An overview of Salmonella enterica metal homeostasis pathways during infection

A Salmonella subset exploits erythrophagocytosis to subvert SLC11A1-imposed iron deprivation

Solute carrier family 11 member 1 (SLC11A1) is critical for host resistance to diverse intracellular pathogens. During infection, SLC11A1 limits Salmonella's access to iron, zinc, and magnesium, but only magnesium deprivation significantly impairs Salmonella replication. To understand the unexpected minor impact of iron, we determined Salmonella's iron access in infected SLC11A1-deficient and normal mice. Using reporter strains and mass spectrometry of Salmonella purified from the spleen, we found that SLC11A1 caused growth-restricting iron deprivation in a subset of Salmonella. Volume electron microscopy revealed that another Salmonella subset circumvented iron restriction by targeting iron-rich endosomes in macrophages degrading red blood cells (erythrophagocytosis). These iron-replete bacteria dominated overall Salmonella growth, masking the effects of the other Salmonella subset's iron deprivation. Thus, SLC11A1 effectively sequesters iron, but heterogeneous Salmonella populations partially bypass this nutritional immunity by targeting iron-rich tissue microenvironments.

The Phase-Specific Dynamics in Gene Expression of Salmonella Typhimurium During Acanthamoeba castellanii Infection

Being facultative intracellular pathogens, the bacterium often is found in the environment. In natural habitats, Salmonella are able to survive and multiply inside free-living protists that support preservation and distribution of the pathogen, its virulence, and resistance to antimicrobial agents. At the same time, the expression profile of Salmonella genes in the eukaryotic cells has been shown not to be stable, but changes dramatically according to the sequential stages of infection. Previously, we had described the gene expression profile of S. enterica serovar Typhimurium 14028S at the early stage of interaction with Acanthamoeba castellanii. In this study, we have revealed the phase-specific dynamics in expression of several clusters and functional groups of S. Typhimurium 14028S genes. The early stage of invasion characterized by a maximum response to oxidative stress, and it was accompanied by activation of SPI-1 genes, which can contribute to the successful internalization into the host cell. At the second stage (8 h) increase in expression of SPI-2 and SPI-3 genes was accompanied with a maximum expression of iron uptake genes and lysozyme inhibitors. At the late stage of the infection (15 h), downregulation of carbon metabolism and oxidative stress response genes, as well as a decrease in the expression of all other genes, was revealed that may be an evidence of adaptation of Salmonella to intracellular conditions. The obtained results might be useful for further search of factors reducing persistence of pathogens like Salmonella in the environment.

Bacterial siderophores: diversity, uptake pathways and applications

Iron is an essential nutrient for the growth, survival and virulence of almost all bacteria. To access iron, many bacteria produce siderophores, molecules with a high affinity for iron. Research has highlighted substantial diversity in the chemical structure of siderophores produced by bacteria, as well as remarkable variety in the molecular mechanisms involved in strategies for acquiring iron through these molecules. The metal-chelating properties of siderophores, characterized by their high affinity for iron and ability to chelate numerous other metals (albeit with lower affinity compared with iron), have also generated interest in diverse fields. Siderophores find applications in the environment, such as in bioremediation and agriculture, in which emerging and innovative strategies are being developed to address pollution and enhance nutrient availability for plants. Moreover, in medicine, siderophores could be used as a tool for novel antimicrobial therapies and medical imaging, as well as in haemochromatosis, thalassemia or cancer treatments. This Review offers insights into the diversity of siderophores, highlighting their potential applications in environmental and medical contexts.

Exploring resource competition by protective lactic acid bacteria cultures to control Salmonella in food: an Achilles' heel to target?

Salmonella is a pathogenic bacterium, being the second most commonly reported foodborne pathogen in Europe, due to the ability of its different serovars to contaminate a wide variety of foods, with differences among countries. Common chemical or physical control methods are not always effective, eco-sustainable and adapted to the diversity of Salmonella serovars. Thus, great attention is given to developing complementary or alternative control methods that can be tailor made for specific situations. One of these methods is biopreservation using lactic acid bacteria, with most studies on their antagonistic activity focused on the production of antimicrobials. Less attention has been given to competition by exploitation of nutrients. This review is thus set to investigate and highlight limiting resources that may be involved in the competitive exclusion of Salmonella in food matrices. To do this the needs for nutrients and microelements and the known homeostatic pathways of Salmonella and lactic acid bacteria are examined. Finally, milk, intended for the manufacture of fermented dairy foods, is pointed out as an example of food to investigate the bioavailable macronutrients, metals and vitamins that could be involved in competition between the different species and serovars, and could be exploited for targeted biopreservation.

Regulation of iron metabolism is critical for the survival of Salmonella Typhimurium in pasteurized milk

Salmonella is known to survive in raw/pasteurized milk and cause foodborne outbreaks. Lactoferrin, present in milk from all animal sources, is an iron-binding glycoprotein that limits the availability of iron to pathogenic bacteria. Despite the presence of lactoferrins, Salmonella can grow in milk obtained from different animal sources. However, the mechanism by which Salmonella overcomes iron scarcity induced by lactoferrin in milk is not evaluated yet. Salmonella employs the DNA binding transcriptional regulator Fur (ferric update regulator) to mediate iron uptake during survival in iron deplete conditions. To understand the importance of Fur in Salmonella milk growth, we profiled the growth of Salmonella Typhimurium Δfur (ST4/74Δfur) in both bovine and camel milk. ST4/74Δfur was highly inhibited in milk compared to wild-type ST4/74, confirming the importance of Fur mediated regulation of iron metabolism in Salmonella milk growth. We further studied the biology of ST4/74Δfur to understand the importance of iron metabolism in Salmonella milk survival. Using increasing concentrations of FeCl3, and the antibiotic streptonigrin we show that iron accumulates in the cytoplasm of ST4/74Δfur. We hypothesized that the accumulated iron could activate oxidative stress via Fenton's reaction leading to growth inhibition. However, the inhibition of ST4/74Δfur in milk was not due to Fenton's reaction, but due to the 'iron scarce' conditions of milk and microaerophilic incubation conditions which made the presence of the fur gene indispensable for Salmonella milk growth. Subsequently, survival studies of 14 other transcriptional mutants of ST4/74 in milk confirmed that RpoE-mediated response to extracytoplasmic stress is also important for the survival of Salmonella in milk. Though we have data only for fur and rpoE, many other Salmonella transcriptional factors could play important roles in the growth of Salmonella in milk, a theme for future research on Salmonella milk biology. Nevertheless, our data provide early insights into the biology of milk-associated Salmonella.

Natural flavonoids disrupt bacterial iron homeostasis to potentiate colistin efficacy

In the face of the alarming rise in global antimicrobial resistance, only a handful of novel antibiotics have been developed in recent decades, necessitating innovations in therapeutic strategies to fill the void of antibiotic discovery. Here, we established a screening platform mimicking the host milieu to select antibiotic adjuvants and found three catechol-type flavonoids-7,8-dihydroxyflavone, myricetin, and luteolin-prominently potentiating the efficacy of colistin. Further mechanistic analysis demonstrated that these flavonoids are able to disrupt bacterial iron homeostasis through converting ferric iron to ferrous form. The excessive intracellular ferrous iron modulated the membrane charge of bacteria via interfering the two-component system pmrA/pmrB, thereby promoting the colistin binding and subsequent membrane damage. The potentiation of these flavonoids was further confirmed in an in vivo infection model. Collectively, the current study provided three flavonoids as colistin adjuvant to replenish our arsenals for combating bacterial infections and shed the light on the bacterial iron signaling as a promising target for antibacterial therapies.

Manganese Transporter Proteins in Salmonella enterica serovar Typhimurium

The metal cofactors are essential for the function of many enzymes. The host restricts the metal acquisition of pathogens for their immunity and the pathogens have evolved many ways to obtain metal ions for their survival and growth. Salmonella enterica serovar Typhimurium also needs several metal cofactors for its survival, and manganese has been found to contribute to Salmonella pathogenesis. Manganese helps Salmonella withstand oxidative and nitrosative stresses. In addition, manganese affects glycolysis and the reductive TCA, which leads to the inhibition of energetic and biosynthetic metabolism. Therefore, manganese homeostasis is crucial for full virulence of Salmonella. Here, we summarize the current information about three importers and two exporters of manganese that have been identified in Salmonella. MntH, SitABCD, and ZupT have been shown to participate in manganese uptake. mntH and sitABCD are upregulated by low manganese concentration, oxidative stress, and host NRAMP1 level. mntH also contains a Mn²⁺-dependent riboswitch in its 5' UTR. Regulation of zupT expression requires further investigation. MntP and YiiP have been identified as manganese efflux proteins. mntP is transcriptionally activated by MntR at high manganese levels and repressed its activity by MntS at low manganese levels. Regulation of yiiP requires further analysis, but it has been shown that yiiP expression is not dependent on MntS. Besides these five transporters, there might be additional transporters that need to be identified.

Natural variation in host feeding behaviors impacts host disease and pathogen transmission potential

Animals ranging from mosquitoes to humans often vary their feeding behavior when infected or merely exposed to pathogens. These so-called "sickness behaviors" are part of the innate immune response with many consequences, including avoiding orally transmitted pathogens. Fully understanding the role of this ubiquitous behavior in host defense and pathogen evolution requires a quantitative account of its impact on host and pathogen fitness across environmentally relevant contexts. Here, we use a zooplankton host and fungal pathogen as a case study to ask if infection-mediated feeding behaviors vary across pathogen exposure levels and natural genetic variation in susceptibility to infection. Then, we connect these changes in behavior to pathogen transmission potential (spore yield) and fitness and growth costs to the host. Our results validate a protective effect of altered feeding behavior during pathogen exposure while also revealing significant variation in the magnitude of this response across host susceptibility and pathogen exposure levels. Across all four host genotypes, feeding rates were negatively correlated with susceptibility to infection and transmission potential. The most susceptible genotypes exhibited either strong anorexia, reducing food intake by 26%-42%, ("Standard") or pronounced hyperphagia, increasing food intake by 20%-54% ("A45"). Together, these results suggest that infection-mediated changes in host feeding behavior-which are traditionally interpreted as immunopathology- may in fact serve as crucial components of host defense strategies and warrant further investigation.

Cation Homeostasis: Coordinate Regulation of Polyamine and Magnesium Levels in Salmonella

Polyamines are organic cations that are important in all domains of life. Here, we show that in Salmonella, polyamine levels and Mg2+ levels are coordinately regulated and that this regulation is critical for viability under both low and high concentrations of polyamines. Upon Mg2+ starvation, polyamine synthesis is induced, as is the production of the high-affinity Mg2+ transporters MgtA and MgtB. Either polyamine synthesis or Mg2+ transport is required to maintain viability. Mutants lacking the polyamine exporter PaeA, the expression of which is induced by PhoPQ in response to low Mg2+, lose viability in the stationary phase. This lethality is suppressed by blocking either polyamine synthesis or Mg2+ transport, suggesting that once Mg2+ levels are reestablished, the excess polyamines must be excreted. Thus, it is the relative levels of both Mg2+ and polyamines that are regulated to maintain viability. Indeed, sensitivity to high concentrations of polyamines is proportional to the Mg2+ levels in the medium. These results are recapitulated during infection. Polyamine synthesis mutants are attenuated in a mouse model of systemic infection, as are strains lacking the MgtB Mg2+ transporter. The loss of MgtB in the synthesis mutant background confers a synthetic phenotype, confirming that Mg2+ and polyamines are required for the same process(es). Mutants lacking PaeA are also attenuated, but deleting paeA has no phenotype in a polyamine synthesis mutant background. These data support the idea that the cell coordinately controls both the polyamine and Mg2+ concentrations to maintain overall cation homeostasis, which is critical for survival in the macrophage phagosome. IMPORTANCE Polyamines are organic cations that are important in all life forms and are essential in plants and animals. However, their physiological functions and regulation remain poorly understood. We show that polyamines are critical for the adaptation of Salmonella to low Mg2+ conditions, including those found in the macrophage phagosome. Polyamines are synthesized upon low Mg2+ stress and partially replace Mg2+ until cytoplasmic Mg2+ levels are restored. Indeed, it is the sum of Mg2+ and polyamines in the cell that is critical for viability. While Mg2+ and polyamines compensate for one another, too little of both or too much of both is lethal. After cytoplasmic Mg2+ levels are reestablished, polyamines must be exported to avoid the toxic effects of excess divalent cations.

Inflammatory bowel disease-associated adherent-invasive Escherichia coli have elevated host-defense peptide resistance

Adherent-invasive Escherichia coli (AIEC) are isolated from inflammatory bowel disease (IBD) patients at a higher rate than from control patients. Using a collection of E. coli strains collected from Crohn's disease (CD), ulcerative colitis (UC), or non-IBD control patients, antibiotic and resistance to the antimicrobial peptides HBD-3 and LL-37 was assessed. Carriage of bacterial-encoded omptin protease genes was assessed by PCR and omptin protease activity was measured using a whole-cell based fluorescence assay. Elevated resistance to antibiotics and host defense peptides in IBD-associated AIEC were observed. IBD-associated strains showed increased (but statistically non-significant) antibiotic resistance. CD-associated strains showed greater (but statistically non-significant) resistance to HBD3-mediated killing while UC-associated strains showed statistically greater resistance to LL-37 mediated killing. High-level resistance to LL-37 was associated with carriage of omptin protease genes and with increased omptin protease activity. Antimicrobial host defense peptide resistance may be an adaptive feature of AIEC leading to enhanced pathogenesis during the initiation or progression of IBD.

FoxR is an AraC-like transcriptional regulator of ferrioxamine uptake in Salmonella enterica

Salmonella enterica spp. produce siderophores to bind iron with high affinity and can also use three xenosiderophores secreted by other microorganisms, ferrichrome, coprogen, and ferrioxamine. Here we focused on FoxA, a TonB-dependent transporter of ferrioxamines. Adjacent to foxA is a gene annotated as a helix-turn-helix (HTH) domain-containing protein, SL0358 (foxR), in the Salmonella enterica serovar Typhimurium SL1344 genome. FoxR shares homology with transcriptional regulators belonging to the AraC/XylS family. foxR is syntenic with foxA in the Enterobacteriaceae family, suggesting their functional relatedness. Both foxA and foxR are repressed by the ferric uptake regulator (Fur) under iron-rich growth conditions. When iron is scarce, FoxR acts as a transcriptional activator of foxA by directly binding to its upstream regulatory region. A point mutation in the HTH domain of FoxR abolished this binding, as did mutation of a direct repeat motif in the foxA upstream regulatory region. Desferrioxamine (DFOE) enhanced FoxR protein stability and foxA transcription but did not affect the affinity of FoxR binding to the foxA regulatory region. In summary, we have identified FoxR as a new member of the AraC/XylS family that regulates xenosiderophore-mediated iron uptake by S. Typhimurium and likely other Enterobacteriaceae members.

Manganese Utilization in Salmonella Pathogenesis: Beyond the Canonical Antioxidant Response

The metal ion manganese (Mn²⁺) is equally coveted by hosts and bacterial pathogens. The host restricts Mn²⁺ in the gastrointestinal tract and Salmonella-containing vacuoles, as part of a process generally known as nutritional immunity. Salmonella enterica serovar Typhimurium counteract Mn²⁺ limitation using a plethora of metal importers, whose expression is under elaborate transcriptional and posttranscriptional control. Mn²⁺ serves as cofactor for a variety of enzymes involved in antioxidant defense or central metabolism. Because of its thermodynamic stability and low reactivity, bacterial pathogens may favor Mn²⁺-cofactored metalloenzymes during periods of oxidative stress. This divalent metal catalyzes metabolic flow through lower glycolysis, reductive tricarboxylic acid and the pentose phosphate pathway, thereby providing energetic, redox and biosynthetic outputs associated with the resistance of Salmonella to reactive oxygen species generated in the respiratory burst of professional phagocytic cells. Combined, the oxyradical-detoxifying properties of Mn²⁺ together with the ability of this divalent metal cation to support central metabolism help Salmonella colonize the mammalian gut and establish systemic infections.