SUMMARY: Seven strains of penicillinase-producing Staphylococcus aureus (coagulase-positive) were serially transferred on streptomycin ditchplates; four of the seven streptomycin-resistant variants showed a decreased ability to produce penicillinase. In three instances this loss in penicillinase production was caused by a decreased growth rate. The fourth strain was unstable in relation to penicillin, and exposure to streptomycin appeared to select the naturally occurring penicillin-sensitive variants.Three of the seven strains were similarly exposed to chloramphenicol; of the three chloramphenicol-resistant variants one was slightly more resistant to penicillin, one considerably less resistant and one unchanged in relation to penicillin. The increased resistance to penicillin was associated with an increase in growth rate. The strain showing a decrease in penicillin-resistance was unstable, and chloramphenicol selected out the naturally occurring penicillin-sensitive variants.
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