PagR mediates the precise regulation of Salmonella pathogenicity island 2 gene expression in response to magnesium and phosphate signals in Salmonella Typhimurium

To establish systemic infections, Salmonella enterica serovar Typhimurium (S. Typhimurium) requires Salmonella pathogenicity island 2 (SPI‐2) to survive and replicate within macrophages. High expression of many SPI‐2 genes during the entire intracellular growth period within macrophages is essential, as it contributes to the formation of Salmonella‐containing vacuole and bacterial replication. However, the regulatory mechanisms underlying the sustained induction of SPI‐2 within macrophages are not fully understood. Here, we revealed a time‐dependent regulation of SPI‐2 expression mediated by a novel regulator PagR (STM2345) in response to the low Mg²⁺ and low phosphate (P_i) signals, which ensured the high induction of SPI‐2 during the entire intramacrophage growth period. Deletion of pagR results in reduced bacterial replication in macrophages and attenuation of systemic virulence in mice. The effects of pagR on virulence are dependent on upregulating the expression of slyA, a regulator of SPI‐2. At the early (0–4 hr) and later (after 4 hr) stage post‐infection of macrophages, pagR is induced by the low P_i via PhoB/R two‐component systems and low Mg²⁺ via PhoP/Q systems, respectively. Collectively, our findings revealed that the PagR‐mediated regulatory mechanism contributes to the precise and sustained activation of SPI‐2 genes within macrophages, which is essential for S. Typhimurium systemic virulence.     

The DUB-ious lack of ALIS in Salmonella infection

Ubiquitinated aggregates are formed in eukaryotic cells in response to several external stimuli, including exposure to bacterial lipopolysaccharide (LPS). Although Salmonella enterica serovar Typhimurium (S. Typhimurium) LPS has been shown to induce aggresome-like induced structures (ALIS) in macrophages, these have not been described in S. Typhimurium-infected macrophages. Given that LPS is present in infection, this suggests that S. Typhimurium might suppress the formation of ALIS. We found that S. Typhimurium induces the formation of ubiquitinated aggregates in epithelial cells and macrophages, but that their presence is masked by the deubiquitinase (DUB) activity of the S. Typhimurium virulence protein, SseL. SseL deubiquitinates SQSTM1/p62-bound proteins found in S. Typhimurium-induced aggregates and ALIS, and reduces the recruitment of autophagic components. While the functions of ALIS and other ubiquitinated aggregates remain unclear, they serve to sequester cytosolic proteins under a variety of stress conditions and are suggested to be involved in host immune defense. During infection, the deubiquitinase activity of SseL reduces autophagic flux in infected cells and favors bacterial replication. This is a new example of how a bacterial pathogen counteracts the autophagy pathway through the action of a translocated virulence protein.     

Global distribution of plasmid-mediated colistin resistance mcr gene in Salmonella: A systematic review

This systematic review focuses on obtaining the most relevant information from multiple studies that detected a mobilized colistin resistance mcr gene in Salmonella for a better comprehension of its global distribution. A group of strategic and systematic keywords were combined to retrieve research data on the detection frequency of the mcr gene globally from four database platforms (Google Scholar, Science Direct, PubMed and Scielo). Forty-eight studies attended all the eligibility criteria and were selected. China was the country with the highest frequency of Salmonella strains with the mcr gene, and Europe exhibited a wide diversity of countries with positive mcr strains. In addition, animals and humans carried the highest frequency of positive strains for the mcr gene. Salmonella Typhimurium was the most frequent serovar carrying the mcr gene. Apparently, colistin overuse in animal husbandry has increased the selective pressure of antimicrobial resistance, resulting in the emergence of a plasmid-mediated colistin resistance mcr gene in China. The mcr-positive Salmonella strains are recently predominant worldwide, which is probably due to the capacity of this gene to be swiftly horizontally transmissible. The transmission ability of mcr-positive Salmonella strains to humans through the consumption of contaminated animal-based food is a public health concern.     

Oxidation-reduction potential regulates RpoS levels in Salmonella Typhimurium

AIMS: The aim of this work was to investigate the connection between oxidation-reduction (redox) potential and stationary phase induction of RpoS in Salmonella Typhimurium. METHODS AND RESULTS: A lux-based reporter was used to evaluate RpoS activity in S. Typhimurium pure cultures. During growth of S. Typhimurium, a drop in the redox potential of the growth medium occurred at the same time as RpoS induction and entry into stationary phase. An artificially induced decrease in redox potential earlier during growth reduced the time to RpoS induction and Salmonella entered the stationary phase prematurely. In contrast, under high redox conditions, Salmonella grew unaffected and entered the stationary growth phase as normal, although RpoS induction did not occur. As a consequence, stationary phase cells grown in the high redox environment were significantly more heat sensitive (P < 0.05) than those grown under normal conditions. CONCLUSIONS: This work suggests that redox potential can regulate RpoS levels in S. Typhimurium and can thus, control the expression of genes responsible for thermal resistance. SIGNIFICANCE AND IMPACT OF THE STUDY: The ability to manipulate RpoS induction and control stationary phase gene expression can have important implications in food safety. Early RpoS induction under low redox potential conditions can lead to enhanced resistance in low cell concentrations to inimical processes such as heat stress. Inhibition of RpoS induction would abolish stationary phase protective properties making cells more sensitive to common food control measures.     

Influence of peracetic acid on adhesion/invasion of Salmonella enterica serotype typhimurium LT2

The influence of peracetic acid (PAA) disinfectant on Salmonella enterica serotype Typhimurium LT2 in sewage effluent was examined by studying its ability to adhere to and invade HeLa cells in vitro. Although the disinfectant produced a decrease of about 5 log units, the bacteria kept their adhesive and invasive abilities. Scanning microscopic observations of the PAA-treated bacteria revealed that PAA caused a loss of external microfilaments and an alteration of membrane structure. Nevertheless, electron-microscopic observations showed that PAA-treated bacteria were still able to adhere to and invade HeLa cells despite the fact that the bacteria seemed to have undergone some structural modifications. With confocal microscopy, the use of anti-actin antibody showed that the contact between the bacteria (with or without PAA treatment) and the HeLa cells activated actinopolymerization of the HeLa cell cytoskeleton.     

Susceptibility of food-borne bacteria to binary combinations of antimicrobials at selected a(w) and pH

AIM: To evaluate the antibacterial susceptibilities of food-borne bacteria to individual and binary mixtures of a synthetic antimicrobial agent with a natural phenolic compound. METHODS AND RESULTS: Antibacterial susceptibilities of Escherichia coli, Listeria innocua, Salmonella Typhimurium and Staphylococcus aureus to individual and binary mixtures of potassium sorbate with a phenolic compound (thymol, carvacrol, or eugenol) were evaluated, at selected water activity (a(w); 0.99 or 0.97) and pH (5.5 or 4.5). The bacteria studied were susceptible to the action of the antimicrobials individually with minimal inhibitory concentrations that varied from 800-ppm potassium sorbate for Staph. aureus at a(w) 0.99, and pH 5.5 to 100-ppm thymol or carvacrol for the four studied bacteria at a(w) 0.97 and pH 4.5. Several binary mixtures of potassium sorbate with thymol, carvacrol or eugenol inhibited bacterial growth. Antimicrobial agent inhibitory concentrations in the mixture varied among bacteria, additionally depending on the a(w) and the pH tested. CONCLUSIONS: Synergistic binary mixtures with fractional inhibitory concentration index <0.6 include 100- or 200-ppm potassium sorbate with 50- or 100-ppm thymol, carvacrol or eugenol. SIGNIFICANCE AND IMPACT OF THE STUDY: The synergistic combinations could be useful in reducing the amounts of antimicrobials needed to inhibit growth, thus diminishing consumer concerns regarding chemical preservatives.     

Salmonella Typhimurium strain expressing OprF‐OprI protects mice against fatal infection by Pseudomonas aeruginosa

Pseudomonas aeruginosa poses a major threat to human health and to the mink industry. Thus, development of vaccines that elicit robust humoral and cellular immunity against P. aeruginosa is greatly needed. In this study, a recombinant attenuated Salmonella vaccine (RASV) that expresses the outer membrane proteins fusion OprF_190–342‐OprI_21–83 (F1I2) from P. aeruginosa was constructed and the potency of this vaccine candidate assessed by measuring F1I2‐specific humoral immune responses upon vaccination through s.c. or oral routes. S.C. administration achieved higher serum IgG titers and IgA titers in the intestine and induced stronger F1I2‐specific IgG and IgA titers in lung homogenate than did oral administration, which resulted in low IgG titers and no local IgA production. High titers of IFN‐γ, IL‐4, and T‐lymphocyte subsets induced a mixed Th1/Th2 response in mice immunized s.c., indicating elicitation of cellular immunity. Importantly, when immunized mice were challenged with P. aeruginosa by the intranasal route 30 days after the initial immunization, s.c. vaccination achieved 77.78% protection, in contrast to 41.18% via oral administration and 66.67% via Escherichia coli‐expressed F1I2 (His‐F1I2) vaccination. These results indicate that s.c. vaccination provides a better protective response against P. aeruginosa infection than do oral administration and the His‐F1I2 vaccine.