COMBINED EFFECTS OF AMPICILLIN AND METHICILLIN ON CELL WALL MORPHOLOGY OF A METHICILLIN-RESISTANT STAPHYLOCOCCUS

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  • MANIAR A. G.
    Fred T. Cadham Public Health Laboratory and Department of Medical Microbiology University of Manitoba

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Some semi-synthetic penicillins and cephalosporins are resistant to hydrolysis by beta-latamases. Hence, penicillin-resistant staphylococci exhibiting beta-lactamase activity become susceptible to these beta-lactam antibiotics. Methicillin (2, 6-dimethoxyphenyl penicillin) in combination with ampicillin (D(-)-alpha-aminobenzyl penicillin) showed either a synergistic (14 strains) or an additive (3 strains) effect (never antagonistic), against 17 methicillin-resistant staphylococcal strains. One of these strains did not produce beta-lactamase. This strain No. 667 grew well in broth containing either 50 mcg/ml of methicillin or half the minimum inhibitory concentration of ampicillin which indicated that inhibition of cell wall synthesis was not apparent. However, ampicillin in combination with methicillin, using half of the above concentrations, exhibited synergism* against the strain 667. In the presence of sub-lethal doses of either ampicillin or methicillin, Escherichia coli became thread-like, while spheroplasts of E. coli were formed under similar conditions when these 2 beta-lactam antibiotics were combined. Since methicillin has no inhibitory action on strain 667, it can be concluded that the inhibition of growth by the combined penicillin might be due to a change in mechanism or site of action (or change of pathways). This is more suggestive as (a) the staphylococcal strain 667 did not produce any beta-lactamase and (b) spheroplasts of E. coli were formed only when the 2 penicillins were combined.

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