Food preservatives influence biofilm formation, gene expression and small RNAs in Salmonella enterica

Common food preservatives impose distinct selective pressures on Salmonella Typhimurium planktonic and biofilm populations

Food preservatives are crucial in controlling microbial growth in processed foods to maintain food safety. Bacterial biofilms pose a threat in the food chain by facilitating persistence on a range of surfaces and food products. Cells in a biofilm are often highly tolerant of antimicrobials and can evolve in response to antimicrobial exposure. Little is known about the efficacy of preservatives against biofilms and their potential impact on the evolution of antimicrobial resistance. In this study we investigated how Salmonella enterica serovar Typhimurium responded to subinhibitory concentrations of four food preservatives (sodium chloride, potassium chloride, sodium nitrite or sodium lactate) when grown planktonically and in biofilms. We found that each preservative exerted a unique selective pressure on S. Typhimurium populations. There was a trade-off between biofilm formation and growth in the presence of three of the four preservatives, where prolonged preservative exposure resulted in reduced biofilm biomass and matrix production over time. All three preservatives selected for mutations in global stress response regulators rpoS and crp. There was no evidence for any selection of cross-resistance to antibiotics after preservative exposure. In conclusion, we showed that preservatives affect biofilm formation and bacterial growth in a compound specific manner. We showed trade-offs between biofilm formation and preservative tolerance, but no antibiotic cross-tolerance. This indicates that bacterial adaptation to continuous preservative exposure, is unlikely to affect food safety or contribute to antibiotic resistance.

Genetic, physiological, and cellular heterogeneities of bacterial pathogens in food matrices: Consequences for food safety

In complex food systems, bacteria live in heterogeneous microstructures, and the population displays phenotypic heterogeneities at the single-cell level. This review provides an overview of spatiotemporal drivers of phenotypic heterogeneity of bacterial pathogens in food matrices at three levels. The first level is the genotypic heterogeneity due to the possibility for various strains of a given species to contaminate food, each of them having specific genetic features. Then, physiological heterogeneities are induced within the same strain, due to specific microenvironments and heterogeneous adaptative responses to the food microstructure. The third level of phenotypic heterogeneity is related to cellular heterogeneity of the same strain in a specific microenvironment. Finally, we consider how these phenotypic heterogeneities at the single-cell level could be implemented in mathematical models to predict bacterial behavior and help ensure microbiological food safety.

Ethanol at Subinhibitory Concentrations Enhances Biofilm Formation in Salmonella Enteritidis

The survival of Salmonella Enteritidis in the food chain is relevant to its biofilm formation capacity, which is influenced by suboptimal environmental conditions. Here, biofilm formation pattern of this bacterium was assessed in the presence of ethanol at sub-minimal inhibitory concentrations (sub-MICs) by microtiter plate assays, cell characteristic analyses, and gene expression tests. It was observed that ethanol at subinhibitory concentrations (1/4 MIC, 2.5%; 1/2 MIC, 5.0%) was able to stimulate biofilm formation in S. Enteritidis. The OD₅₉₅ value (optical density at 595 nm) used to quantify biofilm production was increased from 0.14 in control groups to 0.36 and 0.63 under 2.5% and 5.0% ethanol stresses, respectively. Ethanol was also shown to reduce bacterial swimming motility and enhance cell auto-aggregation ability. However, other cell characteristics such as swarming activity, initial attachment and cell surface hydrophobicity were not remarkedly impacted by ethanol. Reverse transcription quantitative real-time PCR (RT-qPCR) analysis further revealed that the luxS gene belonging to a quorum-sensing system was upregulated by 2.49- and 10.08-fold in the presence of 2.5% and 5.0% ethanol, respectively. The relative expression level of other biofilm-related genes (adrA, csgB, csgD, and sdiA) and sRNAs (ArcZ, CsrB, OxyS, and SroC) did not obviously change. Taken together, these findings suggest that decrease in swimming motility and increase in cell auto-aggregation and quorum sensing may result in the enhancement of biofilm formation by S. Enteritidis under sublethal ethanol stress.

Hydroethanolic Extracts of Haplopappus baylahuen Remy and Aloysia citriodora Palau Have Bactericide Activity and Inhibit the Ability of Salmonella Enteritidis to Form Biofilm and Adhere to Human Intestinal Cells

We analysed whether the hydroethanolic extracts from leaves of Haplopappus baylahuen Remy (bailahuen) and Aloysia citriodora Palau (cedron) inhibit the growth and ability of Salmonella Enteritidis to form biofilms and to adhere to human intestinal epithelial cells. Herein, we first determined the total phenolic content and antioxidant and antibacterial activities of the extracts. Then, Salmonella Enteritidis was treated with the extracts to analyse biofilm formation by scanning electronic microscopy and the violet crystal test. We also measured the efflux pump activity of Salmonella Enteritidis since biofilm formation is associated with this phenomenon. Furthermore, the human intestinal cell line Caco-2 was infected with Salmonella Enteritidis pretreated with the extracts, and 30 min later, the number of bacteria that adhered to the cell surface was quantified. Finally, we determined by qPCR the expression of genes associated with biofilm formation, namely, the diguanilate cyclase AdrA protein gene (adrA) and the BapA protein gene (bapA), and genes associated with adhesion, namely, the transcriptional regulator HilA (hilA). The phenolic content and antioxidant and bactericide activities were higher in bailahuen than in the cedron extract. Biofilm formation was inhibited by the extracts in a dose-dependent manner, while the activity of efflux pumps was decreased only with the cedron extract. Adhesion to Caco-2 cells was also inhibited without differences between doses and extracts. The extracts decreased the expression of adrA; with the cedron extract being the most efficient. The expression of hilA is affected only with the cedron extract. We concluded that hydroethanolic extracts of bailahuen and cedron differentially inhibit the growth of Salmonella Enteritidis and affect its the ability to form biofilms and to adhere to human intestinal epithelial cells. These results highlight the presence of molecules in bailahuen and cedron with a high potential for the control of the Salmonella Enteritidis pathogenesis.

Environmental persistence and virulence of Salmonella spp. Isolated from a poultry slaughterhouse

Salmonella spp. is responsible for severe foodborne disease, and is one of the main agents involved in foodborne outbreaks worldwide. Contamination occurs mainly as a result of poultry and egg consumption since they can carry some serotypes pathogenic to humans. The aim of the study was to evaluate the persistence and pathogenic potential of Salmonella spp. (n = 40) isolated from poultry slaughterhouse mats, using adhesion and invasion assays, antimicrobial susceptibility by disc diffusion, and biofilm production as phenotypic tests and genotypic analyses. Polystyrene mats presented 3.2 times greater chance of isolating Salmonella than canvas mats. Besides, we observed resistance to tetracycline (17.5%), ampicillin (10%), cefotaxime (7.5%), trimethoprim-sulfamethoxazole (5%), and chloramphenicol (2.5%). All strains possessed the invA, sipB, sipD, ssaR, sifA, sitC, iroN, tolC, flgK, fljB, and flgL genes. The genes sopB and sipA were both present in 92.5% of the isolates, while sopD and spvB were observed in 90% and 32.5% of strains, respectively. All strains adhered to and invaded HeLa cells. Regarding biofilm production, 31 (77.5%) strains were able to produce biofilm on polystyrene microplates. Using PFGE, we detected the persistence of clones in the environment for up to 18 fromthe 20 weeks. The ability of these strains to produce a biofilm and thus persist in the environment and disperse through contact surfaces in the processing plant favors the contamination of food, aggravated by the pathogenic potential of these isolates demonstrated by their adhesion capacity, invasion and resistance to various antibiotic agents.

Antimicrobial activity of apitoxin from Apis mellifera in Salmonella enterica strains isolated from poultry and its effects on motility, biofilm formation and gene expression

Salmonella is a major global food-borne pathogen. One of the main concerns related to Salmonella and other food-borne pathogens is their capacity to acquire antimicrobial resistance and produce biofilms. Due to the increased resistance to common antimicrobials used to treat livestock animals and human infections, the discovery of new antimicrobial substances is one of the main challenges in microbiological research. An additional challenge is the development of new methods and substances to inhibit and destruct biofilms. We determined the antimicrobial and antibiofilm activities of apitoxin in 16 Salmonella strains isolated from poultry. In addition, the effect of apitoxin on Salmonella motility and the expression of biofilm- and virulence-related genes was evaluated. The minimum inhibitory concentrations (MIC) of apitoxin ranged from 1,024-256 μg/mL, with 512 μg/mL being the most common. Sub-inhibitory concentrations of apitoxin significantly reduced biofilm formation in 14 of the 16 Salmonella strains tested, with significant increases in motility. MIC concentrations of apitoxin destroyed the pre-formed biofilm by 27.66-68.22% (47.00% ± 10.91). The expression of biofilm- and virulence-related genes and small RNAs was differentially regulated according to the strain and the presence of apitoxin. The transcription of the small RNAs dsrA and csrB, related to antimicrobial resistance, was upregulated in the presence of apitoxin. We suggest that apitoxin is a potential antimicrobial substance that could be used in combination with other substances to develop new drugs and sanitizers against food-borne pathogens.

Current Challenges and Perspectives for the Use of Aqueous Plant Extracts in the Management of Bacterial Infections: The Case-Study of Salmonella enterica Serovars

Worldwide, foodborne diseases are a growing public health problem. Among the infectious bacteria, non-typhoidal Salmonella enterica serovars (NTS) are the major cause of hospitalization and death, and the emergence and spread of their antibiotic-resistance is becoming a worldwide health issue. This, coupled with the restrictions of antibiotics use in agriculture and animal production, calls for alternative approaches to solve this problem. Plant-derived aqueous extracts compounds could provide novel straightforward approaches to control pathogenic bacteria. This review discusses the antimicrobial activity of aqueous plant extracts against Salmonella serovars, the possible mechanisms of action involved, which components/structures might be responsible for such activity, and the current challenges for the use of these extracts/components in Salmonella infection management and their application perspectives.