β-Lactamase

β-Lactamases are enzymes (EC 3.5.2.6) produced by bacteria that provide multi-resistance to β-lactam antibiotics such as penicillins, cephalosporins, cephamycins, and carbapenems, where carbapenems are relatively resistant to β-lactamase. β-Lactamase endows antibiotic resistance by breaking the structure of antibiotics, all of which have a common element in their molecular structure: a four-membered β-lactam ring. The lactamase enzyme opens the β-lactam ring through hydrolysis to deactivate the molecule's antibacterial properties. β-Lactam-based antibiotics are typically applied to treat infections caused by a broad spectrum of Gram-positive and Gram-negative bacteria. β-Lactamases are usually secreted by Gram-negative organisms, particularly when antibiotics are present in the environment.

Evolution

β-Lactamases are ancient bacterial enzymes and β-lactamases (the metallo-β-lactamases, MBLs) of class B are divided into three subclasses: B1, B2 and B3. Subclasses B1 and B2 are hypothesized to have evolved about more than one billion years and subclass B3 is speculated to evolve from the divergence of the Gram-positive and Gram-negative eubacteria about two billion years ago. Papua new guinea metallo-β-lactamase enzyme as the first case of MBL under subclass B3 is derived from a functional metagenomic library of the deep-sea sediments preceding antibiotic era. The other three A, C, and D classes are serine enzymes with little homology to each other. Classes A and D are proved to be sister taxa and class C deviates from A and D, while all of these enzymes, like the β-lactamases in class B, have an ancient origin and are hypothesized to have evolved about two billion years. The OXA (oxacillinase) group in class D especially is thought to have evolved on chromosomes and moved to plasmids at least two separate occasions.

Classification

β-Lactamases can be primarily grouped into penicillinase, extended-spectrum β-lactamase (ESBL), inhibitor-resistant β-lactamase, AmpC-type β-lactamase (Class C), and carbapenemase. ESBLs consist of TEM β-lactamases (class A), SHV β-lactamases (class A), CTX-M β-lactamases (class A), OXA β-lactamases (class D) and other plasmid-mediated ESBLs. Carbapenemases can be further classified into IMP-type carbapenemases (metallo-β-lactamases, class B), VIM (Verona integron-encoded metallo-β-lactamase, class B), OXA group of β-lactamases (class D), KPC (K. pneumoniae carbapenemase, class A), CMY (class C), NDM-1 (New Delhi metallo-β-lactamase) (class B), SME (Serratia marcescens enzymes), IMI (IMIpenem-hydrolysing β-lactamase), NMC and CcrA.

a. Penicillinase

Penicillinase is the first identified β-lactamase and shows specificity for penicillins by destroying the β-lactam ring. The molecular weights of various penicillinases tend to be around 50 kDa. It is first isolated from Gram-negative E. coli, even earlier than penicillin being put into clinical use, while the production of penicillinase is quickly spread to bacteria that previously do not produce it or yield it only at a rare amount. Penicillinase-resistant β-lactams have been developed, but widespread resistance has even happened to these antibiotics.

b. Extended-spectrum β-lactamase (ESBL)

ESBLs that hydrolyze extended-spectrum cephalosporins with an oxyimino side chain are first detected in 1979, and the prevalence of bacteria expressing ESBL has been gradually increasing in acute care hospitals. These cephalosporins are cefotaxime, ceftriaxone, and ceftazidime, as well as the oxyimino-mono β-aztreonam, which thereby are conferred with multi-resistance by ESBLs. Under typical circumstances, they are encoded by genes of TEM-1, TEM-2, or SHV-1 through mutations that change the amino acid configuration around the active sites of these β-lactamases. A broad range of β-lactam antibiotics are sensitive to the degradation of these enzymes. ESBLs are frequently encoded by plasmid that carries genes encoding resistance to other drug classes. Therefore, there are great limitations in choosing antibiotics for the treatment of ESBL-producing bacteria. Carbapenems are always the preferential choices for serious infections caused by organisms producing ESBL. However, administration with such antibiotics has been associated with high failure rates.

c. Inhibitor-resistant β-lactamases

Inhibitor-resistant β-lactamases are often discussed with ESBLs since they are also derived from the traditional TEM- or SHV-type enzymes. At first, these enzymes are given designation IRT for inhibitor-resistant TEM β-lactamase, whereas all have subsequently been retitled with numerical TEM designations. Inhibitor-resistant TEM β-lactamases mainly exist in clinical isolates of E. coli, but also are found in some strains of K. pneumoniae, Klebsiella oxytoca, Citrobacter freundii and P. mirabilis. The inhibitor-resistant TEM variants normally remain susceptility to the inhibition of tazobactam and combination of piperacillin/tazobactam, despite the described resistance.

d. AmpC-type β-lactamases

AmpC-type β-lactamases are commonly extracted from extended-spectrum cephalosporin-resistant Gram-negative bacteria, and are typically carried on the chromosome of many Gram-negative bacteria including Citrobacter, Serratia and Enterobacter species where its expression is usually inducible. AmpC-type β-lactamases may also be encoded on plasmids. In contrast to ESBLs, AmpC β-lactamases could hydrolyze broad and extended-spectrum cephalosporins without being inhibited by β-lactamase inhibitors such as clavulanic acid.

e. Carbapenemases

Carbapenems have won their reputation for being stable to AmpC β-lactamases and extended-spectrum-β-lactamases, while they become invalidated under the catalysis of carbapenemases that are active against the oxyimino-cephalosporins and cephamycins. Carbapenemases are previously believed to derive only from classes A, B, and D, but a class C carbapenemase has also been reported.

Resistance in Gram-negative bacteria

The emergence of resistance to broad-spectrum cephalosporins among Gram-negative bacteria has been a major concern. Initially, only a limited number of bacterial species appear to mutate to hyperly produce their chromosomal class C β-lactamase. A few years later, resistance appeared in bacterial species that not only naturally produce AmpC enzymes, but also secrete TEM- or SHV-type extended-spectrum β-lactamases (ESBLs). Characteristically, such resistance has been blocked by inhibitors such as clavulanate, sulbactam or tazobactam. Chromosomal-mediated AmpC β-lactamases has posed a new threat, for they endow resistance to 7-α-methoxy-cephalosporins such as cefoxitin or cefotetan, but not suppressed by commercially available β-lactamase inhibitors. They thereby can provide resistance to carbapenems in strains with loss of outer membrane porins.

Treatment of ESBL/AmpC/carbapenemases

Generally, an isolate is speculated to be an ESBL producer when it is in vitro susceptible to the second-generation cephalosporins, but is resistant to the third-generation cephalosporins and aztreonam. Once a strain is detected as ESBL producer, it should be described as resistant to all penicillins, cephalosporins, and aztreonam, even though the measured in vitro susceptibility. β-Lactamase inhibitors block the efficacy of most ESBLs in vitro, while the clinical effectiveness of the combinations of β-lactam with β-lactamase inhibitor cannot be trusted consistently for therapy. Cephamycins are not inactivated by majority of ESBLs, but can be hydrolyzed by associated AmpC-type β-lactamase. Similarly, the combination of β-lactam and β-lactamase inhibitor may not be active towards organisms that secret AmpC-type β-lactamase, which sometimes reduce the expression of outer membrane proteins, rendering them resistant to cephamycins. Currently, carbapenems are considered as the desirable agent for the treatment of infections resulted from ESBL-producing organisms. Carbapenems resistant to ESBL-mediated hydrolysis also exhibit excellent in vitro inhibitory activity against Enterobacteriaceae strains expressing ESBLs.

Reference

1. Philippon A, Arlet G, Jacoby G A. Plasmid-determined AmpC-type β-lactamases. Antimicrob Agents Chemother, 2002, 46(1):1–11.