Dissemination and Genetic Structure of Carbapenemase Encoding Genes (bla_OXA-23 and bla_OXA-24) in <i>Acinetobacter baumannii</i> from Southern Texas

Abstract

Acinetobacter baumannii is one of the most important human pathogens causing a variety of nosocomial infections. Carbapenem antibiotics have been primarily used to treat the A. baumannii infections. However, carbapenem resistant A. baumannii producing carbapenemases causes serious treatment problems worldwide. Outbreaks of carbapenem resistant isolates have reported in some area of the United States, but their dissemination and genetic structure of the carbapenemase encoding genes are currently little known. To understand outbreaks, dissemination, and genetic structure of the carbapenemase encoding genes in Southern Texas, 32 clinical isolates collected from Austin and Houston, TX were characterized. Twenty-eight of 32 isolates were resistant to all tested β -lactam antibiotics in cluding carbapenem (imipenem and meropenem). Three of them carried bla OXA-23 as a part of Tn2008 integrated into a known plasmid (pACICU2) and all others carried bla OXA-24 flanked by XerC/XerD-like recombinase binding sites that were adjoined by DNA sequences originated from multiple plasmids. Genotype analysis revealed that the 25 isolates carrying bla OXA-24 were all identical genotypes same as a representative isolate carrying bla OXA-24 from Chicago, IL but the 3 isolates carrying bla OXA-23 was a distinct genotype as compared with isolates carrying bla OXA-23 from Chicago, IL and Washington, D.C. Each of the bla OXA-23 and bla OXA-24 was transferred to carbapenem susceptible A. baumannii and E. coli with similar minimal inhibitory concentration (MIC) of carbapenem as that of their parental isolates but significantly lower levels of MIC in E. coli . Overall results suggest that a unique strain carrying bla OXA-23 and a similar strain carrying bla OXA-24 as seen in other geographic areas are currently disseminated in Southern Texas.