A novel class A extended-spectrum β-lactamase (BES-1) in Serratia marcescens isolated in Brazil

Serratia marcescens Rio-5, one of 18 extended-spectrum p-lactamase (ESBL)-producing strains isolated in several hospitals in Rio de Janeiro (Brazil) in 1996 and 1997, exhibited a high level of resistance to aztreonam (MIC, 512 mug/ml) and a distinctly higher level of resistance to cefotaxime (MIC, 64 mug/ml) than to ceftazidime (MIC, 8 mug/ml). The strain produced a plasmid-encoded ESBL with a pi of 7.5 whose bla gene was not related to those of other plasmid-mediated Ambler class A ESBLs. Cloning and sequencing revealed a bla gene encoding a novel class A beta -lactamase in functional group 2be, designated BES-1 (Brazil extended-spectrum beta -lactamase). This enzyme had 51% identity with chromosomal class A penicillinase of Yersinia enterocolitica Y56, which was the most closely related enzyme and 47 to 48% identity with CTX-M-type beta -lactamases, which were the most closely related ESBLs. In common with CTX-M enzymes, RES-I exhibited high cefotaxime-hydrolyzing activity (k(cat), 425 s(-1)). However, BES-1 differed from CTX-M enzymes by its significant ceftazidime-hydrolyzing activity (k(cat), 25 s(-1)), high affinity for aztreonam (K-i, 1 muM), and lower susceptibility to tazobactam (50% inhibitory concentration [IC50], 0.820 muM) than to clavulanate (IC50, 0.045 muM). Likewise, certain characteristic structural features of CTX-M enzymes, such as Phe-160, Ser-237, and Arg-276, were observed for BES-1, which in addition, harbored different residues (Ala-104, Ser-171, Arg-220, Gly-240) and six additional residues at the end of the sequence. BES-1, therefore, may be an interesting model for further investigations of the structure-function relationships of class A ESBLs.