b Salmonella enterica serovar Typhimurium (S. Typhimurium) is one of the leading causative agents of food-borne bacterial gastroenteritis. Swift invasion through the intestinal tract and successful establishment in systemic organs are associated with the adaptability of S. Typhimurium to different stress environments. Low-pH stress serves as one of the first lines of defense in mammalian hosts, which S. Typhimurium must efficiently overcome to establish an infection. Therefore, a better understanding of the molecular mechanisms underlying the adaptability of S. Typhimurium to acid stress is highly relevant. In this study, we have performed a transcriptome analysis of S. Typhimurium under the acid tolerance response (ATR) and found a large number of genes (ϳ47%) to be differentially expressed (more than 1.5-fold or less than ؊1.5-fold; P < 0.01). Functional annotation revealed differentially expressed genes to be associated with regulation, metabolism, transport and binding, pathogenesis, and motility. Additionally, our knockout analysis of a subset of differentially regulated genes facilitated the identification of proteins that contribute to S. Typhimurium ATR and virulence. Mutants lacking genes encoding the K ؉ binding and transport protein KdpA, hypothetical protein YciG, the flagellar hook cap protein FlgD, and the nitrate reductase subunit NarZ were significantly deficient in their ATRs and displayed varied in vitro virulence characteristics. This study offers greater insight into the transcriptome changes of S. Typhimurium under the ATR and provides a framework for further research on the subject. Salmonella enterica serovar Typhimurium is a neutralophilic, Gram-negative food-and waterborne pathogen that causes diseases ranging from gastroenteritis to systemic infection in humans. The intestinal tract of wild and domestic animals serves as a vehicle by which salmonellae find their way into humans through contaminated food and water. It has been estimated that globally this species accounts for about 80.3 million cases of food-borne gastroenteritis with about 1.5 million deaths (1). A large number of outbreaks have been linked to contaminated fruits and vegetables, including apples, mangoes, lettuce, tomatoes, celery, and unpasteurized juice (2). During host-pathogen interaction, Salmonella constantly encounters various stress conditions, such as changing pH, high osmotic pressure, low oxygen availability, and the presence of bile salts and antimicrobial peptides, that constantly test the adaptability of this pathogen. One such stress condition is low pH, and Salmonella confronts this on transit through the stomach, as well as during survival within the Salmonellacontaining vacuole (SCV) of phagocytic and nonphagocytic cells. Hence, the ability of Salmonella to perceive low-pH environments and respond to such stress is crucial for its survival and pathogenicity.The mechanism by which S. Typhimurium senses acidic environments and adapts to survive under low pH is termed the acid tolerance response (ATR) (3-...

The Gram-negative, enteropathogen Salmonella enterica serovar Typhimurium (S. Typhimurium) is exposed to various stress conditions during pathogenesis, of which acid stress serves as a major defense mechanism in the host. Such environments are encountered in the stomach and Salmonella containing vacuole of phagocytic and non-phagocytic cells. It is only recently that small RNAs (sRNAs) have come to the forefront as major regulators of stress response networks. Consequently, the sRNA DsrA which regulates acid resistance in Escherichia coli, has not been characterized in the acid tolerance response (ATR) of Salmonella. In this study, we show dsrA to be induced two and threefold under adaptation and challenge phases of the ATR, respectively. Additionally, an isogenic mutant lacking dsrA (ΔDsrA) displayed lower viability under the ATR along with reduced motility, feeble adhesion and defective invasion efficacy in vitro. Expression analysis revealed down regulation of several Salmonella pathogenicity island-1 (SPI-1) effectors in ΔDsrA compared to the wild-type, under SPI-1 inducing conditions. Additionally, our in vivo data revealed ΔDsrA to be unable to cause gut inflammation in C57BL/6 mice at 72 h post infection, although intracellular survival and systemic dissemination remained unaffected. A possible explanation may be the significantly reduced expression of flagellin structural genes fliC and fljB in ΔDsrA, which have been implicated as major proinflammatory determinants. This study serves to highlight the role of sRNAs such as DsrA in both acid tolerance and virulence of S. Typhimurium. Additionally the robust phenotype of non-invasiveness could be exploited in developing SPI-I attenuated S. Typhimurium strains without disrupting SPI-I genes.

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