Carbapenemases
Carbapenemases can be classified under Amber Class A, Class B, (MBLs) or Class D. Their presence yields the largest antibiotic resistance range since they hydrolyze carbapenems and also a broad-spectrum of penicillins, oxymino-cephalosporins and cephamycins. Carbapenemases can be found worldwide in organisms such as E.coli, Citrobacter, Enterobacter, Salmonella, Serratia, and P. aeruginosa. (8) The best known class A carbapenemase is KPC, a beta-lactamase first found in K. pneumoniae. (10)
Class B carbarpenemases (B-J-M group 3) are metallo-beta-lactamases (MBLs) which require zinc or another heavy metal for their activity. (10) These enzymes are usually chromosomic, which confer resistance to beta-lactams, carbapenems, are resistant to beta-lactam inhibitors, and can be inhibited by chelating agents such as EDTA. 12 They are found mostly in environmental isolates of Aeromonas, Chryseobacterium, and Stenotrophmonas, species with low pathogenicity. MBLs of clinical importance belong to the IMP, VIM, SPM, GIM, and SIM families (8) and have been described in P. aeruginosa, Acinetobacter, and enteric pathogens. (10)
CLASS D carbapenemases or OXA beta-lactamases are able to hydrolize oxacillin and can give resistance to penicillins, cephalosporins, extended-spectrum cephalosporins, and carbapenems. (1) OXA enzymes have been found more frequently in P.aeruginosa and A. baumannii.(10)
Detection of beta-lactamases
Tests for beta-lactamase production including colorimetric, acidometric and idiometric tests can be useful in the determination of common beta-lactamases produced by organisms such as Staphylococcus, Neisseria, Haemophilus, and Bacteroides. (8)
The detection of ESBL production by clinical microbiology laboratories has proven to be a challenge over the years. (7, 11) The introduction of new breakpoints for third generation cephalosporins and interpretation guidelines has eliminated the need for laboratories to detect ESBLs although this is still controversial. (13)
Test methods for the screening and confirmation for class C beta-lactamase producing strains have presented some difficulties as well. Cefoxitin susceptibility testing has been shown to be a sensitive, but not specific, indicator of class C beta-lactamase.(14)
While PCR techniques are available, they often are limited to reference laboratories. Fortunately, easier methods such as the AmpC disk tests are now available.
Technical issues also exist for tests methods to detect carbapenemase production by organisms. (12) Phenotypic tests like the Modified Hodge Test (MHT), the Carba NP Test, and the double disk synergy test (DDST) have been developed in order to detect the different carbapenemase enzymes. These tests are difficult to interpret and are not recommended for routine use. (12-14)