Scanning electron microscopy of morphological alterations in Proteus mirabilis induced by cephalosporins and semisynthetic penicillins

Abstract

Experiments, using the scanning-beam electron microscope, were conducted to compare the morphologic lesions induced in a strain of Proteus mirabilis by two semisynthetic penicillins, ampicillin and carbenicillin, and two cephalosporins, cephalothin and cephaloridine. The observations demonstrated that the semisynthetic penicillins and cephalosporins did not induce the same sequence of structural alterations. In the presence of cephalothin or cephaloridine, mid-cell spherical defects were induced which rapidly progressed to large bulbous enlargements at low concentrations (2 and 20 μg/ml) but only spheroplast-like forms were observed at high concentrations (200 and 2000 μg/ml). Ampicillin and carbenicillin caused similar alterations in morphology except that marked cell elongation without division occurred with 2 and 20 μg/ml. These variations may reflect subtle differences in sites, rates, sensitivity, or mechanisms of action between these two groups of cell-wall inhibiting antibiotics.