Epidemiologal investigation of an Acinetobacter baumannii outbreak using Core Genome Multilocus Sequence Typing

Carbapenem-resistant (CR) Acinetobacter baumannii is a serious nosocomial pathogen able to cause a variety of serious, often life-threatening infections and outbreaks. We aimed to investigate the molecular epidemiology of clinical isolates of CR-A. baumannii from an outbreak occurred in the intensive care unit (ICU) of our hospital.\n\nFrom January to April 2017, 13 CR-A. baumannii isolates were collected at the \"L. Spallanzani\" hospital, Rome, Italy; typing was performed by repetitive extragenetic palindromic-based (rep)- PCR-based DiversiLab system and WGS data were used for in silico analysis of traditional MLST types, for identifying resistance genes and for core genome multi locus sequence typing (cgMLST) analysis. Epidemiological data were obtained from hospital records.\n\nStrains were cultured from 7 patients treated in the ICU of our hospital; all isolates showed a multi-drug resistant (MDR) profile, carrying the blaOXA-23 carbapenemase. Typing performed by rep-PCR and MLST showed that the isolates clustered into one group while the cgMLST approach, which uses 2690 gene targets to characterize the gene-by-gene allelic profile of A. baumannii, highlighted the presence of two cluster types. These results allowed us to identify two patients who entered the ICU already colonized by two different strains of CR-A. baumannii; we hypothesize that these two patients could be the source of two separate transmission chains.\n\nOur results show that whole-genome-DNA sequencing by cgMLST is a valuable tool, better suited for prompt epidemiological investigations than traditional typing methods because of its higher discriminatory ability in determining clonal relatedness.