Qualitative and quantitative aspects of β-lactamase production as mechanisms of β-lactam resistance in a survey of clinical isolates from faecal samples

A study has been conducted to identify the β-lactamases most likely to contribute to β-lactam resistance in clinical populations and to investigate their interactions with cefuroxime and newer cephalosporins. A total of 217 ampicillin-resistant, Gram-negative isolates from faecal samples of healthy volunteers in Germany, South America and Amman were investigated. Such strains represent the ‘gene pool’ from which infections might arise. Escherichia coli was the prevalent species (59.9%) followed by Klebsiella spp. (20.3%) and Enterobacter cloacae (12.0%). At least 56.7% and possibly as high as 64.5% of strains owed their principal β-lactamase activity to enzymes mediated by R-plasmids. The most prevalent R-plasmid mediated β-lactamase was TEM-1 which was produced by 109 strains. The β-lactamase activity of strains producing only a chromosomal enzyme was often markedly higher than that of strains also producing an R-plasmid mediated enzyme. The qualitative and quantitative aspects of β-lactamase production were investigated in cell free and whole cell tests and this confirmed the superior broad spectrum β-lactamase resistance of ceftazidime over other new cephalosporins.