LMB-1, a novel family of class B3 MBLs from an isolate of Enterobacter cloacae.

Objectives: To identify and characterize a novel MBL gene conferring carbapenem resistance to an isolate of Enterobacter cloacae from Austria.
Methods: The novel MBL gene was heterologously expressed in Escherichia coli TOP10 to conduct comparative MIC studies and biochemical assays. Furthermore, WGS was performed using Illumina MiSeq and Oxford Nanopore MinION instruments to analyse the genetic environment of the novel MBL gene.
Results: The novel MBL showed highest sequence homology to a predicted MBL precursor from the marine bacterium Rheinheimera pacifica and hence belongs to Ambler subgroup B3. The comparative MIC studies and biochemical assays showed activity of the novel enzyme against penicillins, cephalosporins and carbapenems, but not against aztreonam. It was named Linz MBL (LMB-1). The blaLMB-1 gene was shown to be located on a 108 kb plasmid of Inc type IncFIB(K). Of note, a gene adjacent to blaLMB-1 coded for a glycerophosphoryl diester phosphodiesterase that was also previously detected in R. pacifica.
Conclusions: Homologies of the MBL gene itself and another gene located on the same plasmid to genes detected in marine bacterial species strongly suggest that this novel MBL was transferred to E. cloacae from a marine bacterium. This underlines the importance of natural reservoirs supplying hitherto unknown resistance genes to clinically relevant bacterial species and the importance of ongoing surveillance and research.