H-NS Silencing of the Salmonella Pathogenicity Island 6-Encoded Type VI Secretion System Limits Salmonella enterica Serovar Typhimurium Interbacterial Killing

Abstract

The secretion of bacterial toxin proteins is achieved by dedicated machineries called secretion systems. The type VI secretion system (T6SS) is a widespread versatile machine used for the delivery of protein toxins to both prokaryotic and eukaryotic cells. In Salmonella enterica serovar Typhimurium, the expression of the T6SS genes is activated during macrophage or mouse infection. Here, we show that the T6SS gene cluster is silenced by the histone-like nucleoid structuring H-NS protein using a combination of reporter fusions, electrophoretic mobility shift assays, DNase footprinting, and fluorescence microscopy. We further demonstrate that derepression of the S. Typhimurium T6SS genes induces T6SS-dependent intoxication of competing bacteria. Our results suggest that relieving T6SS H-NS silencing may be used as a sense-and-kill mechanism that will help S. Typhimurium to homogenize and synchronize the microbial population to gain efficiency during infection. During the course of infection, bacteria produce and secrete bacterial toxins. Secretion of these toxin proteins is achieved by dedicated, specialized machineries called secretion systems. The type VI secretion system (T6SS) is required for the virulence of several Gram-negative pathogens. In Vibrio cholerae, the T6SS translocates VgrG1, a toxin that carries a domain responsible for actin cross-linking in eukaryotic cells (1-4). However, the role of the T6SS is not limited to virulence toward eukaryotes; an increasing number of reports demonstrate that the T6SS is also involved in interbacterial intoxication (5-10). With bacteriocins and contact-dependent inhibition (CDI), the T6SS is therefore involved in shaping bacterial communities (11-14) by delivering antibacterial toxins, including murein hydrolases, DNases, and phospholipases, directly into the target recipient bacterial cell (5, 6, 15; for recent reviews see references 14, 16 and 17). The attacker cell is protected from these toxins by the coproduction of cognate proteins that confer immunity (15, 18). The T6SS therefore confers a growth advantage to bacteria in mixed cultures and has been suggested to be important in environmental niches where bacterial competition for nutrients is critical for survival or for the uptake of DNA for transformation and acquisition of new traits (14, 19). However, while the role of the T6SS in interbacterial competition has been evidenced and characterized under laboratory conditions, its role in shaping bacterial communities of the microbiota is not yet clearly elucidated.At the molecular level, the T6SS is assembled from 13 proteins, called core components, that form a transenvelope apparatus anchoring a cytoplasmic tubular structure to the membrane (20-24). Based on structural homologies with bacteriophage tail components, this tubular structure has been proposed to be constituted of an inner tube assembled by stacked hexameric rings of the Hcp protein, resembling the tail tube of bacteriophages, tipped by the VgrG protein (21,(25)(26)(27)). The current model descr...

Type VI secretion system contributes to Enterohemorrhagic Escherichia coli virulence by secreting catalase against host reactive oxygen species (ROS)

“…cholerae [27], Edwardsiella tarda [28] and S . Typhimurium [29], we speculate that H-NS may be a repressor of the T6SS in EHEC. We thus constructed the deletion mutant of hns in EHEC by replacing hns gene with chloramphenicol resistance cassette using λ Red recombination system [30].…”

Individual Physiological Adaptations Enable Selected Bacterial Taxa To Prevail during Long-Term Incubations

“…Ribosomally synthesized bacteriocins and nonribosomally synthesized peptides are potent bioactive compounds. Gramnegative bacteriocins can have antibacterial pore-forming activity as well as nuclease and muramidase activities (46). Nonribosomal peptides reveal versatile antimicrobial activities (47)(48)(49).…”

Salmonella Typhimurium utilizes a T6SS-mediated antibacterial weapon to establish in the host gut

“…We were surprised that, in contrast to the E. coli K-12 DH5α or W3110 laboratory strains (this study and ref. 36), the commensal E. coli JB2 strain was resistant to SPI-6 attack, which suggests that a defense mechanism may be present in the commensal strain, such as the presence of a T6SS gene cluster or cross-immunity via a Tai4 homolog. Alternatively, it was shown that resident commensal E. coli and S. Typhimurium may transiently cooperate Feces were collected and plated over time on Salmonella selective medium.…”

“…Tae4 induces bacterial lysis by cleaving the γ-D-glutamyl-L-mesodiaminopimelic acid amide bond of peptidoglycan and is toxic when expressed in a laboratory strain of Escherichia coli (34,35). Furthering this idea, it has recently been reported that the SPI-6 T6SS is required for S. Typhimurium to outcompete a laboratory strain of E. coli in vitro (36). However, the contribution of the T6SS antiprokaryotic activity of S. Typhimurium during infection of the gut remains unknown.…”