Beta-lactamase
Beta-lactamases are enzymes produced by bacteria that provide multi-resistance to beta-lactam antibiotics such as penicillins, cephalosporins, cephamycins, monobactams and carbapenems, although carbapenems are relatively resistant to beta-lactamase. Beta-lactamase provides antibiotic resistance by breaking the antibiotics' structure. These antibiotics all have a common element in their molecular structure: a four-atom ring known as a beta-lactam ring. Through hydrolysis, the enzyme lactamase breaks the β-lactam ring open, deactivating the molecule's antibacterial properties.
Beta-lactamases produced by gram-negative bacteria are usually secreted, especially when antibiotics are present in the environment.
Structure
The structure of a Streptomyces serine β-lactamase is given by. The alpha-beta fold resembles that of a DD-transpeptidase, from which the enzyme is thought to have evolved. β-lactam antibiotics bind to DD-transpeptidases to inhibit bacterial cell wall biosynthesis. Serine β-lactamases are grouped by sequence similarity into types A, C, and D.The other type of beta-lactamase is of the metallo type. Metallo-beta-lactamases need metal ion on their active site for their catalytic activities. The structure of the New Delhi metallo-beta-lactamase 1 is given by. It resembles a RNase Z, from which it is thought to have evolved.
Mechanism of action
The two types of beta-lactamases work on the basis of the two basic mechanisms of opening the β-lactam ring.The SBLs are similar in structure and mechanistically to the β-lactam target penicillin-binding proteins which are necessary for cell wall building and modifying. SBLs and PBPs both covalently change an active site serine residue. The difference between the PBPs and SBLs is that the latter generates free enzyme and inactive antibiotic by the very quick hydrolysis of the acyl-enzyme intermediate.
The MBLs use the Zn2+ ions to activate a binding site water molecule for the hydrolysis of the β-lactam ring. Zinc chelators have recently been investigated as metallo-β-lactamase inhibitors, as they are often able to restore carbapenem susceptibility.
Penicillinase
Penicillinase is a specific type of β-lactamase, showing specificity for penicillins, again by hydrolysing the β-lactam ring. Molecular weights of the various penicillinases tend to cluster near 50 kilodaltons.Penicillinase was the first β-lactamase to be identified. It was first isolated by Abraham and Chain in 1940 from E. coli even before penicillin entered clinical use, but penicillinase production quickly spread to bacteria that previously did not produce it or produced it only rarely. Penicillinase-resistant beta-lactams such as methicillin were developed, but there is now widespread resistance to even these.
Resistance in gram-negative bacteria
Among gram-negative bacteria, the emergence of resistance to extended-spectrum cephalosporins has been a major concern. It appeared initially in a limited number of bacterial species that could mutate to hyperproduce their chromosomal class C β-lactamase. A few years later, resistance appeared in bacterial species not naturally producing AmpC enzymes due to the production of TEM- or SHV-type ESBLs. Characteristically, such resistance has included oxyimino-, but not 7-alpha-methoxy-cephalosporins ; has been blocked by inhibitors such as clavulanate, sulbactam or tazobactam and did not involve carbapenems and temocillin. Chromosomal-mediated AmpC β-lactamases represent a new threat, since they confer resistance to 7-alpha-methoxy-cephalosporins such as cefoxitin or cefotetan but are not affected by commercially available β-lactamase inhibitors, and can, in strains with loss of outer membrane porins, provide resistance to carbapenems.Extended-spectrum beta-lactamase (ESBL)
Members of this family commonly express β-lactamases which confer resistance to expanded-spectrum cephalosporins. In the mid-1980s, this new group of enzymes, the extended-spectrum β-lactamases, was detected. The prevalence of ESBL-producing bacteria have been gradually increasing in acute care hospitals. The prevalence in the general population varies between countries, e.g. approximately 6% in Germany and France, 13% in Saudi Arabia, and 63% in Egypt.ESBLs are beta-lactamases that hydrolyze extended-spectrum cephalosporins with an oxyimino side chain. These cephalosporins include cefotaxime, ceftriaxone, and ceftazidime, as well as the oxyimino-monobactam aztreonam. Thus, ESBLs confer multi-resistance to these antibiotics and related oxyimino-beta lactams. In typical circumstances, they derive from genes for TEM-1, TEM-2, or SHV-1 by mutations that alter the amino acid configuration around the active site of these β-lactamases. A broader set of β-lactam antibiotics are susceptible to hydrolysis by these enzymes. An increasing number of ESBLs not of TEM or SHV lineage have recently been described. The ESBLs are frequently plasmid encoded. Plasmids responsible for ESBL production frequently carry genes encoding resistance to other drug classes. Therefore, antibiotic options in the treatment of ESBL-producing organisms are extremely limited. Carbapenems are the treatment of choice for serious infections due to ESBL-producing organisms, yet carbapenem-resistant isolates have recently been reported. ESBL-producing organisms may appear susceptible to some extended-spectrum cephalosporins. However, treatment with such antibiotics has been associated with high failure rates.