis chemotherapeutic agents and the opaque colony type variant was usually susceptible to these agents, the transition of transparent to opaque colony type was investigated. The rate of the transition was found to be temperature dependent and, in agreement with a previous report, was found to be about 10 −4 to 10 −5 per generation at 37 C. Reversion was found to occur at a rate of 10 −6 to 10 −7 at 37 C. The mutation rate from susceptibility to resistance to rifampin, kanamycin, and erythromycin was about 10 −8 to 10 −9 mutations per bacterium per generation. Judged from our data, the high rate of the transparent to opaque variation was not caused either by mutator effects or by the occurrence of extrachromosomal genes in these bacteria, but could have been due to selective mechanisms still incompletely understood.