The Roles of Two Type VI Secretion Systems in Cronobacter sakazakii ATCC 12868

Min Wang,Hengchun Cao,Qian Wang,Tingting Xu,Xi Guo,Bin Liu

doi:10.3389/fmicb.2018.02499

Min Wang, Hengchun Cao + Show 4 more

Open Access

https://doi.org/10.3389/fmicb.2018.02499

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Abstract

The type VI secretion system (T6SS), which has been found in 25% of gram-negative bacteria, is a crucial virulence factor in several pathogens. Although T6SS gene loci have been discovered in Cronobacter species, one of the major opportunistic foodborne pathogens, its function has not been elucidated. In this study, the roles of two phylogenetically distinct T6SS gene clusters in Cronobacter sakazakii ATCC12868 were investigated. Analysis of 138 genome sequences of C. sakazakii strains, we found that one T6SS gene cluster (T6SS-1) was ubiquitous in all examined strains, whereas another (T6SS-2) was absent or degenerated in a large proportion of the strains (n = 97). In addition, we confirmed the T6SS-1 antibacterial function through an in-frame deletion in the vasK and hcp genes. Compared with the wild-type strain, the T6SS-2-deficient mutant presented a much stronger colonization of organs when infecting neonatal rats. Thus, we proposed that T6SS-2 plays a role in pathogenic processes. This is the first study to investigate the functions of T6SS in C. sakazakii, and the results will extend our understanding of the pathogenic and phylogenetic characteristics of C. sakazakii.

Highlights

Cronobacter spp. is an emerging opportunistic food-borne gram-negative pathogen known to cause severe clinical infections in neonates, including necrotizing enterocolitis (NEC), sepsis, and meningitis (Biering et al, 1989; Gallagher and Ball, 1991; Caubilla-Barron et al, 2007)
The T6SS-1 cluster is ubiquitous among C. sakazakii strains, and its GC content (59.64%) is similar to that of the whole genome (57.02%), which suggests that the gene cluster is part of the inherent genetic material of the species
The antibacterial function of T6SS-1 may be important for the species to gain survival advantages in both environmental and host niches, as it is in several other pathogens such as V. cholerae, P. aeruginosa, and Serratia marcescens (MacIntyre et al, 2010; Russell et al, 2011; Alcoforado Diniz and Coulthurst, 2015)

Summary

Introduction

Cronobacter spp. is an emerging opportunistic food-borne gram-negative pathogen known to cause severe clinical infections in neonates, including necrotizing enterocolitis (NEC), sepsis, and meningitis (Biering et al, 1989; Gallagher and Ball, 1991; Caubilla-Barron et al, 2007). Cronobacter spp. have caused several outbreaks of neonatal meningitis and necrotizing enterocolitis, resulting in a high mortality rate (approximately 33–80%) (Lai, 2001; Healy et al, 2010) and serious sequelae such as brain abscesses and impaired sight and hearing (Kleiman et al, 1981; Muytjens et al, 1983). Type VI secretion system (T6SS) has been found in over 25% of sequenced gram-negative bacterial strains (Bingle et al, 2008).

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