Extended-spectrum beta-lactamase TEM-24 in an Aeromonas clinical strain: acquisition from the prevalent Enterobacter aerogenes clone in France.

The TEM-24 extended-spectrum -lactamase (ESBL), initially characterized in Klebsiella pneumoniae (3), is currently the predominant ESBL in France (4). This could be related to the spread of a TEM-24-producing Enterobacter aerogenes clone present in most French hospitals (1) but also found in Belgium (5) and Spain (2). TEM-24 enzyme has been recovered from an increasing number of species of the family Enterobacteriaceae (6, 8) and in Pseudomonas aeruginosa (7). In contrast, resistance to expanded-spectrum cephalosporins mediated by ESBLs has never been described in the genus Aeromonas, for which -lactam resistance involves three chromosomally mediated enzymes: a cephalosporinase, a penicillinase, and a carbapenemase (9). We report the first characterization of a TEM-24-producing clinical Aeromonas strain. 16S ribosomal DNA (rDNA) and gyrB sequencing (10) showed that the strain was most closely related to Aeromonas caviae. This strain was recovered together with an ESBL-producing E. aerogenes isolate from the diarrheal feces of a 76-year-old man admitted to Montpellier Hospital for intestinal ischemia. Based on the resistance phenotype observed after the disk diffusion assay, both E. aerogenes and Aeromonas sp. isolates were suspected to produce ceftazidimase-type ESBL and AAC(6 )-I enzyme. Synergy was clearly observed between expanded-spectrum cephalosporins and clavulanate. The resistance phenotype was transferred in Escherichia coli C600 by mating experiments using MuellerHinton agar (Sanofi Diagnostics Pasteur, Marnes-la-Coquette, France) containing ceftazidime (4 g/ml). The MICs of -lactams showed high levels of resistance with the E. aerogenes strain and the two transconjugants for penicillins (64 to 2,048 g/ml), ceftazidime (512 to 1,048 g/ml), and aztreonam (64 to 1,048 g/ml). A low level of resistance or decrease in susceptibility was observed for cefotaxime (4 to 8 g/ml). The Aeromonas sp. strain showed lower resistance levels (penicillins, 8 to 256 g/ml; ceftazidime, 64 g/ml; and aztreonam, 8 g/ml). Clavulanate and tazobactam partially or completely restored the susceptibility to penicillins. ESBL characterization was performed for the two isolates by isoelectric focusing, ESBLencoding gene amplification and sequencing, and plasmid content analysis (6, 7). The ESBLs were focused at a pI of 6.5. The sequence of ESBL-encoding genes shared 100% identity with the blaTEM-24 gene. Plasmids of approximately 180 kb, which displayed similar restriction patterns, were isolated from the two strains and their transconjugants. These results suggested an in vivo transfer of TEM-24-encoding plasmid from E. aerogenes to Aeromonas sp. in the intestinal tract. We have previously reported the transfer of 180-kb TEM24-encoding plasmid from E. aerogenes to Providencia rettgeri CIP 107053 (6) and P. aeruginosa CIP 107051 (7). A comparative study of the restriction profiles obtained for the plasmids recovered from these organisms and from the strains analyzed here revealed similar restriction patterns (Fig. 1). Pulsed-field gel electrophoresis (PFGE) analysis of the E. aerogenes strains isolated in this study and previously (6, 7) revealed that they belong to the TEM-24-producing E. aerogenes clone prevalent in France. This clone had disseminated the same TEM-24-encoding plasmid among various bacterial species for several years. The isolation of a TEM-24-producing Aeromonas sp. strain extends the list of TEM-24-harboring bacteria, and this wide host range is one of the factors responsible for the persistence and spread of the TEM-24-encoding plasmid.