Codependent and Independent Effects of Nitric Oxide-Mediated Suppression of PhoPQ andSalmonellaPathogenicity Island 2 on IntracellularSalmonella entericaSerovar Typhimurium Survival

Abstract

Here we show that the Salmonella enterica serovar Typhimurium PhoQ sensor kinase lessens the cytotoxicity of reactive nitrogen species (RNS) generated by inducible nitric oxide synthase (iNOS) in the innate response of mononuclear phagocytic cells. This observation is consistent with the expression patterns of PhoP-activated genes during moderate nitrosative stress in the innate host response. In contrast, RNS synthesized during high-NO fluxes of gamma interferon (IFN-␥)-activated macrophages repress PhoP-activated lpxO, pagP, and phoP gene transcription. Because PhoP-regulated Salmonella pathogenicity island 2 (SPI2) genes are also repressed by high-order RNS (39), we investigated whether the NO-mediated inhibition of PhoPQ underlies the repression of SPI2. Our studies indicate that a third of the expression of the SPI2 spiC gene recorded in nonactivated macrophages depends on PhoQ. Transcription of spiC is repressed in IFN-␥-primed macrophages in an iNOS-dependent manner, irrespective of the phoQ status of the bacteria. Transcription of spiC is restored in IFN-␥-treated, iNOS-deficient macrophages to levels sustained by a phoQ mutant in nonactivated phagocytes, suggesting that most NO-dependent repression of spiC is due to the inhibition of PhoPQ-independent targets. Comparison of the intracellular fitness of spiC, phoQ, and spiC phoQ mutants revealed that PhoPQ and SPI2 have codependent and independent effects on S. Typhimurium survival during innate nitrosative stress. However, the intracellular survival of most S. Typhimurium bacteria is conferred by the PhoPQ two-component regulator, and the SPI2 type III secretion system is repressed by high-order RNS of IFN-␥-activated macrophages.

Low‐molecular‐weight thiol‐dependent antioxidant and antinitrosative defences in Salmonella pathogenesis

“…Because the SPI2 type III secretion system minimizes the contact of Salmonella phagosomes with vesicles harbouring NADPH oxidase or iNOS enzymatic complexes (Vazquez-Torres et al, 2000b;Gallois et al, 2001;Chakravortty et al, 2002;Suvarnapunya and Stein, 2005;Berger et al, 2010), the capacity of LMW thiols to preserve SPI2 transcription in Salmonella enduring moderate levels of nitrosative stress could indirectly boost the antioxidant and antinitrosative arsenal of this intracellular pathogen. Our previous work showed that nitrosative stress represses SsrB-and PhoP-dependent SPI2 transcription in Salmonella (McCollister et al, 2005;Bourret et al, 2009). Accordingly, LMW thiols could protect SsrB and PhoP signalling cascades from the inhibitory effects of RNS.…”

The late endosomal adaptor p14 is a macrophage host defense factor against Salmonella Typhimurium infection

“…Similarly, iNOS-knockout mice were more susceptible to Salmonella infection, and macrophages from these animals displayed a decreased ability to kill intracellular Salmonella (Mastroeni et al, 2000;Vazquez-Torres et al, 2000a). Interestingly, it has been shown that RNS synthesized during high NO fluxes can repress the phoP-phoQ system responsible for survival of intracellular Salmonella (Bourret et al, 2009). Thus, the low expression levels of iNOS and subsequently of NO in p14 2/2 macrophages might favor the survival and replication of Salmonella.…”

The Acid Tolerance Response of Salmonella spp.: An adaptive strategy to survive in stressful environments prevailing in foods and the host

“…Finally, most of these ATR-linked genes have been shown to regulate different virulence factors needed for colonization or survival within host cells (Rychlik & Barrow, 2005). Thus, the regulators Fur, PhoPQ and OmpR have been recently involved in the induction of genes from the Salmonella pathogenicity islands I and II, essential for the invasion of epithelial cells (Bourret et al, 2009;Ellermeier & Slauch, 2008;Fass & Groisman, 2009;Kim & Falkow, 2004;Lee et al, 2000). In fact, Perez et al (2010), studying the influence of acid adaptation on the survival of S. Typhimurium during intestinal infection of Rattus norvegicus, found that non-adapted microorganisms were observed in higher counts in feces than acid-adapted cells, while acid adapted microorganisms demonstrated higher counts in intestinal samples, suggesting a greater intestinal invasion capacity.…”