IMP-4 and OXA β-lactamases in Acinetobacter baumannii from Singapore

Objective: To compare the incidence of carbapenemase genes in Acinetobacter baumannii between two time periods. Methods: We studied 114 isolates of imipenem-resistant A. baumannii collected over two 5 month periods (in 1996 and 2001). Isolates showing carbapenemase activity by plate bioassay were screened for carbapenemase genes using PCR. Chromosomal DNA from strains carrying carbapenemase genes was subjected to PFGE after digestion with Apal. Results: The incidence of imipenem-resistant A. baumannii in our hospital rose from 1.1 per 1000 admissions in 1996 to 2.3 per, 1000 admissions in 2001. However, the number of carbapenemase-producing A. baumannii rose, only slightly in 2001 (0.8 per 1000 admissions) compared to 1996 (0.5 per 1000 admissions). Of 44 isolates with carbapenemase activity, 4 isolates carried bla(IMP-4), 5 carried bla(OXA-58), and 40 carried bla(OXA-23). In addition, most isolates carried a bla(OXA-51)-type beta-lactamase gene. All strains with bla(IMP-4), also carried bla(OXA-58) and bla(PSE-1), but not bla(OXA-51)-type beta-lactamase genes. PCR analysis repeated on seven recent isolates of susceptible A. baumannii showed only the presence of bla(OXA-51)-type beta-lactamase genes. A total of five novel bla(OXA-51)-type beta-lactamase genes (bla(OXA-88,-91,-93,-94), and (-95)) and one new bla(OXA-58)-type beta-lactamase gene (bla(OXA-96)) were found. Conclusions: The incidence of carbapenemase genes did not vary significantly between the two study periods. There is a wide diversity of OXA genes in A. baumannii in Singapore. The most common carbapenemase gene found in our study was bla(OXA-23).