Genetic and Biochemical Characterization of TRU-1, the Endogenous Class C β-Lactamase from Aeromonas enteropelogenes

Aeromonas enteropelogenes (formerly A. tructi) was described to be an ampicillin-susceptible and cephalothin-resistant Aeromonas species, which suggests the production of a cephalosporinase. Strain ATCC 49803 was susceptible to amoxicillin, cefotaxime, and imipenem but resistant to cefazolin (MICs of 2, 0.032, 0.125, and > 256 mu g/ml, respectively) and produced an inducible beta-lactamase. Cefotaxime-resistant mutants (MIC, 32 mu g/ml) that showed constitutive mu-lactamase production could be selected in vitro. The gene coding for the cephalosporinase of A. enteropelogenes ATCC 49803 was cloned, and its biochemical properties were investigated. Escherichia coli transformants showing resistance to various beta-lactams carried a 3.5-kb plasmid insert whose sequence revealed a 1,146-bp open reading frame (ORF) encoding a class C beta-lactamase, named TRU-1, showing the highest identity scores with A. punctata CAV-1 (75%), A. salmonicida AmpC (75%), and A. hydrophila CepH (71%). The bla(TRU-1) locus includes open reading frames (ORFs) showing significant homology with genes found in the genomes of other Aeromonas species, although it exhibits a different organization, as reflected by the presence of additional ORFs located downstream of the beta-lactamase gene in the A. hydrophila and A. salmonicida genomes. Specific PCR assays were negative for cphA-like and bla(OXA-12)-like genes in three A. enteropelogenes ATCC strains. Purified TRU-1 showed a broad substrate profile, efficiently hydrolyzing benzylpenicillin, cephalothin, cefoxitin, and, although with significantly lower turnover rates, oxyiminocephalosporins. Cephaloridine and cefepime were poorly recognized by the enzyme, as reflected by the high K-m values observed with these substrates. Thus far, A. enteropelogenes represents the only known example of an Aeromonas species that produces only one beta-lactamase belonging to molecular class C.