Salmonella Typhimurium PagP- and UgtL-dependent resistance to antimicrobial peptides contributes to the gut colonization

Mucosal barrier formed by cationic antimicrobial peptides (CAMPs) is believed to be crucial for host protection from pathogenic gut infection. However, some pathogens can develop resistance to the CAMPs to survive in hosts. Salmonella enterica is a common cause of acute diarrhea. During the course of this disease, the pathogen must continuously colonize the gut lumen, which contains CAMPs. However, it is incompletely understood whether the resistance of Salmonella strains to CAMPs contributes to the development of gut infections. PhoPQ two-component system-dependent lipid A modifications confer resistance to CAMPs in S. enterica serovar Typhimurium. Therefore, we introduced mutations into the PhoPQ-regulated genes in an S. Typhimurium strain, obtaining pagP ugtL and pmrA mutant strains. Each mutant strain demonstrated a distinct spectrum of the resistance to CAMPs. Using streptomycin mouse model for Salmonella diarrhea, we show that the pagP ugtL, but not pmrA, mutant strain had a gut colonization defect. Furthermore, the pagP ugtL, but not pmrA, mutant strain had decreased outer membrane integrity and susceptibility to magainin 2, an alpha-helical CAMP. Taken together, the PagP- and UgtL-dependent resistance to CAMPs was demonstrated to contribute to sustained colonization in the gut. This may be due to the robust outer membrane of S. Typhimurium, inducing the resistance to alpha-helical CAMPs such as α-defensins. Our findings indicate that the development of resistance to CAMPs is required for the S. Typhimurium gut infection.

由阳离子抗菌肽（cationic antimicrobial peptides, CAMPs）构成的黏膜屏障，被认为在宿主抵御肠道致病性感染中发挥关键保护作用。然而，部分病原体可进化出对CAMPs的耐药性，从而得以在宿主体内存活。肠炎沙门氏菌（Salmonella enterica）是引发急性腹泻的常见致病菌。在该疾病的病程中，该病原体必须持续定殖于含有CAMPs的肠腔环境中，但目前学界尚未完全明确，沙门氏菌菌株对CAMPs的耐药性是否会促进肠道感染的发生发展。鼠伤寒沙门氏菌（Salmonella enterica serovar Typhimurium）的PhoPQ双组分系统依赖型脂质A修饰，可使其获得对CAMPs的耐药性。因此，我们在一株鼠伤寒沙门氏菌菌株的PhoPQ调控基因中引入突变，构建得到pagP、ugtL及pmrA突变菌株。各突变菌株对CAMPs的耐药谱存在显著差异。利用沙门氏菌腹泻的链霉素处理小鼠模型，我们的实验结果显示，pagP与ugtL双突变菌株（而非pmrA单突变菌株）存在肠道定殖缺陷。进一步研究发现，pagP与ugtL双突变菌株（而非pmrA单突变菌株）的外膜完整性降低，且对α螺旋型CAMP蛙皮素2（magainin 2）的敏感性显著升高。综上，PagP与UgtL依赖的CAMPs耐药性被证实有助于鼠伤寒沙门氏菌在肠道内的持续定殖。这一效应可能源于鼠伤寒沙门氏菌坚固的外膜，该结构可诱导其对α防御素（α-defensins）等α螺旋型CAMPs产生耐药性。我们的研究结果表明，对CAMPs的耐药性是鼠伤寒沙门氏菌引发肠道感染所必需的。