Antibiotic sensitivity and mutation rates to antibiotic resistance in Mycoplasma mycoides ssp. mycoides

The antibiotic resistance of Mycoplasma mycoides ssp. mycoides strain T1 was investigated. This strain was resistant to high levels ( > 100 ug/ml) of rifampicin and nalidixic acid. It was sensitive to streptomycin, spectinomycin and novobiocin;however, single step mutants with high levels of resistance (>100 ug/ml) were readily isolated. With erythromycin and tylosin for which the minimum inhibitory concentration (MIC) for the parent strain was < 0.1 ug/ml, mutants resistant to > 100 ug/ml were obtained in two and three steps respectively. The MIC of tetracycline in single step resistant mutants (0-6 ug/ml) 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 x 10-6 and 2 x 10-8. The mutation rate to antibiotic resistance to streptomycin, spectinomycin, novobiocin, erythromycin and tylosin was between 3 x 10-8 and 5 x 10-9 per cell per generation. There was a fivefold decrease in mutation rate to resistance to 60 ug/ml streptomycin compared to that to 20 ug/ml.