, in Buffered Yeast Extract broth, was treated for 5 h at 37°C with rosaramicin, erythromycin, cefotaxime, dibekacin, penicillin, methicillin, cefoxitin, cephalothin, ticarcillin, carbenicillin or polymyxin B at near-MIC levels and above. Electronmicroscopy demonstrated morphological changes to the bacteria in some, but not all, of the antibiotic-treated suspensions. Penicillin, at 1000 µ/ml (40 × MIC) but not less, produced smooth bubble-like structures on cell surfaces; methicillin produced rough bubble-like structures at 100 µ/ml (MIC) but not at 1000 µ/ml. In each case, these structures resembled spheroplasts. Polymyxin B induced small-bleb formation on the bacterial cell surfaces at all concentrations tested (MIC-10 × MIC). The other eight antibiotics did not induce any morphological changes at any concentration tested.


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