Multilocus sequence typing supports the hypothesis that Ochrobactrum anthropi displays a human-associated subpopulation

Background: Ochrobactrum anthropi is a versatile bacterial species with strains living in very diverse habitats. It is increasingly recognized as opportunistic pathogen in hospitalized patients. The population biology of the species particularly with regard to the characteristics of the human isolates is being investigated. To address this issue, we proposed a polyphasic approach consisting in Multi-Locus Sequence Typing (MLST), multi-locus phylogeny, genomic-based fingerprinting by pulsed-field gel electrophoresis (PFGE) and antibiotyping. Results: We tested a population of 70 O. anthropi clinical (n = 43) and environmental (n = 24) isolates as well as the type strain O. anthropi ATCC49188(T) and 2 strains of Ochrobactrum lupini and Ochrobactrum cytisi isolated from plant nodules. A Multi-Locus Sequence Typing (MLST) scheme for O. anthropi is proposed here for the first time. It was based on 7 genes (3490 nucleotides) evolving mostly by neutral mutations. The MLST approach suggested an epidemic population structure. A major clonal complex corresponded to a human-associated lineage since it exclusively contained clinical isolates. Genomic fingerprinting separated isolates displaying the same sequence type but it did not detect a population structure that could be related to the origin of the strains. None of the molecular method allowed the definition of particular lineages associated to the host-bacteria relationship (carriage, colonisation or infection). Antibiotyping was the least discriminative method. Conclusion: The results reveal a human-associated subpopulation in our collection of strains. The emergence of this clonal complex was probably not driven by the antibiotic selective pressure. Therefore, we hypothesise that the versatile species O. anthropi could be considered as a human-specialized opportunistic pathogen.