Complementary activity of mezlocillin and the combination of amoxicillin with clavulanic acid on Enterobacteriaceae

The MICs of amoxicillin, mezlocillin and BRL 25,000, a combination of two parts amoxicillin and one part clavulanic acid (2AM + 1CA), were measured for 331 Enterobacteriaceae strains which produced betalactamases as demonstrated by nitrocefin. When investigated separately according to the bacterial species, three different sensitivity groups were established for the above-mentioned preparations: 1) species with the same or similar sensitivity to mezlocillin and 2AM + 1CA(Escherichia coli and Shigella spp., amoxicillin-resistant strains); 2)species which were more sensitive to mezlocillin than to the combination 2AM + 1CA (Citrobacter spp., Enterobacter cloacae, Serratia spp. and indole-positive Proteus as well as strains of E. coli and Shigella spp.); 3) species which are more sensitive to 2AM + 1CA than to mezlocillin (amoxicillin-resistant Salmonella spp., Proteus mirabilis and Klebsiella pneumoniae ). This complementary activity of mezlocillin and 2AM + 1CA again Enterobacteriaceae depended on the beta-lactamases produced.