Interplay between MgtC and PagC in Salmonella enterica serovar Typhimurium

Multi-locus sequence typing, antimicrobials resistance and virulence profiles of Salmonella enterica isolated from bovine carcasses in Minas Gerais state, Brazil

The aim of this study was to identify virulence and antimicrobial resistance profiles and determine the sequence type (ST) by multilocus sequence typing (MLST) of Salmonella enterica isolates from bovine carcasses from slaughterhouse located in Minas Gerais state, Brazil, and its relationship with bovine isolates obtained on the American continent based on sequence type profile. The MLST results were compared with all Salmonella STs associated with cattle on American continent, and a multi-locus sequence tree (MS tree) was built. Among the 17 S. enterica isolates, five ST profiles identified, and ST10 were the most frequent, grouping seven (41.2%) isolates. The isolates presented 11 different profiles of virulence genes, and six different antibiotics resistance profiles. The survey on Enterobase platform showed 333 Salmonella STs from American continent, grouped into four different clusters. Most of the isolates in the present study (13/17), were concentrated in a single cluster (L4) composed by 74 STs. As a conclusion, five different STs were identified, with ST10 being the most common. The isolates showed great diversity of virulence genes and antibiotics resistance profiles. Most of the isolates of this study were grouped into a single cluster composed by 74 STs formed by bovine isolates obtained on the American continent.

Salmonella Gallinarum mgtC mutant shows a delayed fowl typhoid progression in chicken

Salmonella Gallinarum (SG) provokes fowl typhoid, an infectious disease of acute clinical course that affects gallinaceous of any age and leads to high mortality rates. During the typhoid-like systemic infection of S. Typhimurium (STM) in mice, the bacterium expresses the mgtC gene, which is encoded in the Salmonella Pathogenecity Island - 3 (SPI-3). In this serovar, the function is linked to bacterial replication within macrophages, and its absence attenuates the pathogen. We hypothesized that deleting mgtC from SG genome would alter the microorganism pathogenicity in susceptible commercial poultry in a similar manner. Thus, the present study sought to elucidate the importance of mgtC on SG pathogenicity. For this, a mgtC-mutant lacking S. Gallinarum mutant was constructed (SG ΔmgtC). Its ability to replicate in medium that mimicries the mgtC-related intracellular environment of macrophages as well as in primary macrophages from chicken was evaluated. Moreover, the infection of susceptible chickens was performed to elucidate its pathogenicity and the elicited immune responses by measuring key interleukins by qRT-PCR and the population of macrophages and lymphocytes T CD4+ and CD8+ by means of immunohistochemistry. It was observed that mgtC was required for S. Gallinarum replication in acidified low-Mg2+ media and survival within macrophages. However, unlike its requirement for initial phase of STM infection in mice, lower bacterial counts were only observed at the late stage of macrophage infection without affecting the citotoxicity. Experiments showed that knocking-out the mgtC gene neither altered bacterial uptake by macrophages nor affects bacterial counts in liver and spleen and total chicken mortality. However, plotting a survival curve and analyzing the clinical-pathologic conditions, it was observed a slower progression of the disease in chickens infected by SG ΔmgtC compared to those challenged by the wild-type strain. Furthermore, the mRNA expression of IFN-γ and LITAF were similar between the infected chickens, but higher than in the uninfected group. The same was observed in macrophages and lymphocytes T CD4+ populations. On the other hand, the presence of lymphocytes T CD8+ was increased in the initial phase of the disease provoked by the wild-type strain over the mutant strain. We concluded that the role of mgtC in Fowl Typhoid in susceptible chickens differs from the role in typhoid-like infections in mammals. Thus, the deletion of mgtC gene from S. Gallinarum genome does not affect the overall pathogenicity, but slightly alters the pathogenesis.

RNA chaperone activates Salmonella virulence program during infection

Organisms often harbor seemingly redundant proteins. In the bacterium Salmonella enterica serovar Typhimurium (S. Typhimurium), the RNA chaperones CspC and CspE appear to play redundant virulence roles because a mutant lacking both chaperones is attenuated, whereas mutants lacking only one exhibit wild-type virulence. We now report that CspC—but not CspE—is necessary to activate the master virulence regulator PhoP when S. Typhimurium experiences mildly acidic pH, such as inside macrophages. This CspC-dependent PhoP activation is specific to mildly acidic pH because a cspC mutant behaves like wild-type S. Typhimurium under other PhoP-activating conditions. Moreover, it is mediated by ugtL, a virulence gene required for PhoP activation inside macrophages. Purified CspC promotes ugtL translation by disrupting a secondary structure in the ugtL mRNA that occludes ugtL’s ribosome binding site. Our findings demonstrate that proteins that are seemingly redundant actually confer distinct and critical functions to the lifestyle of an organism.

How the PhoP/PhoQ System Controls Virulence and Mg2+ Homeostasis: Lessons in Signal Transduction, Pathogenesis, Physiology, and Evolution

The PhoP/PhoQ two-component system governs virulence, Mg2+ homeostasis, and resistance to a variety of antimicrobial agents, including acidic pH and cationic antimicrobial peptides, in several Gram-negative bacterial species. Best understood in Salmonella enterica serovar Typhimurium, the PhoP/PhoQ system consists o-regulated gene products alter PhoP-P amounts, even under constant inducing conditions. PhoP-P controls the abundance of hundreds of proteins both directly, by having transcriptional effects on the corresponding genes, and indirectly, by modifying the abundance, activity, or stability of other transcription factors, regulatory RNAs, protease regulators, and metabolites. The investigation of PhoP/PhoQ has uncovered novel forms of signal transduction and the physiological consequences of regulon evolution.

Increased Production of Outer Membrane Vesicles by Salmonella Interferes with Complement-Mediated Innate Immune Attack

Bacterial outer membrane vesicles (OMVs) enriched with bioactive proteins, toxins, and virulence factors play a critical role in host-pathogen and microbial interactions. The two-component system PhoP-PhoQ (PhoPQ) of Salmonella enterica orchestrates the remodeling of outer membrane lipopolysaccharide (LPS) molecules and concomitantly upregulates OMV production. In this study, we document a novel use of nanoparticle tracking analysis to determine bacterial OMV size and number. Among the PhoPQ-activated genes tested, pagC expression had the most significant effect on the upregulation of OMV production. We provide the first evidence that PhoPQ-mediated upregulation of OMV production contributes to bacterial survival by interfering with complement activation. OMVs protected bacteria in a dose-dependent manner, and bacteria were highly susceptible to complement-mediated killing in their absence. OMVs from bacteria expressing PagC bound to complement component C3b in a dose-dependent manner and inactivated it by recruiting complement inhibitor Factor H. As we also found that Factor H binds to PagC, we propose that PagC interferes with complement-mediated killing of Salmonella in the following two steps: first by engaging Factor H, and second, through the production of PagC-enriched OMVs that divert and inactivate the complement away from the bacteria. Since PhoPQ activation occurs intracellularly, the resultant increase in PagC expression and OMV production is suggested to contribute to the local and systemic spread of Salmonella released from dying host cells that supports the infection of new cells. IMPORTANCE Bacterial outer membrane vesicles (OMVs) mediate critical bacterium-bacterium and host-microbial interactions that influence pathogenesis through multiple mechanisms, including the elicitation of inflammatory responses, delivery of virulence factors, and enhancement of biofilm formation. As such, there is a growing interest in understanding the underlying mechanisms of OMV production. Recent studies have revealed that OMV biogenesis is a finely tuned physiological process that requires structural organization and selective sorting of outer membrane components into the vesicles. In Salmonella, outer membrane remodeling and OMV production are tightly regulated by its PhoPQ system. In this study, we demonstrate that PhoPQ-regulated OMV production plays a significant role in defense against host innate immune attack. PhoPQ-activated PagC expression recruits the complement inhibitor Factor H and degrades the active C3 component of complement. Our results provide valuable insight into the combination of tools and environmental signals that Salmonella employs to evade complement-mediated lysis, thereby suggesting a strong evolutionary adaptation of this facultative intracellular pathogen to protect itself during its extracellular stage in the host.

How Pathogens Feel and Overcome Magnesium Limitation When in Host Tissues

Host organisms utilize nutritional immunity to limit the availability of nutrients essential to an invading pathogen. Nutrients may include amino acids, nucleotide bases, and transition metals, the essentiality of which varies among pathogens. The mammalian macrophage protein Slc11a1 (previously Nramp1) mediates resistance to several intracellular pathogens. Slc11a1 is proposed to restrict growth of Salmonella enterica serovar Typhimurium in host tissues by causing magnesium deprivation. This is intriguing because magnesium is the most abundant divalent cation in all living cells. A pathogen's response to factors such as Slc11a1 that promote nutritional immunity may therefore reflect what the pathogen 'feels' in its cytoplasm, rather than the nutrient concentration in host cell compartments.

Detection of Salmonella Infection in Chickens by an Indirect Enzyme-Linked Immunosorbent Assay Based on Presence of PagC Antibodies in Sera

The outcomes of infection of humans and animals with Salmonella range from a persistent asymptomatic carrier state to temporal mild gastroenteritis or severe systemic infection. A rapid and accurate diagnostic test would help formulate strategies for effective prevention of their infections in the animal population. Current sequencing data predict that the outer membrane protein, PagC, is present in all common Salmonella serovars with sequence similarities of more than 98%. PagC sequences in other bacterial species are less than 65% similarity at the amino acid level to those of Salmonella PagC. We hypothesized that PagC could be immunogenic and detection of antibodies to this protein could be an accurate indicator of Salmonella infection. The pagC gene from Salmonella enterica serovar Typhimurium CVCC542 was expressed in Escherichia coli. The purified recombinant PagC protein was immobilized in microtiter plate wells. Sera from SPF chickens infected with Salmonella or other non-Salmonella pathogens by injection were added and binding of PagC protein was detected by the horseradish peroxidase (HRP)-labeled goat anti-chicken antibody. Sera from Salmonella-infected chickens showed high specificity in contrast to the sera from chickens infected with other bacteria. When 87 Salmonella antibody-positive sera from Salmonella Pullorum orally infected SPF chicken and 93 negative sera from uninfected SPF chicken were tested, 98.3% agreement was detected. The rPagC enzyme-linked immunosorbent assay (ELISA) and agglutination had 80.6% agreement in detecting 252 clinical chicken sera samples. These results suggest that PagC antibody-based indirect ELISA can serve as a convenient and novel method for the diagnosis of Salmonella infection.

Intracellular Salmonella metabolism

Growth of Salmonella inside infected host cells is a key aspect of their ability to cause local enteritis or systemic disease. This growth depends on exploitation of host nutrients through a large Salmonella metabolism network with hundreds of metabolites and enzymes. Studies in cell culture infection models are unravelling more and more of the underlying molecular and cellular mechanisms but also show striking Salmonella metabolic plasticity depending on host cell line and experimental conditions. In vivo studies have revealed a qualitatively diverse, but quantitatively poor, host-Salmonella nutritional interface, which on one side makes Salmonella fitness largely resilient against metabolic perturbations, but on the other side severely limits Salmonella biomass generation and growth rates. This review discusses goals and techniques for studying Salmonella intracellular metabolism, summarises main results and implications, and proposes key issues that could be addressed in future studies.

Mycobacterium marinum MgtC plays a role in phagocytosis but is dispensable for intracellular multiplication

MgtC is a virulence factor involved in intramacrophage growth that has been reported in several intracellular pathogens, including Mycobacterium tuberculosis and Salmonella enterica serovar Typhimurium. MgtC participates also in adaptation to Mg2+ deprivation. Herein, we have constructed a mgtC mutant in Mycobacterium marinum to further investigate the role of MgtC in mycobacteria. We show that the M. marinum mgtC gene (Mma mgtC) is strongly induced upon Mg2+ deprivation and is required for optimal growth in Mg2+-deprived medium. The behaviour of the Mma mgtC mutant has been investigated in the Danio rerio infection model using a transgenic reporter zebrafish line that specifically labels neutrophils. Although the mgtC mutant is not attenuated in the zebrafish embryo model based on survival curves, our results indicate that phagocytosis by neutrophils is enhanced with the mgtC mutant compared to the wild-type strain following subcutaneous injection. Increased phagocytosis of the mutant strain is also observed ex vivo with the murine J774 macrophage cell line. On the other hand, no difference was found between the mgtC mutant and the wild-type strain in bacterial adhesion to macrophages and in the internalization into epithelial cells. Unlike the role reported for MgtC in other intracellular pathogens, Mma MgtC does not contribute significantly to intramacrophage replication. Taken together, these results indicate an unanticipated function of Mma MgtC at early step of infection within phagocytic cells. Hence, our results indicate that although the MgtC function is conserved among pathogens regarding adaptation to Mg2+ deprivation, its role towards phagocytic cells can differ, possibly in relation with the specific pathogen's lifestyles.

Biogenesis of Salmonella enterica serovar typhimurium membrane vesicles provoked by induction of PagC

Gram-negative bacteria ubiquitously release membrane vesicles (MVs) into the extracellular milieu. Although MVs are the product of growing bacteria, not of cell lysis or death, the regulatory mechanisms underlying MV formation remained unknown. We have found that MV biogenesis is provoked by the induction of PagC, a Salmonella-specific protein whose expression is activated by conditions that mimic acidified macrophage phagosomes. PagC is a major constituent of Salmonella MVs, and increased expression accelerates vesiculation. Expression of PagC is regulated at the posttranscriptional and/or posttranslational level in a sigmaS (RpoS)-dependent manner. Serial quantitative analysis has demonstrated that MV formation can accelerate when the quantity of the MV constituents, OmpX and PagC, rises. Overproduction of PagC dramatically impacts the difference in the relative amount of vesiculation, but the corresponding overproduction of OmpX was less pronounced. Quantitative examination of the ratios of PagC and OmpX in the periplasm, outer membrane, and MVs demonstrates that PagC is preferentially enriched in MVs released from Salmonella cells. This suggests that specific protein sorting mechanisms operate when MVs are formed. The possible role(s) of PagC-MV in host cells is discussed.

System-level analysis of Salmonella metabolism during infection

Infectious diseases represent a major threat to human health. To develop urgently needed new control strategies, a transition from research focusing on individual factors to a more integrated system-level analysis might be needed. Such an approach faces great challenges and might require development of new concepts in large-scale data analysis. Here, I discuss for the well-characterized model pathogen Salmonella, how extensively studied metabolism can be used as a training field for infection biology at the systems level. Extensive experimental data can be analyzed in context using metabolic network visualization tools and in silico modeling based on genome-scale metabolic reconstructions. Suitable approaches to obtain still missing comprehensive quantitative data on Salmonella nutrition in infected host tissues are described. Such an integrated investigation of Salmonella metabolism during infection will enable an unprecedented large-scale understanding of pathogen in vivo activities, help to evaluate concepts and strategies for system-level analysis of host/pathogen interactions in general, and provide a basis for rational development of novel antimicrobials and efficacious live vaccines.

Proteome of Salmonella Enterica Serotype Typhimurium Grown in a Low Mg/pH Medium

To determine the impact of a low Mg(2+)/pH defined growth medium (MgM) on the proteome of Salmonella enterica serotype Typhimurium, we cultured S. Typhimurium cells in the medium under two different conditions termed MgM Shock and MgM Dilution and then comparatively analyzed the bacterial cells harvested from these conditions by a global proteomic approach. Proteomic results showed that MgM Shock and MgM Dilution differentially affected the S. Typhimurium proteome. MgM Shock induced a group of proteins whose induction usually occurred at low O(2) level, while MgM Dilution induced those related to the type III secretion system (T3SS) of Salmonella Pathogenicity Island 2 (SPI2) and those involved in thiamine or biotin biosynthesis. The metabolic state of the S. Typhimurium cells grown under MgM Shock condition also differed significantly from that under MgM Dilution condition. Western blot analysis not only confirmed the proteomic results, but also showed that the abundances of SPI2-T3SS proteins SsaQ and SseE and biotin biosynthesis proteins BioB and BioD increased after S. Typhimurium infection of RAW 264.7 macrophages. Deletion of the gene encoding BioB reduced the bacterial ability to replicate inside the macrophages, suggesting a biotin-limited environment encountered by S. Typhimurium within RAW 264.7 macrophages.