Abstract
The influence of pH on activity of carbenicillin against a clinical isolate of Proteus mirabilis was studied. Cylinder-plate assays showed greatest activity at pH 7.2, and least activity at pH 5.0. Bactericidal curves for carbenicillin-treated cultures of P. mirabilis also showed least activity at pH 5.0 and greatest at pH 7.2. Similar findings were observed with scanning electron microscopy of bacterial cells. Bizarre morphologic forms, consistent with spheroplasts, were observed after exposure of P. mirabilis to carbenicillin at more neutral pH (6.0, 7.2, 8.0), whereas only minor morphologic defects were noted at pH 5.0. However, standard tubedilution methods for determinations of minimal inhibitory concentrations for P. mirabilis showed no significant differences at pH 5.0, 6.0, 7.2, and 8.0. The results suggest that the effect of pH on carbenicillin activity may involve complex interactions and alterations of drug and bacteria.
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