Agents Chemother . Schweitzer , Catherine Chanal and Richard Bonnet Frédéric Robin , Julien Delmas , Maryse Archambaud , Cédric - Lactamase Detection β Emerging Problem for

The clinical strain Escherichia coli TO799 was resistant to penicillin-clavulanate combinations and ceftazi-dime and was not reproducibly detected as an extended-spectrum ␤-lactamase (ESBL) according to the standards of the Clinical Laboratory Standards Institute (CLSI; formerly NCCLS) and the national guidelines of the French Society for Microbiology (Comité de l'Antibiogramme de la Société Française de Microbiologie). A novel ␤-lactamase, designated TEM-125, was responsible for this phenotype. TEM-125 harbors a complex association of mutations previously described in the ESBL TEM-12 and in the inhibitor-resistant ␤-lactamase TEM-39. TEM-125 is the first complex mutant TEM to present hydrolytic activity against ceftazidime (k cat , 3.7 s ؊1) together with a high level of resistance to clavulanate (50% inhibitory concentration, 13.6 ␮M). The discovery of such an ESBL, which is difficult to detect by the usual ESBL detection methods, confirms the emergence of a complex mutant TEM subgroup and highlights the need to evaluate detection methods so as to avoid possible therapeutic failures. Among Enterobacteriaceae, the most prevalent mechanism of acquired resistance to ␤-lactams is the production of ␤-lac-tamases such as the penicillinases TEM-1 and SHV-1, which hydrolyze penicillins and narrow-spectrum cephalosporins. In order to thwart these ␤-lactamases, two types of ␤-lactams were developed: ␤-lactam antibiotics resistant to the hydroly-sis, such as expanded-spectrum cephalosporins (ceftazidime), and inhibitors of TEM and SHV penicillinases (clavulanic acid and tazobactam). However, the intensive use of these molecules was quickly followed by an evolution of TEM-and SHV-type ␤-lactamases. The first TEM-type extended-spectrum ␤-lactamases (ESBLs) were characterized in 1987 (28). They differ from the SHV and TEM penicillinases by a few amino acid substitutions which confer hydrolytic activity against expanded-spectrum cephalo-sporins. These ESBLs are susceptible to ␤-lactamase inhibi-tors. Clinical laboratories are urged by the CLSI (formerly NCCLS) (8) and the Comité de l'Antibiogramme de la Société Française de Microbiologie (CA-SFM) (9) to use the association of hydrolytic activity against expanded-spectrum cephalo-sporins and susceptibility to inhibitors as a specific means of detecting this type of enzyme. As the use of expanded-spectrum cephalosporins led to the selection of ESBLs, the clinical use of penicillin–␤-lactamase inhibitor combinations led from 1990 onwards to the selection of point mutants of TEM penicillinases resistant to inhibitors (3). However, the strains producing these enzymes, designated inhibitor-resistant TEMs (IRTs), are generally susceptible to cephalosporins (5). A few enzymes that combine ESBL and IRT mutations have recently emerged among the TEM ␤-lactamases. These new enzymes, designated complex mutant TEMs (CMTs), …