A novel Bacteroides metallo-β-lactamase (MBL) and its gene (crxA) in Bacteroides xylanisolvens revealed by genomic sequencing and functional analysis.

Objectives: We sought to characterize the carbapenem resistance mechanism of Bacteroides xylanisolvens 14880, an imipenem-resistant strain from Germany, and assess its prevalence.

Methods: Antimicrobial susceptibilities were determined using agar dilution or Etest methodology and specific imipenemase activity was detected. The genomic sequence of B. xylanisolvens 14880 was determined and analysed for antibiotic resistance genes and genomic islands. We also used gene transfer to a carbapenem susceptible host, along with 5'-RACE, conventional PCR with capillary sequencing and RT-PCR-based screening.

Results: B. xylanisolvens 14880 displayed resistance to carbapenems and produced high specific imipenemase activity. Its genomic sequence was 6.1 Mbp and a class B1 β-lactamase gene (termed crxA) was detected in it. crxA was carried on a putative genomic island with insertion sequence (IS) elements and a putative GNAT (Gcn5-like acetyltransferase) toxin gene. Promoter localization by 5'-RACE and gene targeting to an imipenem-susceptible Bacteroides host indicated that it is activated by an IS1380-like IS element and it can confer carbapenem resistance. The PCR screening of Bacteroides strains showed that crxA was specific to B. xylanisolvens with a carriage rate of 16.7%.

Conclusions: B. xylanisolvens strains can harbour a carbapenem resistance gene, which has many similarities to the 'cfiA system': metallo-β-lactamase (MBL), IS element activation, carriage of a GNAT toxin gene, specific for a unique Bacteroides species with a significant prevalence.