a1 Department of Microbiology, Kings College, University of London, Campden Hill Road, London WS 7AH
The antibiotic resistance of Mycoplasma mycoides ssp. mycoides strain T1 was investigated. This strain was resistant to high levels ( > 100 μg ml−1) of rifampicin and nalidixic acid. It was sensitive to streptomycin, spectinomycin and novobiocin; however, single step mutants with high levels of resistance ( > 100 μg ml−1) were readily isolated. With erythromycin and tylosin for which the minimum inhibitory concentration (MIC) for the parent strain was < 0·1 μg ml−1, mutants resistant to > 100 μg ml−1 were obtained in two and three steps respectively. The MIC of tetracycline in single step resistant mutants (0·6 μg ml−1) was tenfold higher than the parent strain, but could not be increased further. There was only a twofold increase in resistance to chloramphenicol in single step mutants. The frequency of resistant mutants varied with the antibiotic and was between 4× 10minuss;6 and 2× 10−8. The mutation rate to antibiotic resistance to streptomycin, spectinomycin, novobiocin, erythromycin and tylosin was between 3× 10−8 and 5× 10−9 per cell per generation. There was a fivefold decrease in mutation rate to resistance to 60 μg ml−1 streptomycin compared to that to 20 μg ml−1.
(Accepted December 30 1986)
p1 Author for correspondence.