Development of antibiotic resistance in Mycoplasma gallisepticum in vitro

The results of in vitro tests for induction of antibiotic resistance in som e strains of Mycoplasma gallisepticum are reported, The number of passages required to induce resistance varied considerably between different antibio tics. In two groups of tests, with different strains of M. gallisepticum, r esistance (greater than or equal to 1 mg/ml) to streptomycin appeared after two to three passages, to erythromycin and spiramycin after five to eight passages, to tylosin after nine to eleven and to enrofloxacin after eight t o ten passages. With chlortetracycline the increase in resistance was very low (no more than ten times the starting minimal inhibitory concentration). Cross-sensitivity tests using strains with induced resistance to the diffe rent antibiotics demonstrated that those which were resistant to tylosin we re also resistant to other macrolides (>1 mg/ml), whereas strains made resi stant to erythromycin and spiramycin appeared only less sensitive (2 to 200 mu g/ml) to tylosin in comparison with the original strains. Streptomycin, chlortetracycline and enrofloxacin induced very little or no cross-resista nce.