Susceptibility of Mycoplasma pneumoniae to Antibiotics In Vitro

ABSTRACT The antibiotic susceptibility of laboratory strains, Mac and FH, and isolates of Mycoplasma pneumoniae was determined in broth media. With the Mac strain, the minimum inhibitory concentration (MIC) increased with the increase in inoculum size for most antibiotics and the final minimum inhibitory concentration as determined during a month's incubation did for all antibiotics. The antibiotics studied were classified by the MIC into three groups; the first group with the order of 1/100 μg/ml (erythromycin, josamycin, and leucomycin), the second with the order of 1/10 μg/ml (oleandomycin, tetracycline, and spiramycin), and the third with the order of 1 or 10 μg/ml (lincomycin, chloramphenicol, and streptomycin). With the FH strain, the MIC of streptomycin was much lower, showing that the Mac strain was resistant to streptomycin. The susceptibility of 152 isolates from 99 patients to erythromycin, josamycin, tetracycline, and chloramphenicol was essentially similar to that of the Mac strain except for two isolates, which had been isolated from a patient during erythromycin therapy and exhibited resistance to erythromycin and josamycin. The MIC of streptomycin was distributed at 1 μg/ml in 93% of the isolates and at 100 μg/ml or over in 6%. It was considered that streptomycin‐resistant strains occur in M. pneumoniae. Of nine patients who had been given erythromycin, tetracycline, chloramphenicol, or Clotaon for more than a week, the isolates after therapy showed decreased susceptibility to erythromycin in one patient and did not to the antibiotics given in the other eight patients.