Salmonella enterica Serovar Kentucky Flagella Are Required for Broiler Skin Adhesion and Caco-2 Cell Invasion

Whole-genome sequencing-based characterization of Salmonella enterica Serovar Enteritidis and Kentucky isolated from laying hens in northwest of Iran, 2022-2023

BACKGROUND

The transmission of Salmonella spp. to human through the consumption of contaminated food products of animal origin, mainly poultry is a significant global public health concern. The emerging multidrug resistant (MDR) clones of non-typhoidal Salmonella (NTS) serovars, have spread rapidly worldwide both in humans and in the food chain. In this study NTS strains were isolated from diseased laying hens in Iran and were further studied by whole-genome sequencing (WGS) to investigate the prevalent serovars, multilocus sequence types, antimicrobial resistance and virulence genes.

RESULTS

Out of eight isolated Salmonella spp. six were identified as S. Enteritidis serovar ST11 (n = 5) or ST5824 (n = 1), and two isolates were recognized as S. Kentucky serotype ST198 lineages. The aminoglycoside resistance gene aac(6')-Iaa was the most frequently detected gene being present in all serovars, but it did not confer phenotypic resistance to corresponding agents (tobramycin and amikacin). All S. Enteritidis isolates carried a single GyrA D87N/Y substitution. Other identified antimicrobial resistance genes (ARGs) including tetA, floR, sul1, dfrA1, aph(3')-Ia and double gyrA and parC mutations conferring high-level ciprofloxacin resistance (CIPR) (MIC ≥ 16mg/L) were only found in S. Kentucky isolates. The comparison of phenotypic and genotypic antimicrobial resistance (AMR) profiles revealed inconsistent results for some antibiotics. A total of 11 different Salmonella Pathogenicity Islands (SPIs) including SPIs-1, to 5, 9, 10, 13, 14, C63PI, CS54 and several virulence genes related to type III secretion system, adhesins, iron and magnesium uptake, serum and antimicrobial peptide resistance were detected among the isolates.

CONCLUSIONS

Our study reports emergence of a highly MDR- CIPR S. Kentucky ST198 clone form poultry associated sources in Iran. The presence of numerous virulence determinants, SPIs and ARGs in the examined NTS isolates poses a significant risk for food safety. The inconsistencies between the genotypic and phenotypic AMR profiles indicate that WGS data alone may not be always sufficient for guiding therapeutic strategies.

Immune responses of chickens against recombinant Salmonella enterica serotype Heidelberg FimA and FimW fimbriae and FliD and FlgK flagellar proteins

Implementation of a vaccination program is one of the most effective means to control infectious diseases during food animal production. Salmonella, a Gram-negative bacterium, is a leading bacterial cause of human foodborne illnesses worldwide. The major source of this microorganism for human infection is from consumption of unsanitary poultry products. Although live attenuated vaccines are available, these vaccines suffer from problems including persistence and shedding of Salmonella in and from the vaccinated animals. To overcome these problems, the recombinant Salmonella enterica serotype Heidelberg FliD, FlgK, FimA and FimW subunit proteins that are surface-exposed were produced and tested for their immunogenicity in chickens in this study. As expected, there were no detrimental signs observed in chickens after vaccination during the six-week experimental period. These four proteins migrated in a single band to their respective positions. Analysis of immune responses to the proteins reveals that the immunoglobulin (Ig) G, IgM and IgA from most vaccinated chickens reacted strongly to the recombinant FliD and FlgK proteins, but not from unvaccinated chickens. On the other hand, IgG, IgM and IgA antibody responses to FimA and FimW from the vaccinated group were no difference from those from unvaccinated chickens, suggesting that the FimA and FimW proteins may be not good antigens, potentially due to their size, composition, and/or structural complexity. In addition, IgG could be induced by FliD and FlgK after a single vaccination. These antibody studies suggest that recombinant FliD and FlgK have potential as targets for vaccine development. Because of the importance of bacterial fimbriae in pathogenesis and for immunogenicity, a chimeric protein of the FimA and FimW proteins is needed.

Natural anti-adhesive components against pathogenic bacterial adhesion and infection in gastrointestinal tract: case studies of Helicobacter pylori, Salmonella enterica, Clostridium difficile, and diarrheagenic Escherichia coli

Antimicrobial resistance (AMR) poses a global public health concern. Recognizing the critical role of bacterial adhesion in pathogenesis of infection, anti-adhesive therapy emerges as a promising approach to impede initial bacterial attachment, thus preventing pathogenic colonization and infection. Natural anti-adhesive agents derived from food sources are generally safe and have the potential to inhibit the emergence of resistant bacteria. This comprehensive review explored diverse natural dietary components exhibiting anti-adhesive activities against several model enteric pathogens, including Helicobacter pylori, Salmonella enterica, Clostridium difficile, and three key diarrheagenic Escherichia coli (i.e., enterotoxigenic E. coli, enteropathogenic E. coli, and enterohemorrhagic E. coli). Investigating various anti-adhesive products will advance our understanding of current research of the field and inspire further development of these agents as potential nutraceuticals or adjuvants to improve the efficacy of conventional antibiotics.

Recombinant Lactococcus lactis secreting FliC protein nanobodies for resistance against Salmonella enteritidis invasion in the intestinal tract

Salmonella Enteritidis is a major foodborne pathogen throughout the world and the increase in antibiotic resistance of Salmonella poses a significant threat to public safety. Natural nanobodies exhibit high affinity, thermal stability, ease of production, and notably higher diversity, making them widely applicable for the treatment of viral and bacterial infections. Recombinant expression using Lactococcus lactis leverages both acid resistance and mucosal colonization properties of these bacteria, allowing the effective expression of exogenous proteins for therapeutic effects. In this study, nine specific nanobodies against the flagellar protein FliC were identified and expressed. In vitro experiments demonstrated that FliC-Nb-76 effectively inhibited the motility of S. Enteritidis and inhibited its adhesion to and invasion of HIEC-6, RAW264.7, and chicken intestinal epithelial cells. Additionally, a recombinant L. lactis strain secreting the nanobody, L. lactis-Nb76, was obtained. Animal experiments confirmed that it could significantly reduce the mortality rates of chickens infected with S. Enteritidis, together with alleviating the inflammatory response caused by the pathogen. These results provide a novel strategy for the treatment of antibiotic-resistant S. Enteritidis infection in the intestinal tract.

Role of narL gene in the pathogenesis of Salmonella Typhimurium

Salmonella Typhimurium (STM) is a facultative anaerobe and one of the causative agents of nontyphoidal salmonellosis (NTS). Its anaerobic metabolism is enabled under the hypoxic environment that is encountered inside macrophages and the gut lumen of the host. In both of these niches, free radicals and oxidative intermediates are released by neutrophils as an inflammatory response. These chemical species further undergo reactions to produce nitrate, which is preferably taken up by STM as an electron acceptor in the absence of oxygen. NarL, the response regulator of the two-component regulatory system NarX/L, and a transcription factor, gets activated under anaerobic nitrate-rich conditions and upregulates the nitrate reduction during anaerobic respiration of STM. To understand the role of NarL in the pathogenesis of STM, we generated a narL-knockout (STM:ΔnarL) as well as a narL-complemented strain of STM. Anaerobically, the mutant displayed no growth defect but a significant attenuation in the swimming (26%) and swarming (61%) motility, and biofilm-forming ability (73%) in vitro, while these morphotypes got rescued upon genetic complementation. We also observed a downregulation in the expression of genes associated with nitrate reduction (narG) and biofilm formation (csgA and csgD) in anaerobically grown STM:ΔnarL. As compared with wild STM, narL mutant exhibited a threefold reduction in the intracellular replication in both intestinal epithelial cells (INT- 407) and monocyte-derived macrophages of poultry origin. Further, in vivo competitive assay in the liver and spleen of the murine model showed a competitive index of 0.48 ± 0.58 and 0.403668 ± 0.32, respectively, for STM:ΔnarL.

Spermidine constitutes a key determinant of motility and attachment of Salmonella Typhimurium through a novel regulatory mechanism

Spermidine is a poly-cationic molecule belonging to the family of polyamines and is ubiquitously present in all organisms. Salmonella synthesizes, and harbours specialized transporters to import spermidine. A group of polyamines have been shown to assist in Salmonella Typhimurium's virulence and regulation of Salmonella pathogenicity Inslad 1 (SPI-1) genes and stress resistance; however, the mechanism remains elusive. The virulence trait of Salmonella depends on its ability to employ multiple surface structures to attach and adhere to the surface of the target cells before invasion and colonization of the host niche. Our study discovers the mechanism by which spermidine assists in the early stages of Salmonella pathogenesis. For the first time, we report that Salmonella Typhimurium regulates spermidine transport and biosynthesis processes in a mutually inclusive manner. Using a mouse model, we show that spermidine is critical for invasion into the murine Peyer's patches, which further validated our in vitro cell line observation. We show that spermidine controls the mRNA expression of fimbrial (fimA) and non-fimbrial adhesins (siiE, pagN) in Salmonella and thereby assists in attachment to host cell surfaces. Spermidine also regulated the motility through the expression of flagellin genes by enhancing the translation of sigma-28, which features an unusual start codon and a poor Shine-Dalgarno sequence. Besides regulating the formation of the adhesive structures, spermidine tunes the expression of the two-component system BarA/SirA to regulate SPI-1 encoded genes. Thus, our study unravels a novel regulatory mechanism by which spermidine exerts critical functions during Salmonella Typhimurium pathogenesis.

The role and molecular mechanism of flgK gene in biological properties, pathogenicity and virulence genes expression of Aeromonas hydrophila

Aeromonas hydrophila is a highly pathogenic aquatic resident bacterium that can cause co-morbidity in aquatic animals, waterfowl, poultry, and humans. Flagellum is the motility organ of bacteria important for bacterium tissue colonization and invasion. The flgK gene encodes a flagellar hook protein essential for normal flagellar formation. In order to explore the role of flgK in A. hydrophila, a flgK gene mutant strain of A. hydrophila (∆flgK-AH) was constructed using an efficient suicide plasmid-mediated homologous recombination method, and gene sequencing confirmed successful mutation of the flgK gene. The biological properties, pathogenicity and virulence genes expression were compared. The results showed that there was no significant difference in the growth, hemolytic, and swarming abilities, but the swimming and biofilm formation abilities of ∆flgK-AH were significantly reduced and the transmission electron microscope (TEM) results showed that the ∆flgK-AH strain did not have a flagellar structure. The median lethal dose (LD50) value of the ∆flgK-AH in Carassius auratus was 1.47-fold higher than that of the wild-type strain (WT-AH). The quantitative real-time PCR results showed that only the expression level of the lapA gene was up-regulated by 1.47 times compared with the WT-AH, while the expression levels of other genes were significantly down-regulated. In conclusion, flgK gene mutant led to a decline in the pathogenicity possibly by reducing swimming and biofilm formation abilities, these biological properties might result from the down-regulated expression of flagellate and pilus-related genes.

Effects and molecular mechanism of flagellar gene flgK on the motility, adhesion/invasion, and desiccation resistance of Cronobacter sakazakii

Cronobacter sakazakii (C. sakazakii), a food-borne pathogen, can infect neonates, elderly and immunocompromised populations with a high infection and mortality rate. However, the specific molecular mechanism of its motility, biofilm formation, cell adhesion, and desiccation resistance remains unclear, and flagellum hook associated protein (FlgK), a main component of the flagellar complex, may be an important determinant of its virulence and desiccation resistance. In this study, the flgK mutant strain (ΔflgK) was constructed using the homologous recombination method, and the cpflgK complementary strain was obtained by gene complementation, followed by analysis of the difference between the wild type (WT), mutant, and complementary strains in mobility, biofilm formation, cell adhesion, and desiccation resistance. Results indicated that flgK gene played a positive role in motility and invasion, with no significant effect on biofilm formation. Interestingly, flagellar assembly gene deletion showed increased resistance of C. sakazakii to dehydration. The mechanism underlying the negative correlation of flgK gene with dehydration resistance was further investigated by using the high-throughput sequencing technology to compare the gene expression between WT and ΔflgK strains after drying. The results revealed up-regulation in the expression of 54 genes, including genes involved in osmosis and formate dehydrogenase, while down-regulation in the expression of 50 genes, including genes involved in flagellum hook and nitrate reductase. qRT-PCR analysis of the RNA-seq data further indicated that the flgK gene played an important role in the environmental stress resistance of C. sakazakii by up-regulating the formate dehydrogenase, betaine synthesis, and arginine deiminase pathways, due to dynamic proton imbalance caused by lack of flagella. This study facilitates our understanding of the roles of flgK in motion-related functions and the molecular mechanism of desiccation resistance in C. sakazakii.

Transcriptomic and metabolomic insights into the role of the flgK gene in the pathogenicity of Pseudomonas plecoglossicida to orange-spotted grouper (Epinephelus coioides)

Pseudomonas plecoglossicida is the pathogen responsible for visceral white spot disease in large yellow croaker (Larimichthys crocea) and orange-spotted grouper (Epinephelus coioides). Previously, RNA sequencing showed that P. plecoglossicida flgK gene expression was significantly up-regulated in orange-spotted grouper spleens during infection. To explore the role of flgK in P. plecoglossicida pathogenicity, RNA interference (RNAi) was performed to silence the P. plecoglossicida flgK gene, and the mutant (flgK-RNAi strain) with the best silencing efficiency (89.40%) was chosen for further study. Results showed that flgK gene silencing significantly attenuated P. plecoglossicida motility, adhesion, and biofilm formation. Compared to those fish infected with the wild-type strain of P. plecoglossicida, orange-spotted grouper infected with the flgK-RNAi strain showed a 55% increase in the survival rate and a one-day delay in time of first death, with fewer pathogens in the spleen and fewer white spots on the spleen surface. RNAi of flgK significantly affected the transcriptome and metabolome of the spleen in infected orange-spotted grouper. Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis showed that the C-type lectin receptor signaling pathway was the most significantly changed immune-related pathway and the mitogen-activated protein kinase (MAPK) signaling pathway was related to multiple immune-related pathways. Furthermore, arginine biosynthesis and glycerophospholipid metabolism were the most significantly changed metabolism-related pathways. These findings suggest that flgK is a virulence gene of P. plecoglossicida. Furthermore, flgK appears to be involved in the regulation of motility, adhesion, and biofilm formation in P. plecoglossicida, as well as in the regulation of inflammatory and immune responses of orange-spotted grouper to P. plecoglossicida infection.

Secretome analysis of an environmental isolate Enterobacter sp. S-33 identifies proteins related to pathogenicity

Enterobacter species are responsible for causing infections of the lower respiratory tract, urinary tract, meninges, etc. Proteins secreted by these species may act as determinants of host-pathogen interaction and play a role in virulence. Among the secreted proteins, the Type VI secretion system (T6SS) acts as a molecular nanomachine to deliver many effector proteins directly into prey cells in a contact-dependent manner. The secreted proteins may provide an idea for the interaction of bacteria to their environment and an understanding of the role of these proteins for their role in bacterial physiology and behaviour. Therefore, aim of this study was to characterize the secreted proteins in the culture supernatant by a T6SS bacterium Enterobacter sp. S-33 using nano-LC-MS/MS tool. Using a combined mass spectrometry and bioinformatics approach, we identified a total of 736 proteins in the secretome. Bioinformatics analysis predicting subcellular localization identified 110 of the secreted proteins possessed signal sequences. By gene ontology analysis, more than 80 proteins of the secretome were classified into biological or molecular functions. More than 20 percent of secretome proteins were virulence proteins including T6SS proteins, proteins involved in adherence and fimbriae formation, molecular chaperones, outer membrane proteins, serine proteases, antimicrobial, biofilm, exotoxins, etc. In summary, the results of the present study of the S-33 secretome provide a basis for understanding the possible pathogenic mechanisms and future investigation by detailed experimental approach will provide a confirmation of secreted virulence proteins in the exact role of virulence using the in vivo model.

Phase-variable expression of pdcB, a phosphodiesterase, influences sporulation in Clostridioides difficile

Clostridioides difficile is the causative agent of antibiotic-associated diarrhea and is the leading cause of nosocomial infection in developed countries. An increasing number of C. difficile infections are attributed to epidemic strains that produce more toxins and spores. C. difficile spores are the major factor for the transmission and persistence of the organism. Previous studies have identified global regulators that influence sporulation in C. difficile. This study discovers that PdcB, a phosphodiesterase, enhances sporulation in C. difficile strain UK1. Through genetic and biochemical assays, we show that phase-variable expression of pdcB results in hypo- and hyper-sporulation phenotypes. In the "ON" orientation, the identified promotor is in the right orientation to drive the expression of pdcB. Production of the PdcB phosphodiesterase reduces the intracellular cyclic-di-GMP (c-di-GMP) concentration, resulting in a hyper-sporulation phenotype. Loss of PdcB due to the pdcB promoter being in the OFF orientation or mutation of pdcB results in increased c-di-GMP levels and a hypo-sporulation phenotype. Additionally, we demonstrate that CodY binds to the upstream region of pdcB. DNA inversion reorients the CodY binding site so that in the OFF orientation, CodY binds a site that is upstream of the pdcB promoter and can further repress gene expression.

Salmonella Typhimurium encoded cold shock protein E is essential for motility and biofilm formation

The ability of bacteria to form biofilms increases their survival under adverse environmental conditions. Biofilms have enormous medical and environmental impact; consequently, the factors that influence biofilm formation are an important area of study. In this investigation, the roles of two cold shock proteins (CSP) during biofilm formation were investigated in Salmonella Typhimurium, which is a major foodborne pathogen. Among all CSP transcripts studied, the expression of cspE (STM14_0732) was higher during biofilm growth. The cspE deletion strain (ΔcspE) did not form biofilms on a cholesterol coated glass surface; however, complementation with WT cspE, but not the F30V mutant, was able to rescue this phenotype. Transcript levels of other CSPs demonstrated up-regulation of cspA (STM14_4399) in ΔcspE. The cspA deletion strain (ΔcspA) did not affect biofilm formation; however, ΔcspEΔcspA exhibited higher biofilm formation compared to ΔcspE. Most likely, the higher cspA amounts in ΔcspE reduced biofilm formation, which was corroborated using cspA over-expression studies. Further functional studies revealed that ΔcspE and ΔcspEΔcspA exhibited slow swimming but no swarming motility. Although cspA over-expression did not affect motility, cspE complementation restored the swarming motility of ΔcspE. The transcript levels of the major genes involved in motility in ΔcspE demonstrated lower expression of the class III (fliC, motA, cheY), but not class I (flhD) or class II (fliA, fliL), flagellar regulon genes. Overall, this study has identified the interplay of two CSPs in regulating two biological processes: CspE is essential for motility in a CspA-independent manner whereas biofilm formation is CspA-dependent.

The responses of Salmonella enterica serovar Typhimurium to vanillin in apple juice through global transcriptomics

Salmonella enterica serovar Typhimurium can survive some extreme environment in food processing, and vanillin generally recognized as safe is bactericidal to pathogens. Thus, we need to explore the responses of S. Typhimurium to vanillin in order to apply this antimicrobial agent in food processing. In this study, we exposed S. Typhimurium to commercial apple juice with/without vanillin (3.2 mg/mL) at 45 °C for 75 min to determine the survival rate. Subsequently, the 10-min cultures were selected for transcriptomic analysis. Using high-throughput RNA sequencing, genes related to vanillin resistance and their expression changes of S. Typhimurium were identified. The survival curve showed that S. Typhimurium treated with vanillin were inactivated by 5.5 log after 75 min, while the control group only decreased by 2.3 log. Such a discrepancy showed the significant antibacterial effect of vanillin on S. Typhimurium. As a result, 265 differentially expressed genes (DEGs) were found when coping with vanillin, among which, 225 showed up-regulation and 40 DEGs were down-regulated. Treated with vanillin, S. Typhimurium significantly up-regulated genes involved in cell membrane, acid tolerance response (ATR) and oxidative stress response, cold shock cross-protection, DNA repair, virulence factors and some key regulators. Firstly, membrane-related genes, including outer membrane (bamE, mepS, ygdI, lolB), inner membrane (yaiY, yicS) and other proteins (yciC, yjcH), were significantly up-regulated because of the damaged cell membrane. Then, up-regulated proteins associated with arginine synthesis (ArgABCDIG) and inward transportation (ArtI, ArtJ, ArtP and HisP), participated in ATR to pump out the protons inside the cell in this scenario. Next, superoxide stress response triggered by vanillin was found to have a significant up-regulation as well, which was controlled by SoxRS regulon. Besides, NADH-associated (nuoA, nuoB, nuoK, nadE, fre and STM3021), thioredoxin (trxA, trxC, tpx and bcp) and glutaredoxin (grxC and grxD) DEGs led to the increase of the oxidative stress response. Cold shock proteins such as CspA and CspC showed an up-regulation, suggesting it might play a role in cross-protecting S. Typhimurium from vanillin stress. Furthermore, DEGs in DNA repair and virulence factors, including flagellar assembly, adhesins and type III secretion system were up-regulated. Some regulators like fur, rpoE and csrA played a pivotal role in response to the stress caused by vanillin. Therefore, this study sounds an alarm for the risks caused by stress tolerance of S. Typhimurium in food industry.

Effect of lemongrass essential oil against multidrug-resistant Salmonella Heidelberg and its attachment to chicken skin and meat

Salmonella Heidelberg (S. Heidelberg) is a major pathogen implicated in foodborne outbreaks for which poultry products can serve as an epidemiological source. This study determined the efficacy of GRAS-status lemongrass essential oil (LGEO) against S. Heidelberg in vitro and on the pathogen's attachment to skin and meat. At first, employing in vitro assays, the effect of LGEO on multidrug-resistant S. Heidelberg multiplication and motility was examined. Biofilm inhibition and inactivation assays were also performed. The quorum-sensing modulating effect of LGEO was determined. In follow-up experiments, chicken skin or meat samples inoculated with S. Heidelberg were treated with various concentrations of LGEO at different time points at simulated scalding (54°C) and chilling (4°C) temperatures. The samples were incubated, and the surviving populations of S. Heidelberg were enumerated to determine if LGEO could be a potential processing aid in poultry operations. Duplicate samples were included in each treatment, and the experiments were repeated at least 3 times. Significant reductions of S. Heidelberg of at least 4.0 log₁₀ CFU/mL after 24 h in nutrient broth and poultry cecal contents was observed with 0.5% LGEO. Complete inhibition of motility, biofilm formation, and inactivation of pre-formed biofilms was observed with 0.15% LGEO (P ≤ 0.05). Concentrations of LGEO at 0.5% and 1% affected violacein production (P ≤ 0.05). On skin samples, all concentrations significantly reduced S. Heidelberg by 1.2 to 3.9 log₁₀ CFU/sample after 2 min at 54°C. We obtained a significant reduction of the pathogen in meat samples at 54°C and skin samples at 4°C with 2% LGEO. All concentrations significantly reduced S. Heidelberg from the treatment water kept at 4°C and 54°C (P ≤ 0.05). In conclusion, LGEO could potentially serve as a natural antimicrobial strategy in scalding and chilling waters to reduce S. Heidelberg during processing. However, additional studies are warranted before recommending its commercial use.

Salmonella Contamination of Broiler Chicken Carcasses at Critical Steps of the Slaughter Process and in the Environment of Two Slaughter Plants: Prevalence, Genetic Profiles, and Association with the Final Carcass Status

ABSTRACT

Salmonella is a foodborne pathogen commonly associated with poultry products. The aims of this work were to (i) estimate the impact of critical steps of the slaughter process on Salmonella detection from broiler chicken carcasses in two commercial poultry slaughter plants in Quebec, Canada; (ii) investigate the presence of Salmonella in the slaughter plant environment; (iii) describe, using a high-resolution melting (HRM) approach, the HRM Salmonella profiles and serotypes present on carcasses and in the slaughter plant environment; and (iv) evaluate whether the HRM flock status after chilling could be predicted by the flock status at previous steps of the slaughter process, the status of previous flocks, or the status of the processing environment, for the same HRM profile. Eight visits were conducted in each slaughter plant over a 6-month period. In total, 379 carcass rinsates from 79 flocks were collected at five critical steps of the slaughter process. Environmental samples were also collected from seven critical sites in each slaughter plant. The bleeding step was the most contaminated, with >92% positive carcasses. A decrease of the contamination along the slaughtering process was noted, with carcasses sampled after dry-air chilling showing ≤2.5% Salmonella prevalence. The most frequently isolated serotypes were Salmonella Heidelberg, Kentucky, and Schwarzengrund. The detection of the Salmonella Heidelberg 1-1-1 HRM profile on carcasses after chilling was significantly associated with its detection at previous steps of the slaughter process and in previously slaughtered flocks from other farms during a same sampling day. Results highlight the importance of the chilling step in the control of Salmonella on broiler chicken carcasses and the need to further describe and compare the competitive advantage of Salmonella serotypes to survive processing. The current study also illustrates the usefulness of HRM typing in investigating Salmonella contamination along the slaughter process.

HIGHLIGHTS

Salmonella enterica's "Choice": Itaconic Acid Degradation or Bacteriocin Immunity Genes

Itaconic acid is an immunoregulatory metabolite produced by macrophages in response to pathogen invasion. It also exhibits antibacterial activity because it is an uncompetitive inhibitor of isocitrate lyase, whose activity is required for the glyoxylate shunt to be operational. Some bacteria, such as Yersinia pestis, encode enzymes that can degrade itaconic acid and therefore eliminate this metabolic inhibitor. Studies, primarily with Salmonella enterica subspecies enterica serovar Typhimurium, have demonstrated the presence of similar genes in this pathogen and the importance of these genes for the persistence of the pathogen in murine hosts. This minireview demonstrates that, based on Blast searches of 1063 complete Salmonella genome sequences, not all Salmonella serovars possess these genes. It is also shown that the growth of Salmonella isolates that do not possess these genes is sensitive to the acid under glucose-limiting conditions. Interestingly, most of the serovars without the three genes, including serovar Typhi, harbor DNA at the corresponding genomic location that encodes two open reading frames that are similar to bacteriocin immunity genes. It is hypothesized that these genes could be important for Salmonella that finds itself in strong competition with other Enterobacteriacea in the intestinal tract—for example, during inflammation.

Insights into coumarin-mediated inhibition of biofilm formation in Salmonella Typhimurium

Coumarins have been shown to possess antimicrobial, anti-quorum sensing and anti-biofilm properties against a wide range of pathogenic bacteria. This study aimed to shed light on the effects of non-substituted coumarin on biofilm formation by the foodborne pathogen Salmonella Typhimurium. Additionally, its efficacy was tested in combination with another potent anti-biofilm agent, resveratrol. Coumarin inhibited biofilm formation for prolonged periods in millimolar concentrations with marginal effects on planktonic growth. It attenuated curli and cellulose production, likely by downregulating the transcript levels of major biofilm formation genes csgD, csgA and adrA. Coumarin further restricted motility in a dose-dependent manner. In addition, coumarin with resveratrol exhibited improved anti-biofilm properties compared with the individual compounds alone. Thus, coumarin alone or with resveratrol can be employed for inhibiting biofilms in food storage and processing units.

Antivirulence properties and related mechanisms of spice essential oils: A comprehensive review

In recent decades, reduced antimicrobial effectiveness, increased bacterial infection, and newly emerged microbial resistance have become global public issues, leading to an urgent need to find effective strategies to counteract these problems. Strategies targeting bacterial virulence factors rather than bacterial survival have attracted increasing interest, since the modulation of virulence factors may prevent the development of drug resistance in bacteria. Spices are promising natural sources of antivirulence compounds owing to their wide availability, diverse antivirulence phytochemical constituents, and generally favorable safety profiles. Essential oils are the predominant and most important antivirulence components of spices. This review addresses the recent efforts of using spice essential oils to inhibit main bacterial virulence traits, including the quorum sensing system, biofilm formation, motility, and toxin production, with an intensive discussion of related mechanisms. We hope that this review can provide a better understanding of the antivirulence properties of spice essential oils, which have the potential to be used as antibiotic alternatives by targeting bacterial virulence.

Two genomic regions encoding exopolysaccharide production systems have complementary functions in B. cereus multicellularity and host interaction

Bacterial physiology and adaptation are influenced by the exopolysaccharides (EPS) they produce. These polymers are indispensable for the assembly of the biofilm extracellular matrix in multiple bacterial species. In a previous study, we described the profound gene expression changes leading to biofilm assembly in B. cereus ATCC14579 (CECT148). We found that a genomic region putatively dedicated to the synthesis of a capsular polysaccharide (eps2) was overexpressed in a biofilm cell population compared to in a planktonic population, while we detected no change in the transcript abundance from another genomic region (eps1) also likely to be involved in polysaccharide production. Preliminary biofilm assays suggested a mild role for the products of the eps2 region in biofilm formation and no function for the products of the eps1 region. The aim of this work was to better define the roles of these two regions in B. cereus multicellularity. We demonstrate that the eps2 region is indeed involved in bacterial adhesion to surfaces, cell-to-cell interaction, cellular aggregation and biofilm formation, while the eps1 region appears to be involved in a kind of social bacterial motility. Consistent with these results, we further demonstrate using bacterial-host cell interaction experiments that the eps2 region is more relevant to the adhesion to human epithelial cells and the zebrafish intestine, suggesting that this region encodes a bacterial factor that may potentiate gut colonization and enhance pathogenicity against humans.

A Novel Single Domain Antibody Targeting FliC Flagellin of Salmonella enterica for Effective Inhibition of Host Cell Invasion

The enteric pathogen, Salmonella enterica is a major cause of human gastroenteritis globally and with increasing bacterial resistance to antibiotics, alternative solutions are urgently needed. Single domain antibodies (sdAbs), the smallest antibody fragments that retain antigen binding specificity and affinity, are derived from variable heavy-chain only fragments (VHH) of camelid heavy-chain-only immunoglobulins. SdAbs typically contain a single disulfide bond simplifying recombinant protein production in microbial systems. These factors make sdAbs ideally suited for the development of effective anti-bacterial therapeutics. To this end, we generated an anti-Salmonella VHH library from which we screened for high affinity sdAbs. We present a novel sdAb (Abi-Se07) that targets the Salmonella virulence factor, FliC, required for bacterial motility and invasion of host cells. We demonstrate that Abi-Se07 bound FliC with a K D of 16.2 ± 0.1 nM. In addition, Abi-Se07 exhibited cross-serovar binding to whole cells of S. enterica serovar Typhimurium, Heidelberg, and Hadar. Abi-Se07 significantly inhibited bacterial motility and significantly reduced S. enterica colonization in a more native environment of chicken jejunum epithelium. Taken together, we have identified a novel anti-Salmonella sdAb and discuss future efforts toward therapeutic development.

Construction and in vitro characterisation of aroA defective (aroAΔ) mutant Salmonella Infantis

Poultry vaccine programs are important for control of Salmonella infections. Although there are vaccines for Salmonella Enteritidis, Salmonella Typhimurium and Salmonella Typhi, there are no vaccines for Salmonella Infantis which has an increased rate in the world. In this study, it was aimed to generate aroA gene deleted mutant bacteria for the constitution of S. Infantis vaccine prototype and the in vitro characterisation of this bacterium. S. Infantis auxotrophic mutant which has a block at any step of chorismate pathway has been constituted for the first time in the world and it was determined that this bacterium gets susceptibility against some antibiotics and antimicrobial substances. It was also observed that the adhesion and invasion rate of mutant strain tenfold decreased in comparison with the field strain in cell culture assay. It is understood from the in vitro evaluation of this mutant strain that it can be used as a vaccine candidate in further vaccine development studies.

Food Grade Pimenta Leaf Essential Oil Reduces the Attachment of Salmonella enterica Heidelberg (2011 Ground Turkey Outbreak Isolate) on to Turkey Skin

Salmonella attached to the poultry skin is a major source of carcass contamination during processing. Once attached to the poultry skin, it is difficult to detach and inactivate Salmonella by commonly used antimicrobial agents since the pathogen is entrapped deeply in the feather follicles and the crevices on the skin. Essential oils could be natural, safe, and effective alternatives to synthetic antimicrobial agents during commercial and organic processing setup. The present study evaluated the efficacy of pimenta (Pimenta officinalis Lindl.) leaf essential oil (PEO), and its nanoemulsion in reducing Salmonella Heidelberg attachment on to turkey (Meleagris gallopavo) skin during simulated scalding (65°C) and chilling (4°C) steps in poultry processing. A multidrug resistant S. Heidelberg isolate from the 2011 ground turkey outbreak in the United States was used in the study. Results showed that PEO and the nanoemulsion resulted in significant reduction of S. Heidelberg attachment on turkey skin. Turkey skin samples treated with 1.0% PEO for 5 min resulted in >2 log10 CFU/sq. inch reduction of S. Heidelberg at 65 and 4°C, respectively (n = 6; P < 0.05). Similarly, skin samples treated with 1.0% pimenta nanoemulsion (PNE) for 5 min resulted in 1.5- and 1.8- log10 CFU/sq. inch reduction of S. Heidelberg at 65 and 4°C, respectively (n = 6; P < 0.05). In addition, PEO and PNE were effective in reducing S. Heidelberg on skin during short-term storage at 4 and 10°C (temperature abuse) (n = 6; P < 0.05). No Salmonella was detected in the dipping solution containing 0.5 or 1.0% PEO or PNE, whereas a substantial population of the pathogen survived in the control dipping solution. The results were validated using scanning electron -, and confocal - microscopy techniques. PEO or PNE could be utilized as an effective antimicrobial agent to reduce S. Heidelberg attachment to turkey skin during poultry processing.