Cephalosporinases (B-J-M group 1, Ambler group C)
These beta-lactamases are chromosomally encoded and are produced by almost all gram negative bacteria in different degrees; they can be expressed in a constitutive or inducible form. 1 Characteristic of these beta-lactamases is their ability to inactivate extended-spectrum cephalosporins and their resistance to beta-lactam inhibitors. Enterobacter, Citrobacter freundii, Serratia marcescens, Morganella morganii, Providencia, and P. aeruginosa harbour these enzymes, although its production is normally repressed. 8 In the presence of certain antibiotics the production is induced reaching high levels and conferring resistance to the bacteria. The expression levels go back to normal in the absence of the inducing antibiotic.
Mutations may occur in the regulation of the inducible beta-lactamase gene leading to permanent hyper-production or derepression. (8)
Penicillinase
s (B-J-M group 2, Amber class A)
These beta-lactamases have a serine residue in their active site to break the bond of the beta-lactam ring; most are inhibited by beta-lactamase inhibitors.
The B-J-M classification further divides this group based on the scope of action of these enzymes.
Group 2a is comprised of mainly penicillinases present in Staphylococcus species; group 2b consists of beta-lactamases that are primarily penicillinases and have poor activity against cephalosporins. The most important examples in this group are TEM-1 and SHV-1 beta-lactamases known to be the progenitors of extended-spectrum beta lactamases (ESBL) and inhibitor resistant (IRT) beta-lactamases. (10)
Extended-spectrum beta-lactamases
Group 2be are beta-lactamases with an extended substrate profile (ESBL). Class A extended spectrum beta-lactamases (ESBLs) arose in the 1980s following prevalent use of broad-spectrum beta-lactams. 2 These enzymes are derived from TEM-1 and SHV-1 enzymes. TEM-derived ESBLs have been reported in plasmids, transposons, and as chromosomally-mediated beta-lactamases that are usually found in bacteria such as E. coli, Klebsiella pneumoniae, and other gram negative bacteria. (8) SHV derived beta-lactamases, SHV referring to sulfhydryl variable, may be more common in clinical isolates than other types of ESBLs. (11) The SHV-1 enzyme has amino acid changes related to its active site similar to that of TEM-1 enzyme. (10) Point mutations (one or more amino acid substitutions) at its active site also produce its ESBL derivatives. (8,10)
CTX-M derived ESBLs are thought to have developed from plasmids from the chromosomal AmpC enzyme of Kluyvera species. A characteristic of these enzymes is their potent activity against cefotaxime. (11)
Group 2br beta-lactamases are also derived from TEM and SHV beta-lactamases, but the aminoacid substitutions have conferred them with resistance to beta-lactamase inhibitors.
