Relation of β-Lactamase Activity and Cellular Location to Resistance of Enterobacter to Penicillins and Cephalosporins

Abstract

The Enterobacter species E. aerogenes, E. cloacae , and E. hafnia were examined for resistance to penicillin and cephalosporin derivatives. All were resistant to benzyl penicillin, ampicillin, 6 [ d (−)α-amino- p -hydroxyphenylacetamido] penicillanic acid, cephaloridine, cephalothin, and cephalexin. A significant number were sensitive to carbenicillin and 6 [ d (−)α-carboxy-3-thienylacetamido] penicillanic acid. No differences among the three species were noted. The β-lactamase activity was cell-bound, and was not released by osmotic shock, toluene treatment, or diphenylamine treatment. It was rarely released into the growth medium. The β-lactamase activity was primarily directed against cephalosporin derivatives. Synthesis of β-lactamase was chromosomally mediated. Resistance to ampicillin seemed to be partly related to entry of the molecule into the bacteria since exposure to ethylenediaminetetraacetate lowered the minimal inhibitory concentration.