Molecular epidemiology of multidrug-resistant Acinetobacter baumannii in a tertiary care hospital in Naples, Italy, shows the emergence of a novel epidemic clone

Abstract

The molecular epidemiology of multidrug-resistant Acinetobacter baumannii was investigated in two intensive care units of the V. Monaldi university hospital in Naples, Italy, from May 2006 to December 2007. Genotype analysis by pulsed-field gel electrophoresis (PFGE), trilocus sequence-based typing (3LST), and multilocus sequence typing (MLST) of A. baumannii isolates from 71 patients identified two distinct genotypes, one assigned to PFGE group A, 3LST group 1, and ST2 in 14 patients and the other to PFGE group B, 3LST group 6, and ST78 in 71 patients, that we named ST2/A and ST78/B, respectively. Of these, ST2/A corresponded to European clone II identified in the same hospital during 2003 and 2004; ST78/B was a novel genotype that was isolated for the first time in May 2006 but became prevalent during 2007. The ST78/B profile was also identified in five patients from two additional hospitals in Naples during 2007. The ST2/A and ST78/B isolates were resistant to all antimicrobials tested, including carbapenems, but were susceptible to colistin. Both ST2/A and ST78/B isolates possessed a plasmid-borne carbapenem-hydrolyzing oxacillinase gene, bla(OXA-58), flanked by ISAba2 and ISAba3 elements at the 5' and 3' ends, respectively. The selection of the novel ST78/B A. baumannii clone might have been favored by the acquisition of the bla(OXA-58) gene.

FIG. 1.

(A) ApaI PFGE profiles of representative A. baumannii strains included in the study. Capital letters above the lanes indicate PFGE types identified; m, phage lambda DNA molecular mass markers. The isolate number is shown below each lane. Sizes of lambda DNA molecular mass markers are shown on the right. (B) Multiplex PCR to selectively amplify ompA, csuE, and bla_OXA-51-like alleles. ST groups identified are indicated above the lanes; m, 1-kb DNA ladder molecular mass markers (Promega, Milan, Italy). The isolate number is shown below each lane.

FIG. 2.

Molecular epidemiology of A. baumannii in the V. Monaldi hospital, Naples, Italy, during 2006 and 2007. Gray and white bars represent ST2/A isolates from the cardiorespiratory ICU (CR-ICU) and postoperative ICU (PO-ICU), respectively; black and dotted bars represent ST78/B isolates from the CR-ICU and PO-ICU, respectively.

FIG. 3.

(A) Plasmid localization of the bla_OXA-58 gene in A. baumannii ST2/A and ST78/B isolates. Agarose (1%) gel electrophoresis in 1× Tris-acetate-EDTA buffer of HindIII-digested plasmids from A. baumannii isolates 3979 and 3957, respectively, stained with ethidium bromide and visualized under UV light and Southern blot hybridization with the bla_OXA-58 probe are shown. Lane M shows a 1-kb DNA ladder (Promega, Milan, Italy). (B) Schematic map of the genetic structure surrounding the bla_OXA-58 gene in A. baumannii ST2/A and ST78/B isolates. The genes and their corresponding transcription orientations are indicated by horizontal arrows. IS elements are represented by open rectangles filled with black arrows indicating the transposase gene and the direction of transcription. Names of relevant features are reported below or above the map.