Does Acinetobacter baumannii Serve as a Source for bla_NDM Dissemination into Enterobacteriaceae in Hospitalized Patients?

The goal was to study the possibility that Acinetobacter baumannii serve as an epidemiologically significant source for transmission of the bla_NDM gene to Enterobacteriaceae by horizontal gene transfer (HGT) in hospitalized patients. The study was done at the Tel-Aviv Sourasky Medical Center from December 2014 until August 2015. Clinical and surveillance rectal cultures were collected as per hospital policies and were analyzed for the presence of New Delhi metallo-beta-lactamase-producing Enterobacteriaceae (NDME) and A. baumannii (NDMAb). Isolates were typed by pulsed-field gel electrophoresis (PFGE). The location of the bla_NDM gene within the Tn125 transposon was studied by sequencing. A transmission event (TE) was determined if patients shared the same PFGE type of either NDME or NDMAb and were simultaneous in the same ward. HGT-related TE was considered if the two isolates shared identical bla_NDM gene allele and transposon. There were 16 NDMAb- (clinical, 10; surveillance, 4; both, 2) and 13 NDME- (clinical, 3; surveillance, 8; both, 2) infected/colonized patients. All NDMAb isolates except two harbored the bla_NDM-1 allele that was located within a Tn125 transposon and was plasmid borne. The majority of patients (n = 10/16) were infected by one PFGE type of NDMAb, and five clonal TEs were identified. NDME were either Escherichia coli (n = 4) or Klebsiella pneumoniae (n = 9) of different PFGE types with only one NDME TE. The bla_NDM gene was within a Tn125 in three K. pneumoniae isolates. Although one HGT-related TE between NDMAb and K. pneumoniae was epidemiologically suspected, the low similarity between the Tn125 transposons (75.7%) excluded that possibility. In conclusion, whereas NDMAb appears to disseminate by clonal spread, we did not find evidence for HGT-mediated transmission in NDME in hospitalized patients.