Genetic and Biochemical Characterization of the First Extended-Spectrum CARB-Type ß-Lactamase, RTG-4, from Acinetobacter baumannii

Acinetobacter baumannii is an opportunistic pathogen that is an important source of nosocomial infections such as pneumonia, septicemia, urinary tract infections, and wound infections (2). Treatment of infections due to this microorganism is becoming a serious clinical concern since A. baumannii is frequently resistant to multiple classes of antibiotics (23, 37). The main mechanism of resistance to β-lactam molecules in A. baumannii is the production of β-lactamases. Resistance to carbapenems is mostly related to the production of metallo-β-lactamases or carbapenem-hydrolyzing oxacillinases (33), whereas resistance to expanded-spectrum cephalosporins mostly results from the overexpression of the natural AmpC-type enzyme (3) or from the acquisition of extended-spectrum β-lactamases (ESBLs). Those ESBLs may correspond to TEM or SHV derivatives but mostly correspond to β-lactamases of the VEB or PER type in A. baumannii (21). Carbenicillin-hydrolyzing β-lactamases (also named CARB enzymes) are narrow-spectrum class A penicillinases that share less than 50% amino acid identity with SHV and TEM β-lactamases (7). The 10 s-lactamase variants of this family show similar hydrolytic properties but are divided into two subgroups, named the CARB and RTG subgroups, according to their amino acid sequences. The CARB subgroup includes CARB-1 (formerly PSE-4) (18), CARB-2 (formerly PSE-1) (18), CARB-3 (14), CARB-4 (26), Proteus mirabilis N29 β-lactamase (11), CARB-6 (6), CARB-7 (20), and CARB-9 (25). The RTG subgroup includes RTG-1 (P. mirabilis GN79 enzyme) (34), RTG-2 (CARB-5; described in Acinetobacter calcoaceticus var. anitratus [24]), and RTG-3 (CARB-8; identified from an Oligella urethralis clinical isolate [17]). Although those three RTG-type s-lactamases possess low levels of amino acid identity with members of the CARB family (44%), they might be considered the ancestors of that enzymatic group, as proposed by Choury et al. (7). The carbenicillin-hydrolyzing enzymes belonging to the CARB group were first identified from Pseudomonas aeruginosa clinical isolates (38). The blaCARB genes are now widespread among distantly related bacteria, including Vibrio spp. (19), because of their location on mobile genetic structures. Indeed, blaCARB genes such as blaCARB-1 (22), blaCARB-2 (28), and blaCARB-4 (36) were found as a form of gene cassettes of class 1 integrons. In contrast, little is known about the genetic environment of the blaRTG genes. Analysis of the β-lactamase content of an A. baumannii clinical isolate exhibiting an atypical higher level of resistance to cefepime and cefpirome than to ceftazidime and cefotaxime allowed us to identify a novel plasmid-encoded ESBL-encoding gene belonging to the blaRTG group.