Characterization of Bacillus cereus Group Isolates From Human Bacteremia by Whole-Genome Sequencing

Abstract

Members of the <i>Bacillus cereus</i> group are spore-forming organisms commonly associated with food poisoning and intestinal infections. Moreover, some strains of the group (i.e., <i>B. cereus</i> sensu stricto and <i>Bacillus thuringiensis</i>) can cause bacteremia in humans, mainly in immunocompromised individuals. Here we performed the genetic characterization of 17 human clinical strains belonging to <i>B. cereus</i> group isolated from blood culture. The whole-genome sequencing (WGS) revealed that the isolates were closely related to <i>B. cereus</i> sensu stricto and <i>B. thuringiensis</i>-type strain. Multilocus sequence typing analysis performed on the draft genome revealed the genetic diversity of our isolates, which were assigned to different sequence types. Based on <i>panC</i> nucleotide sequence, the isolates were grouped in the phylogenetic groups III and IV. The <i>NHE</i>, <i>cer</i>, and <i>inhA</i> gene cluster, <i>entA</i>, <i>entFM</i>, <i>plcA</i>, and <i>plcB</i>, were the most commonly detected virulence genes. Although we did not assess the ability to generate biofilm by phenotypic tests, we verified the prevalence of biofilm associated genes using an <i>in silico</i> approach. A high prevalence of <i>pur</i> gene cluster, <i>xerC</i>, <i>clpY</i>, <i>codY</i>, <i>tasA</i>, <i>sipW</i>, <i>sinI</i>, and <i>sigB</i> genes, was found. Genes related to the resistance to penicillin, trimethoprim, and ceftriaxone were identified in most of the isolates. Intriguingly, the majority of these virulence and AMR genes appeared to be evenly distributed among <i>B. cereus</i> s.s. isolates, as well as closely related to <i>B. thuringiensis</i> isolates. We showed the WGS represents a good approach to rapidly characterize <i>B. cereus</i> group strains, being able to give useful information about genetic epidemiology, the presence of virulence and antimicrobial genes, and finally about the potential hazard related to this underestimated risk.