Penicillins and Cephalosporins: Differences in Morphologic Effects on Proteus mirabilis

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Abstract
The morphologic effects of penicillins on Proteus mirabilis differed from the effects produced by cephalosporins. The bacilli, when grown in the presence of inhibitory and subinhibitory concentrations of the penicillins, developed into long filamentous forms up to 93 μ in length without any globular outpouchings. In contrast, none of the cephalosporins produced bacilli longer than 14 μ at any concentration of antibiotic tested. The predominant morphologic response to the cephalosporins, especially at and above its inhibitory concentration, consisted of globular outpouchings from short bacilli-presumably an early stage of spheroplast formation. At concentrations of from four to 32 times the minimal inhibitory concentration, the penicillins also produced globular forms identical to those produced by the cephalosporins. Although it is generally presumed that penicillins and cephalosporins have identical modes of action, the data presented indicate that there must be some differences in their action to account for the difference in morphologic response. Possible biochemical mechanisms to account for this difference are suggested.

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