Date Published: October 31, 2018
Publisher: Public Library of Science
Author(s): Mayuka Fujimoto, Ryosuke Goto, Riku Hirota, Masahiro Ito, Takeshi Haneda, Nobuhiko Okada, Tsuyoshi Miki, David Weiss.
Salmonella enterica serovar Typhimurium (S. Tm) is a cause of food poisoning accompanied with gut inflammation. Although mucosal inflammation is generally thought to be protective against bacterial infection, S. Tm exploits the inflammation to compete with commensal microbiota, thereby growing up to high densities in the gut lumen and colonizing the gut continuously at high levels. However, the molecular mechanisms underlying the beneficial effect of gut inflammation on S. Tm competitive growth are poorly understood. Notably, the twin-arginine translocation (Tat) system, which enables the transport of folded proteins outside bacterial cytoplasm, is well conserved among many bacterial pathogens, with Tat substrates including virulence factors and virulence-associated proteins. Here, we show that Tat and Tat-exported peptidoglycan amidase, AmiA- and AmiC-dependent cell division contributes to S. Tm competitive fitness advantage in the inflamed gut. S. Tm tatC or amiA amiC mutants feature a gut colonization defect, wherein they display a chain form of cells. The chains are attributable to a cell division defect of these mutants and occur in inflamed but not in normal gut. We demonstrate that attenuated resistance to bile acids confers the colonization defect on the S. Tm amiA amiC mutant. In particular, S. Tm cell chains are highly sensitive to bile acids as compared to single or paired cells. Furthermore, we show that growth media containing high concentrations of NaCl and sublethal concentrations of antimicrobial peptides induce the S. Tm amiA amiC mutant chain form, suggesting that gut luminal conditions such as high osmolarity and the presence of antimicrobial peptides impose AmiA- and AmiC-dependent cell division on S. Tm. Together, our data indicate that Tat and the Tat-exported amidases, AmiA and AmiC, are required for S. Tm luminal fitness in the inflamed gut, suggesting that these proteins might comprise effective targets for novel antibacterial agents against infectious diarrhea.
Protein translocation constitutes an essential cell function in all types of prokaryotic or eukaryotic cells. Accordingly, bacteria have evolved several sophisticated translocation systems to transport proteins into, or across, the cytoplasmic membrane. In particular, for bacterial pathogens, this process is suspected to contribute to pathogenesis and interbacterial competition because the substrates include virulence factors in certain cases. Most general protein transport occurs via the Sec system, which predominantly transports unfolded proteins across the cytoplasmic membrane . In addition, other additional transport systems have also evolved to facilitate the transport of different types of protein.
The Tat system is widely conserved in many bacterial pathogens and plays crucial roles in virulence . Therefore, it is expected that the Tat system may represent a therapeutic target for bacterial pathogen infection. Earlier studies revealed that the S. Tm tatC mutant exhibits attenuated virulence , consequent to an export defect of certain Tat substrates including AmiA and AmiC . However, the lack of intestinal inflammation in the previously utilized Salmonella typhoid fever mouse model precluded determination of whether the Tat system is involved in Salmonella-induced enterocolitis. Thus, to clarify the role of the Tat system in Salmonella enterocolitis, in this study we utilized the streptomycin mouse model [28, 41, 60] in which infected S. Tm induces severe intestinal inflammation and colonizes the gut [28, 60]. Our results showed that the S. Tm tatC mutant can elicit intestinal inflammation, albeit at a lower level as compared to that of the wild-type strain SL1344. This appears to be correlated with the colonization levels in the cecum, which exhibited attenuated colonization of the S. Tm tatC mutant. As deletion of the tatC gene causes reduced expression of the hilA gene, which encodes a central regulator for ttss-1 , the attenuated inflammation might be due to reduced expression of ttss-1. Moreover, we also demonstrate here that gut colonization of the S. Tm tatC mutant is attenuated in a mixed infection experiment. Based on our data, we concluded that the Tat system in S. Tm is not essential for colitis but is involved in the induction of gut inflammation and colonization in the early infectious course.