Acinetobacter radioresistens as a silent source of carbapenem resistance for Acinetobacter spp

Abstract

Carbapenem resistance results mostly from the expression of acquired carbapenem-hydrolyzing oxacillinases in Acinetobacter baumannii. The bla OXA-23 oxacillinase gene is increasingly reported worldwide and may represent an emerging threat. Our goal was to identify the progenitor of that carbapenemase gene. A collection of 50 Acinetobacter sp. strains corresponding to several Acinetobacter species was screened for bla(OXA-23)-like genes by PCR and hybridization techniques. Five Acinetobacter radioresistens isolates that were susceptible to carbapenems harbored chromosomally encoded bla OXA-23-like genes. A similar plasmid backbone was identified in several bla OXA-23-positive A. baumannii and A. radioresistens isolates, further strengthening the vectors of exchanges for these bla OXA-23-like genes. Therefore, A. radioresistens, a commensal bacterial species which is identified on the skin of hospitalized and healthy patients (a property shared with A. baumannii), was identified as the source of the bla OXA-23 gene.

FIG. 1.

Schematic maps of composite transposon Tn2006 carrying the bla_OXA-23 gene identified on a plasmid in A. baumannii isolates (7) (A) and the sequences identified in the five different A. radioresistens isolates (B). The sequences identical between Tn2006, Tn2007, and the A. radioresistens isolates are indicated by two vertical lines. ATPase is the gene encoding the putative ATPase, and sulf is the gene for a plasmid-borne sulfonamide resistance gene truncated by the insertion of Tn2006. orf1 is the gene encoding a putative protein truncated by the insertion of Tn2007, and mobA is the gene encoding a putative mobilization protein. Sequences indicated by dashes are unknown.

FIG. 2.

Comparison of the amino acid sequences of the OXA-23-like determinants. OXA-23 is from A. radioresistens isolates 3 and 5, OXA-102 is from isolate 1, OXA-103 is from isolate 2, and OXA-105 is from isolate 4. Dashes indicate identical amino acid residues, and the critical motifs for CHDLs are shaded in gray. The numbering is according to the nomenclature for CHDLs (8).

FIG. 3.

PFGE profiles of I-Ceu-I digested whole-cell DNAs of A. radioresistens strains. Lanes 1, A. radioresistens reference strain 1; lanes 2, A. radioresistens clinical isolate 2; lanes 3, A. baumannii Ab13 isolate harboring a plasmid-located bla_OXA-23 gene. Southern hybridization was performed with an internal probe specific for the bla_OXA-23 gene (A) and with a probe specific for the 16S-23S rRNA gene (B). Horizontal arrows indicate the positions of hybridization with the bla_OXA-23 probe. By comparison of the two hybridization patterns, cohybridizations were obtained for strains 1 and 2 (chromosomal location of bla_OXA-23), whereas the bla_OXA-23-positive signal obtained for A. baumannii Ab13 has no corresponding band with the 16S-23S rRNA-specific probe.