The Type VI Secretion System Encoded in Salmonella Pathogenicity Island 19 Is Required for Salmonella enterica Serotype Gallinarum Survival within Infected Macrophages

Abstract

Salmonella enterica serotype Gallinarum is the causative agent of fowl typhoid, a disease characterized by high morbidity and mortality that causes major economic losses in poultry production. We have reported that S. Gallinarum harbors a type VI secretion system (T6SS) encoded in Salmonella pathogenicity island 19 (SPI-19) that is required for efficient colonization of chicks. In the present study, we aimed to characterize the SPI-19 T6SS functionality and to investigate the mechanisms behind the phenotypes previously observed in vivo. Expression analyses revealed that SPI-19 T6SS core components are expressed and produced under in vitro bacterial growth conditions. However, secretion of the structural/secreted components Hcp1, Hcp2, and VgrG to the culture medium could not be determined, suggesting that additional signals are required for T6SS-dependent secretion of these proteins. In vitro bacterial competition assays failed to demonstrate a role for SPI-19 T6SS in interbacterial killing. In contrast, cell culture experiments with murine and avian macrophages (RAW264.7 and HD11, respectively) revealed production of a green fluorescent protein-tagged version of VgrG soon after Salmonella uptake. Furthermore, infection of RAW264.7 and HD11 macrophages with deletion mutants of SPI-19 or strains with genes encoding specific T6SS core components (clpV and vgrG) revealed that SPI-19 T6SS contributes to S. Gallinarum survival within macrophages at 20 h postuptake. SPI-19 T6SS function was not linked to Salmonella-induced cytotoxicity or cell death of infected macrophages, as has been described for other T6SS. Our data indicate that SPI-19 T6SS corresponds to a novel tool used by Salmonella to survive within host cells.

Polyamines are essential for virulence in Salmonella enterica serovar Gallinarum despite evolutionary decay of polyamine biosynthesis genes

“…The level of replication for the wt observed in this assay is similar to the level others have reported (Blondel et al, 2013). In contrast to observation in S. Typhimurium (Jelsbak et al, 2012), these results indicate that intracellular replication of S. Gallinarum is not affected by polyamines (Fig.…”

Patterns of genome evolution that have accompanied host adaptation in Salmonella

“…1), and for S. Gallinarum and S. Pullorum, the retention of a functional SPI-19 is consistent with the T6SS it encodes being important for S. Gallinarum survival in chicken macrophages (16). However, the degradation of SPI-6 and loss of the T6SS, seen within the second clade of S. Enteritidis and the S. Gallinarum/S.…”

Microarray-Based Detection of Salmonella enterica Serovar Enteritidis Genes Involved in Chicken Reproductive Tract Colonization

“…SPI-19 is necessary for survival of Salmonella Gallinarum inside avian macrophages, without contributing to cytotoxicity and cell death (26). Since SPI-19 of Salmonella Enteritidis lost many of the T6SS essential components, it was suggested that this system is not functional within this serotype (27).…”