New β-Lactamases in Gram-Negative Bacteria: Diversity and Impact on the Selection of Antimicrobial Therapy

Karen Bush () 0 1 0 Received 7 September 2000; revised 30 November 2000; electronically published 21 March 2001. Institute , 1000 Route 202, Raritan, NJ 08869 1 The R. W. Johnson Pharmaceutical Research Institute , Raritan, New Jersey Of the 340 discrete b-lactamases that have been identified, the most important groups of enzymes that are continuing to proliferate include the plasmid-encoded cephalosporinases, the metallo-b-lactamases, and the extended-spectrum b-lactamases. Resistance to specific b-lactam-containing antimicrobial agents frequently can be traced to a single b-lactamase, but this task is becoming more difficult for the clinical microbiology laboratory. Other factors, such as multiple b-lactamase production, transferable multidrug-resistance genes, alterations in outer-membrane porins, and possible antibiotic efflux, all may contribute to a resistance phenotype. Appreciation of these factors may help the physician make a more informed decision when choosing therapy to try to avoid selection of even more pathogenic strains. - sequences or differentiated phenotypic behavior (table 1). Many of these enzymes belong to closely related families with similar functions. Because of the relative ease of obtaining genetic sequence data, distinct enzymes are now readily distinguished on the basis of molecular structure, in contrast to early research, in which enzymes were sorted primarily on the basis of biochemical (functional) characteristics. As a result, the number of distinct TEM-related b-lactamases is approaching the century mark, and almost 30 SHV-derived enzymes have been described (http:// www.lahey.org/studies/webt.htm). However, the clinical impact of b-lactamases is related to a combination of factors that rely more heavily on functional rather than structural characteristics, including specificity of hydrolysis and level of expression of enzymatic activity, together with the presence of additional resistance factors in the producing organism. Although amino acid sequence data have provided an attractive means for differentiating b-lactamases, on the clinical level this approach suffers from the fact that we still cannot use molecular biology to predict enzyme function or the subsequent bacterial-susceptibility profile. When a clinician needs to decide how to treat a bacterial infection, the amino acid sequence of a b-lactamase is less important than knowing whether the producing organism is resistant or susceptible to this agent. However, in terms of epidemiology or long-term effects of antibiotic use, identification of specific b-lactamases can play an important role. Today, it is recommended that we determine the microbiological profile of the producing organism, the biochemical properties of a purified enzyme, and the Functional and molecular characteristics of the major groups of b-lactamases. Molecular class 1 2 Attributes of b-lactamases in functional group Often chromosomal enzymes in gram-negative bacteria but may be plasmid-encoded. Confer resistance to all classes of b-lactams, except carbapenems (unless combined with porin changes). Not inhibited by clavulanic acid. Most enzymes responsive to inhibition by clavulanic acid (unless otherwise noted). Staphylococcal and enterococcal penicillinases included. Confer high resistance to penicillins. Broad-spectrum b-lactamases, including TEM-1 and SHV1, primarily from gram-negative bacteria. Extended-spectrum b-lactamases conferring resistance to oxyimino-cephalosporins and monobactams. Inhibitor-resistant TEM (IRT) b-lactamases; one inhibitorresistant SHV-derived enzyme. Carbenicillin-hydrolyzing enzymes. Cloxacillin-(oxacillin)hydrolyzing enzymes; modestly inhibited by clavulanic acid. Cephalosporinases inhibited by clavulanic acid. Carbapenem-hydrolyzing enzymes with active site serine, inhibited by clavulanic acid. Metallob-lactamases conferring resistance to carbapenems and all b-lactam classes except monobactams. Not inhibited by clavulanic acid. Miscellaneous unsequenced enzymes that do not fit into other groups. Estimated no. of enzymesa amino acid sequence of any novel b-lactamase, and then guess its closest relative(s), to understand all of the contributing properties to its observed phenotype. A well-accepted effort to combine structural and functional characteristics of the major classes of b-lactamases was reported in 1995 [3]; it is summarized in table 1, together with an updated summary of the numbers of enzymes and their major attributes. As seen in table 1, from 1995 through 2000, 5 of the 11 (sub)groups of b-lactamases increased in number by at least 50%: groups 1, 2be, 2br, 2d, and 3. Of these, at least 3 different groups of enzymes are increasing in prevalence worldwide: the plasmid-encoded functional group 1 cephalosporinases, the group 3 metallob-lactamases, and the group 2be extended-spectrum b-lactamases (ESBLs), which represent the largest group of b-lactamases described. The other 2 budding groups are somewhat less problematic in North America, in part because of the confinement of the group 2br inhibitorresistant TEM b-lactamases to Western Europe [5] and the restriction of the newer, less efficient group 2d OXA-related enzymes primarily to Eastern Europe [6]. Group 1 cephalosporinases were known originally as the chromosomal enzymes in Pseudomonas aeruginosa or the Enterobacteriaceae that could be induced by b-lactams such as cefoxitin or ampicillin [7]. With the introduction of cephalosporins, such as cefotaxime and ceftazidime, selection of strains with derepressed (hyperproduced) group 1 b-lactamases became more common [8]. Hyperproduction of these enzymes is often associated with the loss of a porin in the outer membrane of the bacteria [9], leading in some cases to highlevel resistancenot only to all cephalosporins, penicillins, and monobactams, but also to the carbapenems [10, 11]. Genes encoding group 1 enzymes are now appearing on highcopy-number plasmids and are being transmitted promiscuously among the Enterobacteriaceae, resulting in organisms such as Escherichia coli and Klebsiella pneumoniae with high levels of group 1 cephalosporinases. Some of the plasmid-encoded enzymes in this class include MIR-1 [12], ACT-1 [11], and the FOX series of enzymes [13], now numbered through FOX-5 [14]. Production of these enzymes has become especially problematic, because these plasmid-containing, cephalosporinaseproducing organisms frequently do not contain an endogenous group 1 b-lactamase but, instead, produce another set of blactamases with a substrate profile that complements that of the group 1 cephalosporinase. Group 3 metallob-lactamases pose a perplexing problem. These enzymes are capable of hydrolyzing b-lactams from all chemical classes except the monobactams, and they are not inhibited by the b-lactamase inhibitors, such as clavulanic acid or tazobactam. The IMP-1 metalloenzyme was initially confined to Japanese isolates and was thought t (...truncated)