SUMMARY Carbapenemases are β-lactamases with versatile hydrolytic capacities. They have the ability to hydrolyze penicillins, cephalosporins, monobactams, and carbapenems. Bacteria producing these β-lactamases may cause serious infections in which the carbapenemase activity renders many β-lactams ineffective. Carbapenemases are members of the molecular class A, B, and D β-lactamases. Class A and D enzymes have a serine-based hydrolytic mechanism, while class B enzymes are metallo-β-lactamases that contain zinc in the active site. The class A carbapenemase group includes members of the SME, IMI, NMC, GES, and KPC families. Of these, the KPC carbapenemases are the most prevalent, found mostly on plasmids in Klebsiella pneumoniae. The class D carbapenemases consist of OXA-type β-lactamases frequently detected in Acinetobacter baumannii. The metallo-β-lactamases belong to the IMP, VIM, SPM, GIM, and SIM families and have been detected primarily in Pseudomonas aeruginosa; however, there are increasing numbers of reports worldwide of this group of β-lactamases in the Enterobacteriaceae. This review updates the characteristics, epidemiology, and detection of the carbapenemases found in pathogenic bacteria.

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A Klebsiella pneumoniae isolate showing moderate to high-level imipenem and meropenem resistance was investigated. The MICs of both drugs were 16 g/ml. The ␤-lactamase activity against imipenem and meropenem was inhibited in the presence of clavulanic acid. The strain was also resistant to extended-spectrum cephalosporins and aztreonam. Isoelectric focusing studies demonstrated three ␤-lactamases, with pIs of 7.2 (SHV-29), 6.7 (KPC-1), and 5.4 (TEM-1). The presence of bla SHV and bla TEM genes was confirmed by specific PCRs and DNA sequence analysis. Transformation and conjugation studies with Escherichia coli showed that the ␤-lactamase with a pI of 6.7, KPC-1 (K. pneumoniae carbapenemase-1), was encoded on an approximately 50-kb nonconjugative plasmid. The gene, bla KPC-1 , was cloned in E. coli and shown to confer resistance to imipenem, meropenem, extended-spectrum cephalosporins, and aztreonam. The amino acid sequence of the novel carbapenem-hydrolyzing ␤-lactamase, KPC-1, showed 45% identity to the pI 9.7 carbapenem-hydrolyzing ␤-lactamase, Sme-1, from Serratia marcescens S6. Hydrolysis studies showed that purified KPC-1 hydrolyzed not only carbapenems but also penicillins, cephalosporins, and monobactams. KPC-1 had the highest affinity for meropenem. The kinetic studies also revealed that clavulanic acid and tazobactam inhibited KPC-1. An examination of the outer membrane proteins of the parent K. pneumoniae strain demonstrated that the strain does not express detectable levels of OmpK35 and OmpK37, although OmpK36 is present. We concluded that carbapenem resistance in K. pneumoniae strain 1534 is mainly due to production of a novel Bush group 2f, class A, carbapenem-hydrolyzing ␤-lactamase, KPC-1, although alterations in porin expression may also play a role.The carbapenems, such as imipenem and meropenem, are used with increasing frequency in the United States and elsewhere for the treatment of multiresistant gram-negative nosocomial pathogens (21,29,30). Resistance to carbapenems is uncommon in enteric organisms; however, resistance can arise by three known mechanisms. First, high-level production of a chromosomal AmpC cephalosporinase combined with decreased outer membrane permeability due to loss or alteration of porins can result in carbapenem resistance. This has been shown for Enterobacter cloacae (28, 54), Enterobacter aerogenes (9, 10, 13, 23), Proteus rettgeri (54), Citrobacter freundii (32), Escherichia coli (11, 64), and Klebsiella pneumoniae (5, 7, 16). The second mechanism is production of a ␤-lactamase that is capable of hydrolyzing carbapenems (8,30,58) The third mechanism of resistance involves changes in the affinity of the target enzymes, the penicillin binding proteins, for carbapenems (15,70).In this study, a K. pneumoniae strain manifesting carbapenem resistance was collected through project ICARE (Intensive Care Antimicrobial Resistance Epidemiology) (4,20) and analyzed for its mechanism(s) of carbapenem resistance. The results presented suggest that the carbapenem resistance phen...

The development and spread of antibiotic resistance in bacteria is a universal threat to both humans and animals that is generally not preventable, but can nevertheless be controlled and must be tackled in the most effective ways possible. To explore how the problem of antibiotic resistance might best be addressed, a group of thirty scientists from academia and industry gathered at the Banbury Conference Centre in Cold Spring Harbor, New York, May 16-18, 2011. From these discussions emerged a priority list of steps that need to be taken to resolve this global crisis.

Antimicrobial-modifying resistance enzymes have traditionally been class specific, having coevolved with the antibiotics they inactivate. Fluoroquinolones, antimicrobial agents used extensively in medicine and agriculture, are synthetic and have been considered safe from naturally occurring antimicrobial-modifying enzymes. We describe reduced susceptibility to ciprofloxacin in clinical bacterial isolates conferred by a variant of the gene encoding aminoglycoside acetyltransferase AAC(6')-Ib. This enzyme reduces the activity of ciprofloxacin by N-acetylation at the amino nitrogen on its piperazinyl substituent. Although approximately 30 variants of this gene have been reported since 1986, the two base-pair changes responsible for the ciprofloxacin modification phenotype are unique to this variant, first reported in 2003 and now widely disseminated. An intense increase in the medical use of ciprofloxacin seems to have been accompanied by a notable development: a single-function resistance enzyme has crossed class boundaries, and is now capable of enzymatically undermining two unrelated antimicrobial agents, one of them fully synthetic.

Structural analysis indicates that affinity for the penta-coordinated zinc can be modulated by neighboring residues, perhaps explaining the absence of the second zinc in the B. cereus structure. Models of bound substrates suggest that the active-site channel can accommodate a wide variety of beta-lactams. We propose that the zinc cluster prepares an hydroxide, probably the hydroxide that ligates both zincs, for nucleophilic attack on the carbonyl carbon atom of the beta-lactam. The resulting negatively charged tetrahedral intermediate implicated in catalysis is stabilized by an oxyanion hole formed by the side chain of the invariant Asn 193 and the tetrahedral zinc.

b-Lactams are the most widely used class of antibiotics. Since the discovery of benzylpenicillin in the 1920s, thousands of new penicillin derivatives and related b-lactam classes of cephalosporins, cephamycins, monobactams, and carbapenems have been discovered. Each new class of b-lactam has been developed either to increase the spectrum of activity to include additional bacterial species or to address specific resistance mechanisms that have arisen in the targeted bacterial population. Resistance to b-lactams is primarily because of bacterially produced b-lactamase enzymes that hydrolyze the b-lactam ring, thereby inactivating the drug. The newest effort to circumvent resistance is the development of novel broad-spectrum b-lactamase inhibitors that work against many problematic b-lactamases, including cephalosporinases and serine-based carbapenemases, which severely limit therapeutic options. This work provides a comprehensive overview of b-lactam antibiotics that are currently in use, as well as a look ahead to several new compounds that are in the development pipeline.