Penicillins and cephalosporins. Differences in morphologic effects on Proteus mirabilis.

The morpologic effects of penicillins on P. mirabilis differed from the effects produced by cephalosporins. The bacteria when grown in the presence of inhibitory and subinhibitory concentrations of the penicillins, developed into long filamentous forms up to 93 mu in length without any globular outpouchings. In contrast, none of the cephalosporins produced bacteria longer than 14 mu 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 bacteria, presumably an early stage of spheroplast formation. At concentrations of from 4 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.