Cellular and molecular responses of Salmonella Typhimurium to antimicrobial-induced stresses during the planktonic-to-biofilm transition

Influence of temperature and pH on induction of Shiga toxin Stx1a in Escherichia coli

Shiga toxin-producing strains represent pathogenic group that is of concern in food production. The present study evaluated forty-eight E. coli isolates (11 with intact stx gene, while remaining isolates presented only stx-fragments) for Shiga toxin production. The four most expressive stx-producers (O26, O103, O145, and O157) were selected to evaluate effects of pH (3.5, 4.5, and 7) and temperature (35, 40, and 50°C). After determining acid stress effects in media on Stx-induction, we mimicked "in natura" conditions using milk, apple, and orange juices. Only isolates that showed the presence of intact stx gene (11/48) produced Shiga toxin. In addition, acid pH had a role in down-regulating the production of Shiga toxin, in both lactic acid and juices. In contrast, non-lethal heating (40°C), when in neutral pH and milk was a favorable environment to induce Shiga toxin. Lastly, two isolates (O26 and O103) showed a higher capacity to produce Shiga toxin and were included in a genomic cluster with other E. coli involved in worldwide foodborne outbreaks. The induction of this toxin when subjected to 40°C may represent a potential risk to the consumer, since the pathogenic effect of oral ingestion of Shiga toxin has already been proved in an animal model.

Molecular mechanisms associated with quinolone resistance in Enterobacteriaceae: review and update

BACKGROUND

Quinolones are broad-spectrum antibiotics, which are used for the treatment of different infectious diseases associated with Enterobacteriaceae. During recent decades, the wide use as well as overuse of quinolones against diverse infections has led to the emergence of quinolone-resistant bacterial strains. Herein, we present the development of quinolone antibiotics, their function and also the different quinolone resistance mechanisms in Enterobacteriaceae by reviewing recent literature.

METHODS

All data were extracted from Google Scholar search engine and PubMed site, using keywords; quinolone resistance, Enterobacteriaceae, plasmid-mediated quinolone resistance, etc.

RESULTS AND CONCLUSION

The acquisition of resistance to quinolones is a complex and multifactorial process. The main resistance mechanisms consist of one or a combination of target-site gene mutations altering the drug-binding affinity of target enzymes. Other mechanisms of quinolone resistance are overexpression of AcrAB-tolC multidrug-resistant efflux pumps and downexpression of porins as well as plasmid-encoded resistance proteins including Qnr protection proteins, aminoglycoside acetyltransferase (AAC(6')-Ib-cr) and plasmid-encoded active efflux pumps such as OqxAB and QepA. The elucidation of resistance mechanisms will help researchers to explore new drugs against the resistant strains.

Pore-forming treatments induce aggregation of Salmonella Senftenberg through protein leakage

Fresh herbs are not commonly associated with foodborne pathogens, due to the production of essential oils with antimicrobial activity. Recalls of contaminated basil, and basil outbreaks caused by Salmonella motivated studies aimed to comprehend the antimicrobial activity of basil essential oils, and to explore the mechanisms in which Salmonella can overcome them. Linalool, a major constituent of basil oil, increases the permeability of Salmonella Senftenberg cells by damaging their membrane. Linalool also induces bacterial aggregation. We hypothesized that the membrane perforation effect triggers cell aggregation through leakage of intracellular substances from live and dead cells. By exposing S. Senftenberg to additional physical (sonication) or chemical (eugenol, Triton-X-100) treatments, we showed that the aggregation is caused by various membrane-targeted treatments. Enzymatic degradation of leaked proteins restricted the bacterial aggregation, and disassembled existing aggregates. Moreover, supplemented proteins such as bacterial intracellular proteins or BSA also caused aggregation, further supporting the hypothesis that non-specific proteins trigger the bacterial aggregation. This study provides a novel understanding of the role of protein leakage in promoting bacterial aggregation. Since aggregation has significant roles in food safety and microbial ecology, this finding may establish future studies about microbial resistance via formation of clusters similar to biofilm development.

Biofilms as Promoters of Bacterial Antibiotic Resistance and Tolerance

Multidrug resistant bacteria are a global threat for human and animal health. However, they are only part of the problem of antibiotic failure. Another bacterial strategy that contributes to their capacity to withstand antimicrobials is the formation of biofilms. Biofilms are associations of microorganisms embedded a self-produced extracellular matrix. They create particular environments that confer bacterial tolerance and resistance to antibiotics by different mechanisms that depend upon factors such as biofilm composition, architecture, the stage of biofilm development, and growth conditions. The biofilm structure hinders the penetration of antibiotics and may prevent the accumulation of bactericidal concentrations throughout the entire biofilm. In addition, gradients of dispersion of nutrients and oxygen within the biofilm generate different metabolic states of individual cells and favor the development of antibiotic tolerance and bacterial persistence. Furthermore, antimicrobial resistance may develop within biofilms through a variety of mechanisms. The expression of efflux pumps may be induced in various parts of the biofilm and the mutation frequency is induced, while the presence of extracellular DNA and the close contact between cells favor horizontal gene transfer. A deep understanding of the mechanisms by which biofilms cause tolerance/resistance to antibiotics helps to develop novel strategies to fight these infections.

Plant-Based Phytochemicals as Possible Alternative to Antibiotics in Combating Bacterial Drug Resistance

The unprecedented use of antibiotics that led to development of resistance affect human health worldwide. Prescription of antibiotics imprudently and irrationally in different diseases progressed with the acquisition and as such development of antibiotic resistant microbes that led to the resurgence of pathogenic strains harboring enhanced armors against existing therapeutics. Compromised the treatment regime of a broad range of antibiotics, rise in resistance has threatened human health and increased the treatment cost of diseases. Diverse on metabolic, genetic and physiological fronts, rapid progression of resistant microbes and the lack of a strategic management plan have led researchers to consider plant-derived substances (PDS) as alternative or in complementing antibiotics against the diseases. Considering the quantitative characteristics of plant constituents that attribute health beneficial effects, analytical procedures for their isolation, characterization and phytochemical testing for elucidating ethnopharmacological effects has being worked out for employment in the treatment of different diseases. With an immense potential to combat bacterial infections, PDSs such as polyphenols, alkaloids and tannins, present a great potential for use, either as antimicrobials or as antibiotic resistance modifiers. The present study focuses on the mechanisms by which PDSs help overcome the surge in resistance, approaches for screening different phytochemicals, methods employed in the identification of bioactive components and their testing and strategies that could be adopted for counteracting the lethal consequences of multidrug resistance.

Virulence Factors of Clostridioides (Clostridium) difficile Linked to Recurrent Infections

From 20 to 30% of Clostridioides (Clostridium) difficile infection (CDI), patients might develop recurrence of the infection (RCDI) and, after the first recurrence, the risk of further episodes increases up to 60%. Several bacterial virulence factors have been associated with RCDI, including the elevated production of toxins A and B, the presence of a binary toxin CDT, and mutations in the negative regulator of toxin expression, tcdC. Additional factors have shown to regulate toxin production and virulence in C. difficile in RCDI, including the accessory-gene regulator agr, which acts as a positive switch for toxin transcription. Furthermore, adhesion and motility-associated factors, such as Cwp84, SlpA, and flagella, have shown to increase the adhesion efficiency to host epithelia, cell internalization, and the formation of biofilm. Finally, biofilm confers to C. difficile protection from antibiotics and acts as a reservoir for spores that allow the persistence of the infection in the host. In this review, we describe the key virulence factors of C. difficile that have been associated with recurrent infections.

The resistance mechanism of Escherichia coli induced by ampicillin in laboratory

Background

Multi-drug-resistant Escherichia coli poses a great threat to human health, especially resistant to ampicillin (AMP), but the mechanism of drug resistance is not very clear.

Purpose

To understand the mechanism of resistance of E. coli to beta-lactam antibiotics by inducing drug resistance of sensitive bacteria in laboratory.

Methods

Clinical sensitive E. coli strain was induced into resistance strain by 1/2 minimum inhibitive concentration (MIC) induced trails of AMP. The drug resistance spectrum was measured by modified K-B susceptibility test. Whole-genome sequencing analysis was used to analyze primary sensitive strain, and resequencing was used to analyze induced strains. Protein tertiary structure encoded by the gene containing single nucleotide polymorphism (SNP) was analyzed by bioinformatics.

Results

After 315 hrs induced, the MIC value of E. coli 15743 reached to 256 µg/mL, 64 times higher than that of the sensitive bacteria. During the induction process, the bacterial resistance process is divided into two stages. The rate of drug resistance occurs rapidly before reaching the critical concentration of 32 µg/mL, and then the resistance rate slows down. Sequencing of the genome of resistant strain showed that E. coli 15743 drug-resistant strain with the MIC values of 32 and 256 µg/mL contained four and eight non-synonymous SNPs, respectively. These non-synonymous SNPs were distributed in the genes of frdD, ftsI, acrB, OmpD, marR, VgrG, and envZ.

Conclusion

These studies will improve our understanding of the molecular mechanism of AMP resistance of E. coli, and may provide the basis for prevention and control of multi-drug-resistant bacteria and generation of new antibiotics to treat E. coli infection.

Local Repressor AcrR Regulates AcrAB Efflux Pump Required for Biofilm Formation and Virulence in Acinetobacter nosocomialis

Multidrug efflux systems contribute to antimicrobial resistance and pathogenicity in bacteria. Here, we report the identification and characterization of a transcriptional regulator AcrR controlling the yet uncharacterized multidrug efflux pump, AcrAB in Acinetobacter nosocomialis. In silico analysis revealed that the homologs of AcrR and AcrAB are reported in the genomes of many other bacterial species. We confirmed that the genes encoding the AcrAB efflux pump, acrA and acrB forms a polycistronic operon which is under the control of acrR gene upstream of acrA. Bioinformatic analysis indicated the presence of AcrR binding motif in the promoter region of acrAB operon and the specific binding of AcrR was confirmed by electrophoretic mobility shift assay (EMSA). The EMSA data showed that AcrR binds to −89 bp upstream of the start codon of acrA. The mRNA expression analysis depicted that the expression of acrA and acrB genes are elevated in the deletion mutant compared to that in the wild type confirming that AcrR acts as a repressor of acrAB operon in A. nosocomialis. The deletion of acrR resulted in increased motility, biofilm/pellicle formation and invasion in A. nosocomialis. We further analyzed the role of AcrR in A. nosocomialis pathogenesis in vivo using murine model and it was shown that acrR mutant is highly virulent inducing severe infection in mouse leading to host death. In addition, the intracellular survival rate of acrR mutant was higher compared to that of wild type. Our data demonstrates that AcrR functions as an important regulator of AcrAB efflux pump and is associated with several phenotypes such as motility, biofilm/pellicle formation and pathogenesis in A. nosocomialis.

Inhibition of Initial Attachment of Injured Salmonella Typhimurium onto Abiotic Surfaces

Following sanitation interventions in food processing facilities, sublethally injured bacterial cells can remain on food contact surfaces. We investigated whether injured Salmonella Typhimurium cells can attach onto abiotic surfaces, which is the initial stage for further biofilm development. We utilized heat, UV, hydrogen peroxide, and lactic acid treatments, which are widely utilized by the food industry. Our results showed that heat, UV, and hydrogen peroxide did not effectively change populations of attached Salmonella Typhimurium. Cells treated with hydrogen peroxide had a slightly higher tendency to adhere to abiotic surfaces, although there was no significant difference between the populations of control and hydrogen peroxide-treated cells. However, lactic acid effectively reduced the number of Salmonella Typhimurium cells attached to stainless steel. We also compared physicochemical changes of Salmonella Typhimurium after application of lactic acid and used hydrogen peroxide as a positive control because only lactic acid showed a decreased tendency for attachment and hydrogen peroxide induced slightly higher numbers of attached bacteria cells. Extracellular polymeric substance produced by Salmonella Typhimurium was not detected in any treatment. Significant differences in hydrophobicity were not observed. Surface charges of cell membranes did not show relevant correlation with numbers of attached cells, whereas autoaggregation showed a positive correlation with attachment to stainless steel. Our results highlight that when lactic acid is applied in a food processing facility, it can effectively interfere with adhesion of injured Salmonella Typhimurium cells onto food contact surfaces.

Use of carvacrol, thymol, and eugenol for biofilm eradication and resistance modifying susceptibility of Salmonella enterica serovar Typhimurium strains to nalidixic acid

The Aims of the study was to evaluate the antibacterial susceptibility and the biofilm eradication of three natural compounds carvacrol (CAR), thymol (TH) and eugenol (EUG), alone or in combination with nalidixic acid (NA) against twelve Salmonella Typhimurium strains. The minimum inhibitory concentration (MIC) and the minimum biofilm eradication concentration (BEC₅₀) of the tested compounds (CAR, TH and EUG) and their combinations with NA were evaluated. In order to assess whether these bacteria had active efflux pumps, ethidium bromide (EtBr) accumulation assays was achieved using spectrophotometric accumulation assays. Moreover, scanning electron microscopy was used to visualize the bacterial biofilm formation on stainless steel surfaces after exposed to NA, CAR, TH and EUG alone and in combination. TH was the most effective essential oil, with the lowest MICs values ranging from 32 to 128 μg/mL followed by EUG and CAR. In addition, the combination of NA with the different compounds enhances antibiotic susceptibility of the tested bacterial strains. These results were confirmed by EtBr accumulation assays. A pronounced effect in decreasing biofilm mass was also noticed. Moreover, SEM revealed that bacterial membrane was disrupted and a complete loss of membrane integrity was also evident. The combination of natural compounds with antibiotic enhances bacterial susceptibility to NA. This combination ameliorates eradication of biofilm formed by S. Typhimurium on polystyrene microtitre plates. Additionally, this synergy induces an alteration of the bacterial cell surface visualized by SEM.

Streptomycin Induced Stress Response in Salmonella enterica Serovar Typhimurium Shows Distinct Colony Scatter Signature

We investigated the streptomycin-induced stress response in Salmonella enterica serovars with a laser optical sensor, BARDOT (bacterial rapid detection using optical scattering technology). Initially, the top 20 S. enterica serovars were screened for their response to streptomycin at 100 μg/mL. All, but four S. enterica serovars were resistant to streptomycin. The MIC of streptomycin-sensitive serovars (Enteritidis, Muenchen, Mississippi, and Schwarzengrund) varied from 12.5 to 50 μg/mL, while streptomycin-resistant serovar (Typhimurium) from 125–250 μg/mL. Two streptomycin-sensitive serovars (Enteritidis and Mississippi) were grown on brain heart infusion (BHI) agar plates containing sub-inhibitory concentration of streptomycin (1.25–5 μg/mL) and a streptomycin-resistant serovar (Typhimurium) was grown on BHI containing 25–50 μg/mL of streptomycin and the colonies (1.2 ± 0.1 mm diameter) were scanned using BARDOT. Data show substantial qualitative and quantitative differences in the colony scatter patterns of Salmonella grown in the presence of streptomycin than the colonies grown in absence of antibiotic. Mass-spectrometry identified overexpression of chaperonin GroEL, which possibly contributed to the observed differences in the colony scatter patterns. Quantitative RT-PCR and immunoassay confirmed streptomycin-induced GroEL expression while, aminoglycoside adenylyltransferase (aadA), aminoglycoside efflux pump (aep), multidrug resistance subunit acrA, and ribosomal protein S12 (rpsL), involved in streptomycin resistance, were unaltered. The study highlights suitability of the BARDOT as a non-invasive, label-free tool for investigating stress response in Salmonella in conjunction with the molecular and immunoassay methods.

Participation of the Salmonella OmpD porin in the infection of RAW264.7 macrophages and BALB/c mice

Salmonella Typhimurium is the etiological agent of gastroenteritis in humans and enteric fever in mice. Inside these hosts, Salmonella must overcome hostile conditions to develop a successful infection, a process in which the levels of porins may be critical. Herein, the role of the Salmonella Typhimurium porin OmpD in the infection process was assessed for adherence, invasion and proliferation in RAW264.7 mouse macrophages and in BALB/c mice. In cultured macrophages, a ΔompD strain exhibited increased invasion and proliferation phenotypes as compared to its parental strain. In contrast, overexpression of ompD caused a reduction in bacterial proliferation but did not affect adherence or invasion. In the murine model, the ΔompD strain showed increased ability to survive and replicate in target organs of infection. The ompD transcript levels showed a down-regulation when Salmonella resided within cultured macrophages and when it colonized target organs in infected mice. Additionally, cultured macrophages infected with the ΔompD strain produced lower levels of reactive oxygen species, suggesting that down-regulation of ompD could favor replication of Salmonella inside macrophages and the subsequent systemic dissemination, by limiting the reactive oxygen species response of the host.

Combating pathogenic microorganisms using plant-derived antimicrobials: a minireview of the mechanistic basis

The emergence of antibiotic resistance in pathogenic bacteria has led to renewed interest in exploring the potential of plant-derived antimicrobials (PDAs) as an alternative therapeutic strategy to combat microbial infections. Historically, plant extracts have been used as a safe, effective, and natural remedy for ailments and diseases in traditional medicine. Extensive research in the last two decades has identified a plethora of PDAs with a wide spectrum of activity against a variety of fungal and bacterial pathogens causing infections in humans and animals. Active components of many plant extracts have been characterized and are commercially available; however, research delineating the mechanistic basis of their antimicrobial action is scanty. This review highlights the potential of various plant-derived compounds to control pathogenic bacteria, especially the diverse effects exerted by plant compounds on various virulence factors that are critical for pathogenicity inside the host. In addition, the potential effect of PDAs on gut microbiota is discussed.

Physicochemical, mechanical, and molecular properties of nonlysogenic and p22-lysogenic Salmonella typhimurium treated with citrus oil

The aim of this study was to evaluate the phenotypic and genotypic properties of nonlysogenic Salmonella Typhimurium (ST(P22-)) and lysogenic Salmonella Typhimurium (ST(P22+)) in the presence of sublethal concentrations (SLC2D) of citrus essential oils (CEOs), which were used to evaluate antimicrobial susceptibility, cell surface hydrophobicity, autoaggregation ability, bacterial motility, lysogenic conversion, gene expression patterns, and antibiofilm formation. The SLC2D values of non-heat-treated (N-CEO) and heat-treated (H-CEO) CEO in an autoclave at 121°C for 20 min were 2.0 to 2.1 mg/ml against ST(P22-) and 1.7 to 1.9 mg/ml against STP(22+). The rates of injured ST(P22-) and ST(P22+) cells treated with SLC2D of N-CEO and H-CEO ranged from 67 to 83%. The hydrophobicity and autoaggregation were decreased to 2.5 and 19.5% for ST(P22-) and 4.7 and 21.7% for ST(P22+), respectively, in the presence of N-CEO. A noticeable reduction in the swarming motility was observed in ST(P22-) with N-CEO (14.5%) and H-CEO (13.3%). The numbers of CEO-induced P22 were 5.40 log PFU/ml for N-CEO and 5.65 log PFU/ml for H-CEO. The relative expression of hilA, hilC, hilD, invA, invC, invE, invF, sirA, and sirB was down-regulated in ST(P22-) and ST(P22+) with N-CEO and H-CEO. The numbers of adherent ST(P22-) and ST(P22+) were effectively reduced by more than 1 log in the presence of CEO. These results suggest that CEO has potential to be used to control bacterial attachment, colonization, and invasion.

Role of efflux pumps in the antibiotic resistance of bacteria embedded in a biofilm

Biofilms are complex microbial associations anchored to abiotic or biotic surfaces, embedded in extracellular matrix produced by the biofilms themselves where they interact with each other and the environment. One of the main properties of biofilms is their capacity to be more resistant to antimicrobial agents than planktonic cells. Efflux pumps have been reported as one of the mechanisms responsible for the antimicrobial resistance in biofilm structures. Evidence of the role of efflux pump in biofilm resistance has been found in several microorganisms such as Pseudomonas aeruginosa, Escherichia coli and Candida albicans. However, in spite of the studies on the importance of efflux pumps in biofilm growth and about their relevance in antimicrobial resistance forming biofilm, the exact role of these efflux systems has not been determined as yet.

Probiotic-mediated competition, exclusion and displacement in biofilm formation by food-borne pathogens

The objective of this study was to examine the inhibitory effect of probiotic strains on pathogenic biofilm formation in terms of competition, exclusion and displacement. Probiotic strains (Lactobacillus acidophilus KACC 12419, Lact. casei KACC 12413, Lact. paracasei KACC 12427 and Lact. rhamnosus KACC 11953) and pathogens (Salmonella Typhimurium KCCM 40253 and Listeria monocytogenes KACC 12671) were used to evaluate the auto-aggregation, hydrophobicity and biofilm formation inhibition. The highest auto-aggregation abilities were observed in Lact. rhamnosus (17·5%), Lact. casei (17·2%) and Lact. acidophilus (15·1%). Salm. Typhimurium had the highest affinity to xylene, showing the hydrophobicity of 53·7%. The numbers of L. monocytogenes biofilm cells during the competition, exclusion and displacement assays were effectively reduced by more than 3 log when co-cultured with Lact. paracasei and Lact. rhamnosus. The results suggest that probiotic strains can be used as alternative way to effectively reduce the biofilm formation in pathogenic bacteria through competition, exclusion and displacement.