Antimicrobial susceptibility of clinical isolates of Enterobacteriaceae producing complex β-lactamase patterns including extended-spectrum enzymes

The antimicrobial susceptibility of 103 clinical isolates of Enterobacteriaceae to 11 antibiotics, was investigated, using a conventional inoculum size (5×105 CFU) and a higher inoculum size (5×108 CFU). All the isolates produced complex β-lactamase patterns, including an extended-spectrum β-lactamase (ESBL) of the TEM- or SHV-type plus other enzymes (a TEM-type or an SHV-type non-ESBL and/or a class C enzyme). The following repertoire of ESBLs was produced by the isolates: TEM-15, TEM-19, TEM-26, TEM-52, TEM-72, TEM-87, TEM-92, SHV-2a, SHV-5 and SHV-12, as assessed by sequencing. Production of the other enzymes was showed by analytical isoelectric focusing. Overall, meropenem was the most active agent and less influenced by inoculum size, while other β-lactams showed a lower activity and a significant inoculum size effect. In conclusion, from its in vitro performance, meropenem could be considered as the last resource drug against strains producing complex β-lactamase patterns including an ESBL. © 2004 Elsevier B.V. and the International Society of Chemotherapy. All rights reserved.