Overview of the Epidemiological Profileand Laboratory Detection of Extended-Spectrum β-Lactamases

Michael A. Pfaller 1 John Segreti 0 0 Department of Medicine, Rush Medical College of Rush University , Chicago, Illinois 1 Department of Pathology, Roy J. and Lucille A. Carver College of Medicine, University of Iowa , Iowa City Extended-spectrum b-lactamases (ESBLs) are plasmid-mediated bacterial enzymes that confer resistance to a broad range of b-lactams. They are descended by genetic mutation from native b-lactamases found in gramnegative bacteria, especially infectious strains of Escherichia coli and Klebsiella species. Genetic sequence modifications have broadened the substrate specificity of the enzymes to include third-generation cephalosporins, such as ceftazidime. Because ESBL-producing strains are resistant to a wide variety of commonly used antimicrobials, their proliferation poses a serious global health concern that has complicated treatment strategies for a growing number of hospitalized patients. Another resistance mechanism, also common to Enterobacteriaceae, results from the overproduction of chromosomal or plasmid-derived AmpC b-lactamases. These organisms share an antimicrobial resistance pattern similar to that of ESBL-producing organisms, with the prominent exception that, unlike most ESBLs, AmpC enzymes are not inhibited by clavulanate and similar b-lactamase inhibitors. Recent technological improvements in testing and in the development of uniform standards for both ESBL detection and confirmatory testing promise to make accurate identification of ESBLproducing organisms more accessible to clinical laboratories. - Extended-spectrum b-lactamases (ESBLs) are plasmidmediated bacterial enzymes that are able to hydrolyze a wide variety of penicillins and cephalosporins. Most ESBLs have evolved by genetic mutation from native b-lactamases, particularly TEM-1, TEM-2, and SHV-1. These parent enzymes are commonly found in gramnegative bacteria, particularly Enterobacteriaceae [1]; they are highly active against penicillins and modestly active against early-generation cephalosporins [2]. The genetic mutations that give rise to ESBLs broaden the parental resistance pattern to a phenotype that includes resistance to third-generation cephalosporins (e.g., cefotaxime and ceftazidime) and monobactams (e.g., aztreonam) [3]. In general, ESBL-producing isolates remain susceptible to cephamycins (e.g., cefoxitin) and carbapenems [3]. Nevertheless, their resistance to a wide variety of common antimicrobials has made the proliferation of ESBL-producing strains a serious global health concern that has complicated treatment strategies for a growing number of patients. In this context, routine screening for ESBL-producing organisms is of great importance. Unfortunately, the overall adherence to routine screening among diagnostic microbiology laboratories is relatively low. Efforts are now under way to improve this situation. ESBLS: CLASSIFICATION AND PROPERTIES Although ESBLs have been reported most frequently in Escherichia coli and Klebsiella species [2], they have been found in other bacterial species as well, including Salmonella enterica, Pseudomonas aeruginosa, and Serratia marcescens [46]. The first definitively characterized ESBL, TEM-3 (cefotaxime-hydrolyzing enzyme type 1), was discovered in Klebsiella pneumoniae isolates recovered from intensive care unit patients in France [7]. Since that initial report, TEM-type enzymes have become the most abundant class of ESBLs, with 1100 genetic variants now reported [1]. A limited number of mutations are sufficient to convert a parental b-lactamase into an ESBL; TEM-3 is illustrative of the process. It is a plasmid-mediated b-lactamase with a complex resistance phenotype toward antibiotics. The amino acid sequence of TEM-3 differs from that of its parent, TEM-2, by substitutions at just 2 positions [8]. In general, the mutations that give rise to ESBLs tend to be clustered at discrete foci within the nucleotide sequence. There are at least 4 hot spots in the coding sequence of TEM-1, where specific amino acid substitutions in TEM-1, or in a descendant of TEM-1, contribute to the ESBL phenotype [1]. Members of the SHV family of b-lactamases trace their descent to SHV-1, a plasmid-encoded enzyme that confers to K. pneumoniae high levels of resistance against ampicillin [1]. With 150 unique genetic variants, there are significantly fewer SHV-type b-lactamases than there are enzymes of the TEM type [1]. The earliest reported ESBL belonging to the SHV family (SHV-2) differs from SHV-1 by a single amino acid, a glycine-to-serine substitution at position 213 [9]. Comparing the sequences of the SHV family of ESBLs reveals that the amino acid changes that give rise to the extended-spectrum phenotype are confined to relatively few regions of the enzyme [1]. Two substitutions in particular are important for determining the specificity of SHV-type b-lactamases. Both occur within the catalytic site of the enzyme: one substitution (serine 238 for glycine) is important for degrading cefotaxime, whereas the other substitution (lysine 240 for glutamate in combination with serine 238) strongly increases activity against ceftazidime [10]. On the basis of characterizations of numerous b-lactamases, a classification scheme devised by Bush, Jacoby, and Medeiros [2] assigns most ESBLs to group 2be (table 1)that is, blactamases that are inhibited by clavulanic acid, which can hydrolyze penicillins, narrow- and extended-spectrum cephalosporins, and monobactams [2]. Although susceptibility to b-lactamase inhibitors is a defining property of ESBLs, there are several examples of enzymes derived from TEM and SHV that have a resistance spectrum similar to that of ESBLs but are resistant to inhibitors [1]. In addition to the TEM- and SHV-types, 2 other classes of ESBLs have been identified (table 1). The cefotaxime-hydrolyzing (CTX-M)type b-lactamases are carried on plasmids and have been found in Klebsiella species [11], Salmonella typhimurium, and E. coli [1, 13]. These enzymes are not closely related to TEM and SHV b-lactamases [1]. Instead, they show a very high degree of sequence homology with the chromosome-encoded AmpC-type b-lactamase of Kluyvera georgiana, suggesting that the CTX-Mtype b-lactamases might represent genetic variants descended from the b-lactamase of Kluyvera species [14]. The CTX-M enzymes show a preference for hydrolyzing cefotaxime, and members of the class are susceptible to inhibition by clavulanate, sulbactam, and tazobactam [1, 15, 16]. The oxacillin-hydrolyzing (OXA)type b-lactamases are unique among the ESBLs because they are most often found in P. aeruginosa, rather than in members of the Enterobacteriaceae [1]. In the Bush-Jacoby-Medeiros classification scheme, the OXA enzymes are assigned to group 2d, apart from most other ESBLs [2]. Their preferred substrates are penicillins and cloxacillin [17, 18], rather than third-generation cephalosporins. The OXA class of ESBLs exhibits appreciable diversity in (...truncated)