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ABSTRACT
The use of third-generation cephalosporins led to the selection of mutants of Enterobacter, Citrobacter, Serratia and Morganella that were resistant by hyperproduction of chromosomal AmpC beta-lactamases. Cefepime is a new fourth-generation cephalosporin that is stable in the presence of Bush group 1 AmpC beta-lactamases. In vitro activity of cefepime against derepressed mutants of beta-lactamase-inducible species of Enterobacteriaceae was studied. Chromosomal beta-lactamase derepression (constitutive hyperproduction) was detected in 25.9% of isolates (165 of 636). Constitutive AmpC beta-lactamase hyperproducers included: E. cloacae (109), C.freundii (25), S.marcescens (18), M.morganii (6), E.aerogenes (4) and Providentia spp. (3). They were isolated from: urine (n=65), respiratory tract (n=50), wound (n=28) and blood (n=22). No resistance to cefepime was found among the tested isolates. Due to its in vitro activity and low propensity for selection of resistant mutants, cefepime is a suitable therapy for the treatment of serious hospital-acquired infections, caused by Bush group 1 beta-lactamase producing Enterobacteriaceae.