ATP reduction by MgtC and Mg2+ homeostasis by MgtA and MgtB enables Salmonella to accumulate RpoS upon low cytoplasmic Mg2+ stress

The bacterial magnesium transporter MgtA reveals highly selective interaction with specific cardiolipin species

The bacterial magnesium transporter A (MgtA) is a specialized P-type ATPase important for Mg2+ import into the cytoplasm; disrupted magnesium homeostasis is linked to intrinsic ribosome instability and antibacterial resistance in Salmonella strains. Here, we show that MgtA has functional specificity for cardiolipin 18:1. Still, it reaches maximum activity only in combination with cardiolipin 16:0, equivalent to the major components of native cardiolipin found in E. coli membranes. Native mass spectrometry indicates the presence of two binding sites for cardiolipin, agreeing with the kinetic studies revealing that a cooperative relationship likely exists between the two cardiolipin variants. This is the first experimental evidence of cooperative effects between lipids of the same class, with only minor variations in their acyl chain composition, acting on a membrane protein. In summary, our results reveal that MgtA exhibits a highly complex interaction with one cardiolipin 18:1 and one cardiolipin 16:0, affecting protein activity and stability, contributing to our understanding of the particular interactions between lipid environment and membrane proteins. Further, a better understanding of Mg2+ homeostasis in bacteria, due to its role as a virulence regulator, will provide further insights into the regulation and mechanism of bacterial infections.

Effects of 405 ± 5-nm LED Illumination on Environmental Stress Tolerance of Salmonella Typhimurium in Sliced Beef

Salmonella Typhimurium is a widely distributed foodborne pathogen and is tolerant of various environmental conditions. It can cause intestinal fever, gastroenteritis and bacteremia. The aim of this research was to explore the effect of illumination with 405 nm light-emitting diodes (LEDs) on the resistance of S. Typhimurium to environmental stress. Beef slices contaminated with S. Typhimurium were illuminated by 405 nm LEDs (18.9 ± 1.4 mW/cm²) for 8 h at 4 °C; controls were incubated in darkness at 7 °C. Then, the illuminated or non-illuminated (control) cells were exposed to thermal stress (50, 55, 60 or 65 °C); oxidative stress (0.01% H₂O₂ [v/v]); acid stress (simulated gastric fluid [SGF] at pH 2 or 3); or bile salts (1%, 2%, or 3% [w/v]). S. Typhimurium treated by 405 nm LED irradiation showed decreased resistance to thermal stress, osmotic pressure, oxidation, SGF and bile salts. The transcription of eight environmental tolerance-related genes were downregulated by the illumination. Our findings suggest the potential of applying 405 nm LED-illumination technology in the control of pathogens in food processing, production and storage, and in decreasing infection and disease related to S. Typhimurium.

Cellular Adaptations to Cytoplasmic Mg2+ Limitation

Mg²⁺ is the most abundant divalent cation in living cells. It is essential for charge neutralization, macromolecule stabilization, and the assembly and activity of ribosomes and as a cofactor for enzymatic reactions. When experiencing low cytoplasmic Mg²⁺, bacteria adopt two main strategies: They increase the abundance and activity of Mg²⁺ importers and decrease the abundance of Mg²⁺-chelating ATP and rRNA. These changes reduce regulated proteolysis by ATP-dependent proteases and protein synthesis in a systemic fashion. In many bacterial species, the transcriptional regulator PhoP controls expression of proteins mediating these changes. The 5' leader region of some mRNAs responds to low cytoplasmic Mg²⁺ or to disruptions in translation of open reading frames in the leader regions by furthering expression of the associated coding regions, which specify proteins mediating survival when the cytoplasmic Mg²⁺ concentration is low. Microbial species often utilize similar adaptation strategies to cope with low cytoplasmic Mg²⁺ despite relying on different genes to do so.

Limitation of phosphate assimilation maintains cytoplasmic magnesium homeostasis

Phosphorus (P) is essential for life. As the fifth-most-abundant element in living cells, P is required for the synthesis of an array of biological molecules including (d)NTPs, nucleic acids, and membranes. Organisms typically acquire environmental P as inorganic phosphate (Pi). While essential for growth and viability, excess intracellular Pi is toxic for both bacteria and eukaryotes. Using the bacterium Salmonella enterica serovar Typhimurium as a model, we establish that Pi cytotoxicity is manifested following its assimilation into adenosine triphosphate (ATP), which acts as a chelating agent for Mg²⁺ and other cations. Our findings identify physiological processes disrupted by excessive Pi and how bacteria tune P assimilation to cytoplasmic Mg²⁺ levels.

Phosphorus (P) is an essential component of core biological molecules. In bacteria, P is acquired mainly as inorganic orthophosphate (Pi) and assimilated into adenosine triphosphate (ATP) in the cytoplasm. Although P is essential, excess cytosolic Pi hinders growth. We now report that bacteria limit Pi uptake to avoid disruption of Mg²⁺-dependent processes that result, in part, from Mg²⁺ chelation by ATP. We establish that the MgtC protein inhibits uptake of the ATP precursor Pi when Salmonella enterica serovar Typhimurium experiences cytoplasmic Mg²⁺ starvation. This response prevents ATP accumulation and overproduction of ribosomal RNA that together ultimately hinder bacterial growth and result in loss of viability. Even when cytoplasmic Mg²⁺ is not limiting, excessive Pi uptake increases ATP synthesis, depletes free cytoplasmic Mg²⁺, inhibits protein synthesis, and hinders growth. Our results provide a framework to understand the molecular basis for Pi toxicity. Furthermore, they suggest a regulatory logic that governs P assimilation based on its intimate connection to cytoplasmic Mg²⁺ homeostasis.

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.

Analysis of In Vivo Transcriptome of Intracellular Bacterial Pathogen Salmonella enterica serovar Typhmurium Isolated from Mouse Spleen

Salmonella enterica serovar Typhimurium (S. Typhimurium) is an important intracellular pathogen that poses a health threat to humans. This study tries to clarify the mechanism of Salmonella survival and reproduction in the host. In this study, high-throughput sequencing analysis was performed on RNA extracted from the strains isolated from infected mouse spleens and an S. Typhimurium reference strain (ATCC 14028) based on the BGISEQ-500 platform. A total of 1340 significant differentially expressed genes (DEGs) were screened. Functional annotation revealed DEGs associated with regulation, metabolism, transport and binding, pathogenesis, and motility. Through data mining and literature retrieval, 26 of the 58 upregulated DEGs (FPKM > 10) were not reported to be related to the adaptation to intracellular survival and were classified as candidate key genes (CKGs) for survival and proliferation in vivo. Our data contribute to our understanding of the mechanisms used by Salmonella to regulate virulence gene expression whilst replicating inside mammalian cells.

Reduced ATP-dependent proteolysis of functional proteins during nutrient limitation speeds the return of microbes to a growth state

When cells run out of nutrients, the growth rate greatly decreases. Here, we report that microorganisms, such as the bacterium Salmonella enterica serovar Typhimurium, speed up the return to a rapid growth state by preventing the proteolysis of functional proteins by ATP-dependent proteases while in the slow-growth state or stationary phase. This reduction in functional protein degradation resulted from a decrease in the intracellular concentration of ATP that was nonetheless sufficient to allow the continued degradation of nonfunctional proteins by the same proteases. Protein preservation occurred under limiting magnesium, carbon, or nitrogen conditions, indicating that this response was not specific to low availability of a particular nutrient. Nevertheless, the return to rapid growth required proteins that mediate responses to the specific nutrient limitation conditions, because the transcriptional regulator PhoP was necessary for rapid recovery only after magnesium starvation. Reductions in intracellular ATP and in ATP-dependent proteolysis also enabled the yeast Saccharomyces cerevisiae to recover faster from stationary phase. Our findings suggest that protein preservation during a slow-growth state is a conserved microbial strategy that facilitates the return to a growth state once nutrients become available.

Residue L193P Mutant of RpoS Affects Its Activity During Biofilm Formation in Salmonella Pullorum

The role of alternative sigma factor RpoS in regulating biofilm formation may differ in various Salmonella Pullorum strains. In this study, the biofilm-forming ability of two Salmonella Pullorum strains S6702 and S11923-3 were compared. The biofilm forming ability of S11923-3 was much stronger than that of S6702. After knocking out the rpoS gene, S11923-3ΔrpoS had significantly reduced biofilm while S6702ΔrpoS demonstrated similar biofilm compared with each parent strain. The analysis of RpoS sequences indicated two amino acid substitutions (L193P and R293C) between S6702 and S11923-3 RpoS. A complementation study confirmed that the expression of S11923-3 RpoS rather than S6702 RpoS could restore the biofilm-forming ability of ΔrpoS strains and the L193P mutation contributed to the restoration of the biofilm-forming ability. Further study indicated that RpoS with the L193P mutant had significantly improved expression level and binding activity to RNAP and csgD gene promoter, which increased the efficacy of the csgD gene promoter and biofilm-forming ability. Therefore, the L193P mutation of RpoS is critical for stronger biofilm formation of Salmonella Pullorum.

The mgtCBR mRNA Leader Secures Growth of Salmonella in Both Host and Non-host Environments

Upon intracellular cues, bacterial mRNA leaders often form secondary structures that determine expression of a downstream protein-coding region(s), thereby providing bacteria with a mechanism to control the amounts of necessary proteins in the right locales. Here we describe a polycistronic mRNA leader that secures bacterial growth by preventing dysregulated expression of the protein-coding regions. In Salmonella, the mgtCBR mRNA encodes the virulence protein MgtC and the Mg²⁺ transporter MgtB. A mutant designed to produce leaderless mgtCBR mRNA induced MgtC and MgtB in conditions that promote mgtC transcription. The dysregulated expression of MgtC and MgtB impaired bacterial growth under all such non-host environments. While MgtC, but not MgtB, normally reduces ATP levels in a process requiring the F₁F₀ ATP synthase, dysregulated MgtC and MgtB reduced ATP levels independently of the F₁F₀ ATP synthase, which correlated with the mutant’s growth defect. The mutant showed dysregulated MgtC expression and attenuated survival inside macrophages. While MgtB normally does not affect the phenotype, MgtB impaired intramacrophage survival of the mutant in the presence of MgtC. We provide an example showing that a polycistronic mRNA leader prevents the dysregulated function of protein-coding regions to allow bacteria to proliferate across complex niches.

Magnesium Is a Key Player in Neuronal Maturation and Neuropathology

Magnesium (Mg) is the second most abundant cation in mammalian cells, and it is essential for numerous cellular processes including enzymatic reactions, ion channel functions, metabolic cycles, cellular signaling, and DNA/RNA stabilities. Because of the versatile and universal nature of Mg²⁺, the homeostasis of intracellular Mg²⁺ is physiologically linked to growth, proliferation, differentiation, energy metabolism, and death of cells. On the cellular and tissue levels, maintaining Mg²⁺ within optimal levels according to the biological context, such as cell types, developmental stages, extracellular environments, and pathophysiological conditions, is crucial for development, normal functions, and diseases. Hence, Mg²⁺ is pathologically involved in cancers, diabetes, and neurodegenerative diseases, such as Parkinson's disease, Alzheimer's disease, and demyelination. In the research field regarding the roles and mechanisms of Mg²⁺ regulation, numerous controversies caused by its versatility and complexity still exist. As Mg²⁺, at least, plays critical roles in neuronal development, healthy normal functions, and diseases, appropriate Mg²⁺ supplementation exhibits neurotrophic effects in a majority of cases. Hence, the control of Mg²⁺ homeostasis can be a candidate for therapeutic targets in neuronal diseases. In this review, recent results regarding the roles of intracellular Mg²⁺ and its regulatory system in determining the cell phenotype, fate, and diseases in the nervous system are summarized, and an overview of the comprehensive roles of Mg²⁺ is provided.

The role of rpoS in the regulation of Vibrio alginolyticus virulence and the response to diverse stresses

Vibrio alginolyticus is a leading aquatic pathogen, causing huge losses to aquaculture. rpoS has been proven to play a variety of important roles in stress response and virulence in several bacteria. In our previous study, upon treatment with Cu²⁺ , Pb²⁺ , Hg²⁺ and low pH, the expression levels of rpoS were downregulated as assessed by RNA-seq, while impaired adhesion ability was observed, indicating that rpoS might play roles in the regulation of adhesion. In the present study, the RNAi technology was used to knockdown rpoS in V. alginolyticus. In comparison with wild-type V. alginolyticus, RNAi-treated bacteria showed significantly impaired abilities of adhesion, growth, haemolytic, biofilm production, movement and virulence. Meanwhile, alterations of temperature, salinity, pH and starvation starkly affected rpoS expression. The present data suggested that rpoS is a critical regulator of virulence in V. alginolyticus; in addition, rpoS regulates bacterial adhesion in response to temperature, pH and nutrient content changes. These are helpful to explore its pathogenic mechanism and provide reference for disease control.