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Antone A. Medeiros, Thomas F. O'Brien, Mechanisms of Resistance to Cephalosporins in Ampicillin-resistant Escherichia Coli, The Journal of Infectious Diseases, Volume 128, Issue Supplement_2, October 1973, Pages S335–S340, https://doi.org/10.1093/infdis/128.Supplement_2.S335
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Abstract
Three distinct classes of ampicillin-resistant Escherichia coli possessing different β-lactamases varied in their levels of resistance to cephalothin, cephaloridine, cephalexin, and cefazolin. The β-lactamase of class I E. coli hydrolyzed the different cephalosporins at widely different rates. When tested with large inocula of organisms, MICs of all the cephalosporins except cephalexin were elevated to resistant levels and generally correlated with rates of hydrolysis. When tested with small inocula, MICs of all four cephalosporins were low. This is probably explained by the observation that, unlike ampicillin, permeability of cells to cephalosporins was high, and affinity of the β-lactamase for them was low. Class II E. coli had much more cephalosporinase than ampicillinase activity and were resistant to all four cephalosporins even when tested with small inocula. The β-lactamase of class III E. coli had low levels of specific activity, and bacteria of this class were only slightly more resistant to the cephalosporins than were ampicillin-susceptible E. coli. In classes II and III, as well as in E. coli susceptible to ampicillin, MICs of cephaloridine and cefalozin were slightly lower than those of the other cephalosporins.
