The nosocomial spread of six genetically related Klebsiella pneumoniae strains producing GES-type β-lactamases was found in a neonatal intensive care unit, and we previously reported that one of the six strains, strain KG525, produced a new β-lactamase, GES-3. In the present study, the molecular mechanism of cephamycin resistance observed in strain KG502, one of the six strains described above, was investigated. This strain was found to produce a variant of GES-3, namely, GES-4, which was responsible for resistance to both cephamycins (cefoxitin MIC, >128 μg/ml) and β-lactamase inhibitors (50% inhibitory concentration of clavulanic acid, 15.2 ± 1.7 μM). The GES-4 enzyme had a single G170S substitution in the Ω-loop region compared with the GES-3 sequence. This single amino acid substitution was closely involved with the augmented hydrolysis of cephamycins and carbapenems and the decreased affinities of β-lactamase inhibitors to GES-4. A cloning experiment and sequencing analysis revealed that strain KG502 possesses duplicate blaGES-4 genes mediated by two distinct class 1 integrons with similar gene cassette configurations. Moreover, the genetic environments of the blaGES-4 genes found in strain KG502 were almost identical to that of blaGES-3 in strain KG525. From these findings, these two phenotypically different strains were suggested to belong to a clonal lineage. The blaGES-4 gene found in strain KG502 might well emerge from a point mutation in the blaGES-3 gene harbored by its ancestor strains, such as strain KG525, under heavy antibiotic stress in order to acquire extended properties of resistance to cephamycins and carbapenems.
All Science Journal Classification (ASJC) codes
- Pharmacology (medical)
- Infectious Diseases