TolC is important for bacterial survival and oxidative stress response in Salmonella enterica serovar Choleraesuis in an acidic environment

Role of bacterial multidrug efflux pumps during infection

Multidrug efflux pumps are protein complexes located in the cell envelope that enable bacteria to expel, not only antibiotics, but also a wide array of molecules relevant for infection. Hence, they are important players in microbial pathogenesis. On the one hand, efflux pumps can extrude exogenous compounds, including host-produced antimicrobial molecules. Through this extrusion, pathogens can resist antimicrobial agents and evade host defenses. On the other hand, efflux pumps also have a role in the extrusion of endogenous compounds, such as bacterial intercommunication signaling molecules, virulence factors or metabolites. Therefore, efflux pumps are involved in the modulation of bacterial behavior and virulence, as well as in the maintenance of the bacterial homeostasis under different stresses found within the host. This review delves into the multifaceted roles that efflux pumps have, shedding light on their impact on bacterial virulence and their contribution to bacterial infection. These observations suggest that strategies targeting bacterial efflux pumps could both reinvigorate the efficacy of existing antibiotics and modulate the bacterial pathogenicity to the host. Thus, a comprehensive understanding of bacterial efflux pumps can be pivotal for the development of new effective strategies for the management of infectious diseases.

Molecular mechanism of Hfq-dependent sRNA1039 and sRNA1600 regulating antibiotic resistance and virulence in Shigella sonnei

Bacillary dysentery caused by Shigella spp. is a significant concern for human health. Small non-coding RNA (sRNA) plays a crucial role in regulating antibiotic resistance and virulence in Shigella spp. However, the specific mechanisms behind this phenomenon are still not fully understood. This study discovered two sRNAs (sRNA1039 and sRNA1600) that may be involved in bacterial resistance and virulence. By constructing deletion mutants (WT/ΔSR1039 and WT/ΔSR1600), this study found that the WT/ΔSR1039 mutants caused a two-fold increase in sensitivity to ampicillin, gentamicin and cefuroxime, and the WT/ΔSR1600 mutants caused a two-fold increase in sensitivity to cefuroxime. Furthermore, the WT/ΔSR1600 mutants caused a decrease in the adhesion and invasion of bacteria to HeLa cells (P<0.01), and changed the oxidative stress level of bacteria to reduce their survival rate (P<0.001). Subsequently, this study explored the molecular mechanisms by which sRNA1039 and sRNA1600 regulate antibiotic resistance and virulence. The deletion of sRNA1039 accelerated the degradation of target gene cfa mRNA and reduced its expression, thereby regulating the expression of pore protein gene ompD indirectly and negatively to increase bacterial sensitivity to ampicillin, gentamicin and cefuroxime. The inactivation of sRNA1600 reduced the formation of persister cells to reduce resistance to cefuroxime, and reduced the expression of type-III-secretion-system-related genes to reduce bacterial virulence by reducing the expression of target gene tomB. These results provide new insights into Hfq-sRNA-mRNA regulation of the resistance and virulence network of Shigella sonnei, which could potentially promote the development of more effective treatment strategies.

Implication of the σE Regulon Members OmpO and σN in the ΔompA299-356-Mediated Decrease of Oxidative Stress Tolerance in Stenotrophomonas maltophilia

Outer membrane protein A (OmpA) is the most abundant porin in bacterial outer membranes. KJΔOmpA299-356, an ompA C-terminal in-frame deletion mutant of Stenotrophomonas maltophilia KJ, exhibits pleiotropic defects, including decreased tolerance to menadione (MD)-mediated oxidative stress. Here, we elucidated the underlying mechanism of the decreased MD tolerance mediated by ΔompA299-356. The transcriptomes of wild-type S. maltophilia and the KJΔOmpA299-356 mutant strain were compared, focusing on 27 genes known to be associated with oxidative stress alleviation; however, no significant differences were identified. OmpO was the most downregulated gene in KJΔOmpA299-356. KJΔOmpA299-356 complementation with the chromosomally integrated ompO gene restored MD tolerance to the wild-type level, indicating the role of OmpO in MD tolerance. To further clarify the possible regulatory circuit involved in ompA defects and ompO downregulation, σ factor expression levels were examined based on the transcriptome results. The expression levels of three σ factors were significantly different (downregulated levels of rpoN and upregulated levels of rpoP and rpoE) in KJΔOmpA299-356. Next, the involvement of the three σ factors in the ΔompA299-356-mediated decrease in MD tolerance was evaluated using mutant strains and complementation assays. rpoN downregulation and rpoE upregulation contributed to the ΔompA299-356-mediated decrease in MD tolerance. OmpA C-terminal domain loss induced an envelope stress response. Activated σE decreased rpoN and ompO expression levels, in turn decreasing swimming motility and oxidative stress tolerance. Finally, we revealed both the ΔompA299-356-rpoE-ompO regulatory circuit and rpoE-rpoN cross regulation. IMPORTANCE The cell envelope is a morphological hallmark of Gram-negative bacteria. It consists of an inner membrane, a peptidoglycan layer, and an outer membrane. OmpA, an outer membrane protein, is characterized by an N-terminal β-barrel domain that is embedded in the outer membrane and a C-terminal globular domain that is suspended in the periplasmic space and connected to the peptidoglycan layer. OmpA is crucial for the maintenance of envelope integrity. Stress resulting from the destruction of envelope integrity is sensed by extracytoplasmic function (ECF) σ factors, which induce responses to various stressors. In this study, we revealed that loss of the OmpA-peptidoglycan (PG) interaction causes peptidoglycan and envelope stress while simultaneously upregulating σP and σE expression levels. The outcomes of σP and σE activation are different and are linked to β-lactam and oxidative stress tolerance, respectively. These findings establish that outer membrane proteins (OMPs) play a critical role in envelope integrity and stress tolerance.

Transcriptomic analysis of chloride tolerance in Leptospirillum ferriphilum DSM 14647 adapted to NaCl

Chloride ions are toxic for most acidophilic microorganisms. In this study, the chloride tolerance mechanisms in the acidophilic iron-oxidizing bacterium Leptospirillum ferriphilum DSM 14647 adapted to 180 mM NaCl were investigated by a transcriptomic approach. Results showed that 99 genes were differentially expressed in the adapted versus the non-adapted cultures, of which 69 and 30 were significantly up-regulated or down-regulated, respectively. Genes that were up-regulated include carbonic anhydrase, cytochrome c oxidase (ccoN) and sulfide:quinone reductase (sqr), likely involved in intracellular pH regulation. Towards the same end, the cation/proton antiporter CzcA (czcA) was down-regulated. Adapted cells showed a higher oxygen consumption rate (2.2 x 10⁻⁹ ppm O₂ s^-1cell^-1) than non-adapted cells (1.2 x 10⁻⁹ ppm O₂ s^-1cell^-1). Genes coding for the antioxidants flavohemoprotein and cytochrome c peroxidase were also up-regulated. Measurements of the intracellular reactive oxygen species (ROS) level revealed that adapted cells had a lower level than non-adapted cells, suggesting that detoxification of ROS could be an important strategy to withstand NaCl. In addition, data analysis revealed the up-regulation of genes for Fe-S cluster biosynthesis (iscR), metal reduction (merA) and activation of a cellular response mediated by diffusible signal factors (DSFs) and the second messenger c-di-GMP. Several genes related to the synthesis of lipopolysaccharide and peptidoglycan were consistently down-regulated. Unexpectedly, the genes ectB, ectC and ectD involved in the biosynthesis of the compatible solutes (hydroxy)ectoine were also down-regulated. In line with these findings, although hydroxyectoine reached 20 nmol mg^-1 of wet biomass in non-adapted cells, it was not detected in L. ferriphilum adapted to NaCl, suggesting that this canonical osmotic stress response was dispensable for salt adaptation. Differentially expressed transcripts and experimental validations suggest that adaptation to chloride in acidophilic microorganisms involves a multifactorial response that is different from the response in other bacteria studied.

Aeromonas veronii virulence and adhesion attenuation mediated by the gene aodp

Aeromonas veronii (A. veronii) is an opportunistic pathogen of fish-human-livestock, which poses a threat to the development of aquaculture. Based on our previous studies on proteomics and genomics, we found out that the aodp gene may be related to the virulence of A. veronii TH0426. However, aodp gene encodes a hypothetical protein with an unknown function, and its role in A. veronii TH0426 is not clear. Here, we first constructed a mutant strain (△-aodp) to investigate the functional role of aodp in A. veronii TH0426. Compared with the wild strain A. veronii TH0426, the growth rate of strain △-aodp was slower and was resistant to neomycin and kanamycin, but sensitive to cephalexin. The swimming and swarming ability of △-aodp strain decreased, and the pathogenicity to mice decreased by 15.84-fold. Besides, the activity of caspase-3 in EPCs infected with △-aodp strain was 1.49-fold lower than that of the wild strain. We examined 20 factors closely related to A. veronii virulence, among them 17 genes were down-regulated as a result of aodp deficiency. This study laid a foundation for further studies on the pathogenesis of A. veronii.

Influence of the Metabolic Activity of Microorganisms on Disinfection Efficiency of the Visible Light and P25 TiO2 Photocatalyst

The beneficial photocatalytic properties of UV light activated TiO2 powder are well-known and have been demonstrated with various pollutants and pathogens. However, traditionally observed photocatalytic activity of visible light activated pristine TiO2 is insignificant but there are a few studies which have reported that under some specific conditions commercially available TiO2 powder could at least partially disinfect microorganisms even under visible light. To better understand this phenomenon, in the current study we focused on bacteria response to the treatment by visible light and P25 TiO2 powder. More specifically, we analyzed the relationship between the bacteria viability, outer membrane permeability, metabolism, and its capacity to generate intracellular reactive oxygen species. During the study we assayed the viability of treated bacteria by the spread plate technique and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) reduction method. Changes in bacterial outer membrane permeability were determined by measuring the fluorescence of N-phenyl-1-naphthylamine (NPN). To detect intracellular reactive oxygen species formation, the fluorescence of dichlorodihydrofluorescein diacetate (DCFH-DA) was assayed. Results of our study indicated that TiO2 and wide spectrum visible light irradiation damaged the integrity of the outer membrane and caused oxidative stress in the metabolizing bacteria. When favorable conditions were created, these effects added up and unexpectedly high bacterial inactivation was achieved.

Genetic Determinants of Stress Resistance in Desiccated Salmonella enterica

Enteric pathogens, including Salmonella, are capable of long-term survival after desiccation and resist heat treatments that are lethal to hydrated cells. The mechanisms of dry-heat resistance differ from those of wet-heat resistance. To elucidate the mechanisms of dry-heat resistance in Salmonella, screening of the dry-heat resistance of 108 Salmonella strains, representing 39 serotypes, identified the 22 most resistant and the 8 most sensitive strains for comparative genome analysis. A total of 289 genes of the accessory genome were differently distributed between resistant and sensitive strains. Among these genes, 28 proteins with a putative relationship to stress resistance were selected for to quantify relative gene expression before and after desiccation and expression by solid-state cultures on agar plates relative to cultures growing in liquid culture media. Of these 28 genes, 15 genes were upregulated (P < 0.05) after desiccation or by solid-state cultures on agar plates. These 15 genes were cloned into the low-copy-number vector pRK767 under the control of the lacZ promoter. The expression of 6 of these 15 genes increased (P < 0.05) resistance to dry heat and to treatment with pressure of 500 MPa. Our finding extends the knowledge of mechanisms of stress resistance in desiccated Salmonella to improve control of this bacterium in dry food. IMPORTANCE This study directly targeted an increasing threat to food safety and developed knowledge and targeted strategies that can be used by the food industry to help reduce the risk of foodborne illness in their dry products and thereby reduce the overall burden of foodborne illness. Genomic and physiological analyses have elucidated mechanisms of bacterial resistance to many food preservation technologies, including heat, pressure, disinfection chemicals, and UV light; however, information on bacterial mechanisms of resistance to dry heat is scarce. Mechanisms of tolerance to desiccation likely also contribute to resistance to dry heat, but this assumption has not been verified experimentally. It remains unclear how mechanisms of resistance to wet heat relate to dry-heat resistance. Thus, this study will fill a knowledge gap to improve the safety of dry foods.

The virulence of Salmonella Enteritidis in Galleria mellonella is improved by N-dodecanoyl-homoserine lactone

Salmonella is a food and waterborne pathogen responsible for outbreaks worldwide, and it can survive during passage through the stomach and inside host phagocytic cells. Virulence genes are required for infection and survival in macrophages, and some are under the regulation of the quorum sensing (QS) system. This study investigated the influence of the autoinducer 1 (AI-1), N-dodecanoyl-homoserine lactone (C12-HSL), on the virulence of Salmonella PT4 using Galleria mellonella as an infection model. Salmonella PT4 was grown in the presence and absence of C12-HSL under anaerobic conditions for 7 h, and the expression of rpoS, arcA, arcB, and invA genes was evaluated. After the inoculation of G. mellonella with the median lethal dose (LD₅₀) of Salmonella PT4, the survival of bacteria inside the larvae and their health status (health index scoring) were monitored, as well as the pigment, nitric oxide (NO), superoxide dismutase (SOD), and catalase (CAT) production. Also, the hemocyte viability, the induction of caspase-3, and microtubule-associated light chain 3 (LC3) protein in hemocytes were evaluated. Salmonella PT4 growing in the presence of C12-HSL showed increased rpoS, arcA, arcB, and invA expression and promoted higher larvae mortality and worse state of health after 24 h of infection. The C12-HSL also increased the persistence of Salmonella PT4 in the hemolymph and in the hemocytes. The highest pigmentation, NO production, and antioxidant enzymes were verified in the larva hemolymph infected with Salmonella PT4 grown with C12-HSL. Hemocytes from larvae infected with Salmonella PT4 grown with C12-HSL showed lower viability and higher production of caspase-3 and LC3. Taken together, these findings suggest that C12-HSL could be involved in the virulence of Salmonella PT4.

The Evolutionary Conservation of Escherichia coli Drug Efflux Pumps Supports Physiological Functions

Bacteria harness an impressive repertoire of resistance mechanisms to evade the inhibitory action of antibiotics. One such mechanism involves efflux pump-mediated extrusion of drugs from the bacterial cell, which significantly contributes to multidrug resistance. Intriguingly, most drug efflux pumps are chromosomally encoded components of the intrinsic antibiotic resistome. In addition, in terms of xenobiotic detoxification, bacterial efflux systems often exhibit significant levels of functional redundancy. Efflux pumps are also considered to be highly conserved; however, the extent of conservation in many bacterial species has not been reported and the majority of genes that encode efflux pumps appear to be dispensable for growth. These observations, in combination with an increasing body of experimental evidence, imply alternative roles in bacterial physiology. Indeed, the ability of efflux pumps to facilitate antibiotic resistance could be a fortuitous by-product of ancient physiological functions. Using Escherichia coli as a model organism, we here evaluated the evolutionary conservation of drug efflux pumps and we provide phylogenetic analysis of the major efflux families. We show the E. coli drug efflux system has remained relatively stable and the majority (∼80%) of pumps are encoded in the core genome. This analysis further supports the importance of drug efflux pumps in E. coli physiology. In this review, we also provide an update on the roles of drug efflux pumps in the detoxification of endogenously synthesized substrates and pH homeostasis. Overall, gaining insight into drug efflux pump conservation, common evolutionary ancestors, and physiological functions could enable strategies to combat these intrinsic and ancient elements.

Nisin and acid resistance in Salmonella is enhanced by N-dodecanoyl-homoserine lactone

Salmonella is a foodborne pathogen that can develop resistance to different stresses, which is essential for successful infection of the host. Some genes directly related to acid resistance are also involved in cationic peptide resistance in Gram-negative bacteria and could be under the control of quorum sensing (QS) mediated by autoinducer 1, known as acyl-homoserine lactone. Here, we investigated the influence of autoinducer 1, N-dodecanoyl-homoserine lactone (C12-HSL) on the resistance of Salmonella enterica subspecies enterica serovar Enteritidis to nisin and acid stress. Salmonella cells growing in anaerobic tryptic soy agar (TSB) at a pH of 7.0 for 7 h were submitted to acid stress at a pH of 4.5 in the presence and absence of nisin and were either supplemented or not with C12-HSL. Viable cell counts, gene expression, membrane charge alterations, fatty acid composition, and intracellular content leakage were observed. The autoinducer C12-HSL increased nisin resistance and survival at a pH of 4.5 in Salmonella. Also, C12-HSL increased the expression of the genes, phoP, phoQ, pmrA, and pmrB, which are involved with antimicrobial and acid resistance. The positive charge on the cell surface and concentration of cyclopropane fatty acid of the cellular membrane were increased in the presence of C12-HSL under acidic conditions, whereas membrane fluidity decreased. The loss of K⁺ and NADPH, promoted by nisin, was reduced in the presence of C12-HSL at a pH of 4.5. Taken together, these findings suggest that quorum sensing plays an important role in enhanced nisin and acid resistance in Salmonella.

Inactivation of the T6SS inner membrane protein DotU results in severe attenuation and decreased pathogenicity of Aeromonas veronii TH0426

Background

The inner membrane protein DotU of Aeromonas veronii is an important component of the minimal core conserved membrane proteome required for the formation of an envelope-transmembrane complex. This protein functions in a type VI secretion system (T6SS), and the role of this T6SS during the pathogenic process has not been clearly described.

Results

A recombinant A. veronii with a partial disruption of the dotU gene (720 bp of the in-frame sequence) (defined as ∆dotU) was constructed by two conjugate exchanges. We found that the mutant ∆dotU allele can be stably inherited for more than 50 generations. Inactivation of the A. veronii dotU gene resulted in no significant changes in growth or resistance to various environmental changes. However, compared with the wild-type strain colony, the mutant ∆dotU colony had a rough surface morphology. In addition, the biofilm formation ability of the mutant ∆dotU was significantly enhanced by 2.1-fold. Conversely, the deletion of the dotU gene resulted in a significant decrease in pathogenicity and infectivity compared to those of the A. veronii wild-type strain.

Conclusions

Our findings indicated that the dotU gene was an essential participant in the pathogenicity and invasiveness of A. veronii TH0426, which provides a novel perspective on the pathogenesis of TH0426 and lays the foundation for discovering potential T6SS effectors.

Resistance Profiles of Salmonella Isolates Exposed to Stresses and the Expression of Small Non-coding RNAs

Salmonella can resist various stresses and survive during food processing, storage, and distribution, resulting in potential health risks to consumers. Therefore, evaluation of bacterial survival profiles under various environmental stresses is necessary. In this study, the resistance profiles of five Salmonella isolates [serotypes with Agona, Infantis, Typhimurium, Enteritidis, and a standard strain (ATCC 13076, Enteritidis serotype)] to acidic, hyperosmotic, and oxidative stresses were examined, and the relative expressions of non-coding small RNAs were also evaluated, including CyaR, MicC, MicA, InvR, RybB, and DsrA, induced by specific stresses. The results indicated that although all tested strains displayed a certain resistance to stresses, there was great diversity in stress resistance among the strains. According to the reduction numbers of cells exposed to stress for 3 h, S. Enteritidis showed the highest resistance to acidic and hyperosmotic stresses, whereas ATCC 13076 showed the greatest resistance to oxidative stress, with less than 0.1 Log CFU/ml of cell reduction. Greater resistance of cells to acidic, hyperosmotic, and oxidative stresses was observed within 1 h, after 2 h, and from 1 to 2 h, respectively. The relative expression of sRNAs depended on the isolate for each stress; acidic exposure for the tested isolates induced high expression levels of DsrA, MicC, InvR, RybB, MicA, and CyaR. The sRNA RybB, associated with sigma E and outer membrane protein in bacteria, showed a fold change of greater than 7 in S. Enteritidis exposed to the tested stresses. CyaR and InvR involved in general stress responses and stress adaptation were also induced to show high expression levels of Salmonella exposed to hyperosmotic stress. Overall, these findings demonstrated that the behaviors of Salmonella under specific stresses varied according to strain and were likely not related to other profiles. The finding also provided insights into the survival of Salmonella subjected to short-term stresses and for controlling Salmonella in the food industry.

Contribution of the colicin receptor CirA to biofilm formation, antibotic resistance, and pathogenicity of Salmonella Enteritidis

Salmonella Enteritidis is an important foodborne pathogen that can infect a wide range of animal species including human beings, resulting in great losses to commercial husbandry and human health. CirA is an outer membrane receptor involved in iron uptake and colicin1A/B-mediated competitive killing. Although iron uptake is crucial to bacterial virulence, limited literature is available about the role of CirA in infection. In the present work, we aimed to evaluate the role of CirA during S. Enteritidis infection. For this purpose, we generated a CirA-deficient mutant of the S. Enteritidis strain C50336 and examined its biological characteristics. The results showed that cirA gene inactivation caused sharply decreased biofilm formation and apparently impaired antibiotic resistance. Furthermore, the cirA gene deletion mutant showed markedly reduced adhesion and invasion to human epithelial cell line Caco-2 cells and decreased proliferation in mouse macrophage cell line RAW264.7 cells. Moreover, attenuated virulence was determined by a mouse model, with an LD₅₀ increase of approximately 1,000-fold. These data indicated that CirA plays critical roles in the S. Enteritidis infection process.

Variability in the Adaptive Response of Antibiotic-Resistant Salmonella Typhimurium to Environmental Stresses

This study was designed to evaluate the resistance phenotype and genotype of wild type (WT)-, cefotaxime (CET)-, and ciprofloxacin (CIP)-induced Salmonella Typhimurium ATCC 19585, CIP-resistant Salmonella Typhimurium ATCC 19585, Salmonella Typhimurium CCARM 8009, and Salmonella Typhimurium KCCM 40253 before and after exposure to pH 4.5, 4% NaCl, and heat at 42°C. The susceptibilities of WT Salmonella Typhimurium ATCC 19585 and WT Salmonella Typhimurium KCCM 40253 to all antibiotics tested in this study were decreased after CET and CIP induction with the exception with kanamycin, meropenem, and polymyxin B. The highest β-lactamase activities were 2.8 and 3.3 nmol/(min·mL), respectively, at the WT- and CET-induced Salmonella Typhimurium CCARM 8009. FT-IR spectra were found to be dominant at the region from 1,700 to 1,500 cm^-1 corresponding to proteins such as amides I, II, and III. The relative expression levels of efflux pump-related genes (acrA, acrB, and TolC), porin-related gene (ompC), virulence-related gene (stn), adhesion-related gene (fimA), and stress-induced alternative sigma factor (rpoS) varied in the antibiotic resistance and stress exposure. This study provides useful information for understanding the antibiotic resistance profile, physicochemical property, and gene expression pattern in Salmonella Typhimurium in association with the induction of antibiotic resistance and exposure to environmental stresses.

Effects of L-arabinose efflux on λ Red recombination-mediated gene knockout in multiple-antimicrobial-resistant Salmonella enterica serovar Choleraesuis

In this study, six swine-derived multiple-antimicrobial-resistant (MAR) strains of Salmonella Choleraesuis (S. Choleraesuis) were demonstrated to possess higher efflux pump activity than the wild-type (WT). L-Arabinose, a common inducer for gene expression, modulated S. Choleraesuis efflux pump activity in a dose-dependent manner. At low L-arabinose concentrations, increasing L-arabinose led to a corresponding increase in fluorophore efflux, while at higher L-arabinose concentrations, increasing L-arabinose decreased fluorophore efflux activity. The WT S. Choleraesuis that lacks TolC (ΔtolC), an efflux protein associated with bacterial antibiotic resistance and virulence, was demonstrated to possess a significantly reduced ability to extrude L-arabinose. Further, due to the rapid export of L-arabinose, an efficient method for recombination-mediated gene knockout, the L-arabinose-inducible bacteriophage λ Red recombinase system, has a reduced recombination frequency (~ 12.5%) in clinically isolated MAR Salmonella strains. An increased recombination frequency (up to 60%) can be achieved using a higher concentration of L-arabinose (fivefold) for genetic manipulation and functional analysis for MAR Salmonella using the λ Red system. The study suggests that L-arabinose serves not only as an inducer of the TolC-dependent efflux system but also acts as a competitive substrate of the efflux system. In addition, understanding the TolC-dependent efflux of L-arabinose should facilitate the optimization of L-arabinose induction in strains with high efflux activity.