Drug sensitivity and mutability to drug resistance associated with the presence of an R factor

Abstract

SUMMARY: Studies on the growth kinetics of R⁺and R⁻cultures ofEscherichia coliin the presence of nalidixic acid (NA), acriflavine (AF) and kanamycin (Kan) showed that each drug caused a decline in viability of both R⁺and R⁻cells for several hours. During further incubation the viability rose rapidly for the R⁺cultures, but either rose less rapidly (AF and Kan) or continued to decline (NA) for R⁻cultures. Distribution curves of the resistances of individual clones of R⁺and R⁻bacteria to atabrine, NA, AF and Kan suggested that the presence of an R factor in the host bacterium increased its mutation rate to resistance to these drugs: this would account for the more rapid growth rate of R⁺cells during the latter stages of incubation in their presence. The mutations causing increased resistance to NA and to Kan were located in the bacterial chromosome and not in theRfactor.