Acid environments affect biofilm formation and gene expression in isolates of Salmonella enterica Typhimurium DT104

Sublethal chlorine stress promotes the biofilm-forming ability of Salmonella enterica serovars enteritidis and expression of the related genes

Chlorine treatment is the most common disinfection method in food-related environments. In addition to being simple and inexpensive, this method is very effective if used properly. However, insufficient chlorine concentrations only cause a sublethal oxidative stress in the bacterial population and may alter the growth behavior of stressed cells. In the present study, the effect of sublethal chlorine stress on the biofilm formation characteristics of Salmonella Enteritidis was evaluated. Our results demonstrated that, sublethal chlorine stress (350 ppm total chlorine) activates the biofilm (csgD, agfA, adrA and bapA) and quorum-sensing (sdiA and luxS) related genes in planktonic cells of S. Enteritidis. The higher expression of these genes illustrated that the chlorine stress induced the initiation of the biofilm formation process in S. Enteritidis. Results of the initial attachment assay confirmed this finding. In addition, the number of chlorine-stressed biofilm cells was significantly higher than non-stressed biofilm cells after 48 h incubation at 37 °C. In S. Enteritidis ATCC 13076 and S. Enteritidis KL19, the number of chlorine-stressed biofilm cells were 6.93 ± 0.48 and 7.49 ± 0.57 log CFU/cm², while the number of non-stressed biofilm cells were 5.12 ± 0.39 and 5.63 ± 0.51 log CFU/cm², respectively. These findings were confirmed by measurements of the major components of biofilm, i.e., eDNA, protein and carbohydrate. The amount of these components in 48-h biofilms was higher when the cells were initially subjected to sublethal chlorine stress. However, the up-regulation of the biofilm and quorum sensing genes was not observed in 48-h biofilm cells, indicating that the effect of chlorine stress had vanished in the subsequent generations of Salmonella. In total, these results revealed that sublethal chlorine concentrations can promote the biofilm-forming ability of S Enteritidis.

Effect of pH and Salinity on the Ability of Salmonella Serotypes to Form Biofilm

Salmonella is a major cause of food-borne infections in Europe, and the majority of human infections are caused by only a few serotypes, among them are Salmonella enterica subsp. enterica serotype Enteritidis (hereafter Salmonella Enteritidis), Salmonella Typhimurium, and the monophasic variant of S. Typhimurium. The reason for this is not fully understood, but could include virulence factors as well as increased ability to transfer via the external environment. Formation of biofilm is considered an adaptation strategy used by bacteria to overcome environmental stresses. In order to assess the capability of different Salmonella serotypes to produce biofilm and establish whether this is affected by pH and salinity, 88 Salmonella isolates collected from animal, food, and human sources and belonging to 15 serotypes, including those most frequently responsible for human infections, were tested. Strains were grown in tryptic soy broth (TSB), TSB with 4% NaCl pH 4.5, TSB with 10% NaCl pH 4.5, TSB with 4% NaCl pH 7, or TSB with 10% NaCl pH 7, and biofilm production was assessed after 24 h at 37°C using crystal violet staining. A linear mixed effect model was applied to compare results from the different experimental conditions. Among the tested serotypes, S. Dublin showed the greatest ability to form biofilm even at pH 4.5, which inhibited biofilm production in the other tested serotypes. Salmonella Senftenberg and the monophasic variant of S. Typhimurium showed the highest biofilm production in TSB with 10% NaCl pH 7. In general, pH had a high influence on the ability to form biofilm, and most of the tested strains were not able to produce biofilm at pH 4.5. In contrast, salinity only had a limited influence on biofilm production. In general, serotypes causing the highest number of human infections showed a limited ability to produce biofilm in the tested conditions, indicating that biofilm formation is not a crucial factor in the success of these clones.

Differences in biofilm formation of Salmonella serovars on two surfaces under two temperature conditions

AIMS

Salmonella is extremely diverse, with >2500 serovars that are genetically and phenotypically diverse. The aim of this study was to build a collection of Salmonella isolates that are genetically diverse and to evaluate their ability to form biofilm under different conditions relevant to a processing environment.

METHODS AND RESULTS

Twenty Salmonella isolates representative of 10 serovars were subtyped using Clustered regularly interspaced short palindromic repeats (CRISPR)-typing to assess the genetic diversity between isolates of each serovar. Biofilm formation of the isolates on both plastic and stainless-steel surfaces at 25 and 15°C was assessed. At 25°C, 8/20 isolates each produced strong and moderate biofilm on plastic surface compared to stainless-steel (3/20 and 13/20 respectively). At 15°C, 5/20 produced strong biofilm on plastic surface and none on stainless-steel. Several isolates produced weak biofilm on plastic (11/20) and stainless-steel (16/20) surfaces. Serovar Schwarzengrund consistently produced strong biofilm while serovars Heidelberg and Newport produced weak biofilm.

CONCLUSION

These results suggest that Salmonellae differ in their attachment depending on the surface and temperature conditions encountered, which may influence persistence in the processing environment.

SIGNIFICANCE AND IMPACT OF STUDY

These differences in biofilm formation could provide useful information for mitigation of Salmonella in processing environments.

The combined effect of stressful factors (temperature and pH) on the expression of biofilm, stress, and virulence genes in Salmonella enterica ser. Enteritidis and Typhimurium

Salmonella enterica is a major food borne pathogen that creates biofilm. Salmonella biofilm formation under different environmental conditions is a public health problem. The present study was aimed to evaluate the combined effects of stressful factors (temperature and pH) on the expression of biofilm, stress, and virulence genes in Salmonella Enteritidis and Salmonella Typhimurium. In this study, the effect of temperature (2, 8, 22.5, 37, 43 °C) and pH (2.4, 3, 4.5, 6, 6.6) on the expression of biofilm production genes (adr A, bap A), virulence genes (hil A, inv A) and the stress gene (RpoS) of S. Enteritidis and S. Typhimurium was evaluated. The response surface methodology (RSM) approach was used to evaluate the combined effect of the above factors. The highest expression of adr A, bap A, hil A, and RpoS gene for S. Typhimurium was at 22 °C-pH 4.5 (6.39-fold increase), 37 °C-pH 6 (3.92-fold increase), 37 °C-pH 6 (183-fold increase), and 37 °C-pH 3 (43.8-fold increase), respectively. The inv A gene of S. Typhimurium was decreased in all conditions. The adr A, bap A, hil A, inv A, and RpoS gene of S. Enteritidis had the highest expression level at 8 °C-pH 3 (4.09-fold increase), 22 °C-pH 6 (2.71-fold increase), 8 °C pH 3 (190-fold increase), 22 °C-pH 4.5 (9.21-fold increase), and 8 °C-pH 3 (16.6-fold), respectively. Response surface methodology (RSM) indicated that the temperature and pH had no significant effect on the expression level of adr A, bap A, hil A, Inv A, and RpoS gene in S. Enteritidis and S. Typhimurium. The expression of biofilm production genes (adr A, bap A), virulence genes (hil A, inv A) and the stress gene (RpoS) of S. Enteritidis and S. Typhimurium is not directly and exclusively associated with temperature and pH conditions.

Effects of acid, alkaline, cold, and heat environmental stresses on the antibiotic resistance of the Salmonella enterica serovar Typhimurium

Antibiotic resistance in Salmonella enterica serovar Typhimurium (S. ser. Typhimurium) has become a critical safety hazard in food. Sublethal environmental stresses can influence resistance in Salmonella during food processing. This study simulated environmental stresses in food processing. The antibiotic resistance of three strains of S. ser. Typhimurium (the ATCC 14028 strain and two wild-type isolates from chicken and pork product processing) was evaluated under different pH levels (5.0, 5.5, 6.0, 8.0, and 9.0). Also, dynamic changes in resistance with treatment duration under cold (4 °C, -20 °C) and heat (55 °C) treatment were studied. The results showed that acid and alkaline stresses reduced the resistance of S. ser. Typhimurium to eight antibiotics; meanwhile, the resistance of meropenem (MERO) increased. The minimal inhibitory concentration (MIC) of MERO was increased 16- to 64-fold. With acid or alkaline stress, the extracellular ATP content increased, and the scanning electron microscopy (SEM) result clearly revealed the appearance of wrinkles and holes on the outer membrane of Salmonella. These observations imply changes in membrane permeability, which may decrease the antibiotic resistance of Salmonella. Cold or heat stress increased the resistance of S. ser. Typhimurium to tetracycline, cefotaxime, ceftazidime, nalidixic acid, azithromycin, and ampicillin; the MIC increased 2- to 4-fold. The antibiotic resistance only changed when cold and heat stresses occurred over a certain period of time and remained unchanged when the stress persisted. This study reports on the ability of S. ser. Typhimurium to develop antibiotic resistance after environmental stresses. It can provide valuable information for meat processing to improve interventions and risk management.

Impact of the Resistance Responses to Stress Conditions Encountered in Food and Food Processing Environments on the Virulence and Growth Fitness of Non-Typhoidal Salmonellae

The success of Salmonella as a foodborne pathogen can probably be attributed to two major features: its remarkable genetic diversity and its extraordinary ability to adapt. Salmonella cells can survive in harsh environments, successfully compete for nutrients, and cause disease once inside the host. Furthermore, they are capable of rapidly reprogramming their metabolism, evolving in a short time from a stress-resistance mode to a growth or virulent mode, or even to express stress resistance and virulence factors at the same time if needed, thanks to a complex and fine-tuned regulatory network. It is nevertheless generally acknowledged that the development of stress resistance usually has a fitness cost for bacterial cells and that induction of stress resistance responses to certain agents can trigger changes in Salmonella virulence. In this review, we summarize and discuss current knowledge concerning the effects that the development of resistance responses to stress conditions encountered in food and food processing environments (including acid, osmotic and oxidative stress, starvation, modified atmospheres, detergents and disinfectants, chilling, heat, and non-thermal technologies) exerts on different aspects of the physiology of non-typhoidal Salmonellae, with special emphasis on virulence and growth fitness.

Proteomics study unveils ROS balance in acid-adapted Salmonella Enteritidis

Salmonella Enteritidis is a major cause of foodborne gastroenteritis and is thus a persistent threat to global public health. The acid adaptation response helps Salmonella survive exposure to gastric environment during ingestion. In a previous study we highlighted the damage caused to cell membrane and the regulation of intracellular reactive oxygen species (ROS) in S. Enteritidis. In this study, we applied both physiologic and iTRAQ analyses to explore the regulatory mechanism of acid resistance in Salmonella. It was found that after S. Enteritidis was subject to a 1 h period of acid adaptation at pH 5.5, an additional 1 h period of acid shock stress at pH 3.0 caused less Salmonella cell death than in non-acid adapted Salmonella cells. Although there were no significant differences between adapted and non-adapted cells in terms of cell membrane damage (e.g., membrane permeability or lipid peroxidation) after 30 min, intracellular ROS level in acid adapted cells was dramatically reduced compared to that in non-acid adapted cells, indicating that acid adaption promoted less ROS generation or increased the ability of ROS scavenging with little reduction in the integrity of the cell membrane. These findings were confirmed via an iTRAQ analysis. The adapted cells were shown to trigger incorporation of exogenous long-chain fatty acids into the cellular membrane, resulting in a different membrane lipid profile and promoting survival rate under acid stress. S. Enteritidis experiences oxidative damage and iron deficiency under acid stress, but after acid adaption S. Enteritidis cells were able to balance their concentrations of intracellular ROS. Specifically, SodAB consumed the free protons responsible for forming reactive oxygen intermediates (ROIs) and KatE protected cells from the toxic effects of ROIs. Additionally, acid-labile proteins released free unbound iron promoting ferroptotic metabolism, and NADH reduced GSSH to G-SH, protecting cells from acid/oxidative stress.

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.

Salmonella enterica subsp. enterica Serovar Heidelberg Food Isolates Associated with a Salmonellosis Outbreak Have Enhanced Stress Tolerance Capabilities

Salmonella enterica serovar Heidelberg is currently the 12th most common serovar of Salmonella enterica causing salmonellosis in the United States and results in twice the average incidence of blood infections caused by nontyphoidal salmonellae. Multiple outbreaks of salmonellosis caused by Salmonella Heidelberg resulted from the same poultry processor, which infected 634 people during 2013 and 2014. The hospitalization and invasive illness rates were 38% and 15%, respectively. We hypothesized that the outbreak strains of Salmonella Heidelberg had enhanced stress tolerance and virulence capabilities. We sourced nine food isolates collected during the outbreak investigation and three reference isolates to assess their tolerance to heat and sanitizers, ability to attach to abiotic surfaces, and invasiveness in vitro We performed RNA sequencing on three isolates (two outbreak-associated isolates and a reference Salmonella Heidelberg strain) with various levels of heat tolerance to gain insight into the mechanism behind the isolates' enhanced heat tolerance. We also performed genomic analyses to determine the genetic relationships among the outbreak isolates. Ultimately, we determined that (i) six Salmonella Heidelberg isolates associated with the foodborne outbreak had enhanced heat tolerance, (ii) one outbreak isolate with enhanced heat tolerance also had an enhanced biofilm-forming ability under stressful conditions, (iii) exposure to heat stress increased the expression of Salmonella Heidelberg multidrug efflux and virulence genes, and (iv) outbreak-associated isolates were likely transcriptionally primed to better survive processing stresses and, potentially, to cause illness.IMPORTANCE This study provides a deep analysis of the intrinsic stress tolerance and virulence capabilities of Salmonella Heidelberg that may have contributed to the length and severity of a recent salmonellosis outbreak. Additionally, this study provides a comprehensive analysis of the transcriptomic response of S. enterica strains to heat stress conditions and compares baseline stationary-phase gene expression among outbreak- and non-outbreak-associated Salmonella Heidelberg isolates. These data can be used in assay development to screen isolates for stress tolerance and subsequent survival. This study adds to our understanding of the strains associated with the outbreak and informs ongoing regulatory discussions on Salmonella in poultry.

The Effect of Previous Life Cycle Phase on the Growth Kinetics, Morphology, and Antibiotic Resistance of Salmonella Typhimurium DT104 in Brain Heart Infusion and Ground Chicken Extract

Growth models are predominately used in the food industry to estimate the potential growth of selected microorganisms under environmental conditions. The growth kinetics, cellular morphology, and antibiotic resistance were studied throughout the life cycle of Salmonella Typhimurium. The effect of the previous life cycle phase [late log phase (LLP), early stationary phase (ESP), late stationary phase (LSP), and early death phase (EDP)] of Salmonella after reinoculation in brain heart infusion broth (BHI), ground chicken extract (GCE), and BHI at pH 5, 7, and 9 and salt concentrations 2, 3, and 4% was investigated. The growth media and previous life cycle phase had significant effects on the lag time (λ), specific growth rate (μ max), and maximum population density (Y max). At 2 and 4% salt concentration, the LLP had the significantly (p < 0.05) fastest μ max (1.07 and 0.69 log CFU/ml/h, respectively). As the cells transitioned from the late log phase (LLP) to the early death phase (EDP), the λ significantly (p < 0.05) increased. At pH 5 and 9, the EDP had a significantly (p < 0.05) lower Y max than the LLP, ESP, and LSP. As the cells transitioned from a rod shape to a coccoid shape in the EDP, the cells were more susceptible to antibiotics. The cells regained their resistance as they transitioned back to a rod shape from the EDP to the log and stationary phase. Our results revealed that growth kinetics, cell's length, shape, and antibiotic resistance were significantly affected by the previous life cycle phase. The results of this study also demonstrate that the previous life cycle should be considered when developing growth models of foodborne pathogens to better ensure the safety of poultry and poultry products.

An ordinal logistic regression approach to predict the variability on biofilm formation stages by five Salmonella enterica strains on polypropylene and glass surfaces as affected by pH, temperature and NaCl

This study assessed the adhesion and formation of biofilm by five Salmonella enterica strains (S. Enteritidis 132, S. Infantis 176, S. Typhimurium 177, S. Heidelberg 281 and S. Corvallis 297) on polypropylene (PP) and glass (G) surfaces as affected by pH (4-7), NaCl concentration (0-10% w/v) and temperature (8-35 °C). Sessile counts <3 log CFU/cm² were considered lack of adhesion (category 1), while counts ≥ 3 and < 5 log CFU/cm² corresponded to adhesion (category 2) and counts ≥ 5 log CFU/cm² corresponded biofilm formation (category 3). The obtained results categorized in these three responses were used to develop ordinal regression models to predict the probability of biofilm stages on PP- and G-surfaces. The experimental outcomes for lack of adhesion were >90% on PP- and G-surfaces. Generally, adhesion outcomes corresponded to approximately 36% of the total, whereas biofilm outcomes were close to 65% in both PP- and G-surfaces. The biofilm stages varied among the strains studied and with the material surface under the same experimental conditions. According to the generated ordinal models, the probability of adhesion and biofilm formation on PP-surface by the five S. enterica strains tested decreased at pH 4 or 5 in NaCl concentrations >4% and at a temperature <20 °C. On G-surface, the probability of adhesion increased pH 6 or 7, in the absence of NaCl and temperatures <20 °C, while, the probability of biofilm formation increased in the same pH, NaCl concentration up to 4% and temperatures ≥20 °C. This is the first study assessing the biofilm formation through categorical, ordinal responses and it shows that ordinal regression models can be useful to predict biofilm stages of S. enterica as a function of pH, NaCl, and temperature or their interactions.

Biofilms: Architecture, Resistance, Quorum Sensing and Control Mechanisms

Biofilm is a mode of living employed by many pathogenic and environmental microbes to proliferate as multicellular aggregates on inert inanimate or biological substrates. Several microbial diseases are associated with biofilms that pose challenges in treatment with antibiotics targeting individual cells. Bacteria in biofilms secrete exopolymeric substances that contribute to architectural stability and provide a secure niche to inhabiting cells. Quorum sensing (QS) plays essential roles in biofilm development. Pathogenic bacteria in biofilms utilize QS mechanisms to activate virulence and develop antibiotic resistance. This review is a brief overview of biofilm research and provides updates on recent understandings on biofilm development, antibiotic resistance and transmission, and importance of QS mechanisms. Strategies to combat biofilm associated diseases including anti-biofilm substances, quorum quenching molecules, bio-surfactants and competitive inhibitors are briefly discussed. The review concludes with updates on recent approaches utilized for biofilm inhibition and provides perspectives for further research in the field.

Response to Acid Adaptation in Salmonella enterica Serovar Enteritidis

Acid adaptation in Salmonella Enteritidis was characterized by phenotypic and gene-expression analyses. S. Enteritidis cells at log-phase and stationary-phase were kept at pH 4.5 to 6.0 for 1 to 4 hours. All treatments induced various levels of acid tolerance response that were dependent on pH, exposure time and growth phase. This acid adaptation resulted in tolerance to 50 °C and 8% NaCl regardless of the growth phase. However, the tolerance of log-phase and stationary-phase cells to low temperatures (4 and -20 °C) was increased and decreased, respectively. RT-qPCR analysis revealed that genes involved in tolerance to acid (SEN1564A and cfa), heat (rpoH, uspB, and htrA), salt (proP, proV, and osmW), and cold (cspA, cspC, and csdA) stress were generally upregulated after acid adaptation. These results provide an initial insight into mechanisms of acid adaptation and induced cross protection in S. Enteritidis. PRACTICAL APPLICATION: Stress tolerance acquisition resulting from acid adaptation in foodborne pathogens poses a great threat to food safety. The current work showed that acid adaptation induced direct tolerance and cross-tolerance to high temperature, low temperature, and salt in Salmonella Enteritidis, possibly due to the upregulation of stress tolerance-related genes. These results provide key insights into acid adaptation mechanisms and efficient control of S. Enteritidis.

Highly Prevalent Multidrug-Resistant Salmonella From Chicken and Pork Meat at Retail Markets in Guangdong, China

This study aimed to investigate the prevalence, serotype distribution, and antibiotic resistance, and to characterize the extended spectrum β-lactamases (ESBLs) producing Salmonella isolates from chicken and pork meats from retail markets in Guangdong province, China. A total of 903 retail meat samples (475 chicken and 428 pork meats) were obtained from six cities (Guangzhou, Shenzhen, Heyuan, Shaoguan, Foshan, and Yunfu) of Guangdong province between May 2016 and April 2017. High levels of Salmonella contamination were detected in chicken (302/475, 63.6%) and pork (313/428, 73.1%). Thirty-eight serotypes were identified in 615 detected Salmonella, and the serotypes varied greatly between chicken and pork samples. Agona (55/302, 18.2%), Corvallis (45/302, 14.9%), Kentucky (38/302, 12.6%), Mbandaka (32/302, 10.6%) was the dominant serotypes in chicken samples. However, Typhimurium (78/313, 24.9%), Rissen (67/313, 24.1%), Derby (66/313, 21.1%), and London (48, 15.3%) were the most common in pork samples. High rates of antibiotic resistance were found to sulfisoxazole (468/615, 76.1%), tetracycline (463/615, 75.3%), ampicillin (295/615, 48.0%), and ofloxacin (275/615, 44.7%). Notably, antimicrobial susceptibility tests identified resistance to polymyxin B (12/615, 2.0%) and imipenem (3/615, 0.5%). Multidrug-resistance (MDR) was detected in Salmonella isolated from chicken (245/302, 81.1%) and pork (229/313, 73.2%). The resistance rate of different Salmonella serotypes varied widely. Especially, isolates such as Typhimurium, Agona, Corvallis and Kentucky exhibited highly resistance to antibiotics. The MDR rate of Salmonella isolates from chicken was significantly higher than that from pork isolates (P < 0.05). Twenty-one Salmonella isolates were identified as ESBLs-producing, covering six Salmonella serotypes and displaying different pulse field gel electrophoresis (PFGE) genotypes. Bla_OXA-1 was the dominant ESBLs gene (9/21, 42.9%), followed by bla_CTX-M-55 (5/21, 23.8%). This study indicated that Salmonella was widespread in chicken and pork from retail markets in Guangdong province and the isolates showed high multidrug-resistance, especially the known multidrug-resistant Salmonella serotypes. Therefore, it is important to focus on Salmonella serotypes and strengthen the long-term monitoring of MDR Salmonella serotypes in animal-derived foods.

Salmonella and Campylobacter biofilm formation: a comparative assessment from farm to fork

It takes several steps to bring food from the farm to the fork (dining table), and contamination with food-borne pathogens can occur at any point in the process. Campylobacter spp. and Salmonella spp. are the main microorganisms responsible for foodborne disease in the EU. These two pathogens are able to persist throughout the food supply chain thanks to their ability to form biofilms. Owing to the high prevalence of Salmonella and especially of Campylobacter in the food supply chain and the huge efforts of food authorities to reduce these levels, it is of great importance to fully understand their mechanisms of persistence. Diverse studies have evaluated the biofilm-forming capacity of foodborne pathogens isolated at different steps of food production. Nonetheless, the principal obstacle of these studies is to reproduce the real conditions that microorganisms encounter in the food supply chain. While there are a wide number of Salmonella biofilm studies, information on Campylobacter biofilms is still limited. A comparison between the two microorganisms could help to develop new research in the field of Campylobacter biofilms. Therefore, this review evaluates relevant work in the field of Salmonella and Campylobacter biofilms and the applicability of the data obtained from these studies to real working conditions. © 2018 Society of Chemical Industry.

Predicting adhesion and biofilm formation boundaries on stainless steel surfaces by five Salmonella enterica strains belonging to different serovars as a function of pH, temperature and NaCl concentration

This study aimed to assess the capability of 97 epidemic S. enterica strains belonging to 18 serovars to form biofilm. Five strains characterized as strong biofilm-producers, belonging to distinct serovars (S. Enteritidis 132, S. Infantis 176, S. Typhimurium 177, S. Heidelberg 281 and S. Corvallis 297) were assayed for adhesion/biofilm formation on stainless steel surfaces. The experiments were conducted in different combinations of NaCl (0, 2, 4, 5, 6, 8 and 10% w/v), pH (4, 5, 6 and 7) and temperatures (8 °C, 12 °C, 20 °C and 35 °C). Only adhesion was assumed to occur when S. enterica counts were ≥3 and <5 log CFU/cm², whereas biofilm formation was defined as when the counts were ≥5 log CFU/cm². The binary responses were used to develop models to predict the probability of adhesion/biofilm formation on stainless steel surfaces by five strains belonging to different S. enterica serovars. A total of 99% (96/97) of the tested S. enterica strains were characterized as biofilm-producers in the microtiter plate assays. The ability to form biofilm varied (P < 0.05) within and among the different serovars. Among the biofilm-producers, 21% (20/96), 45% (43/96), and 35% (34/96) were weak, moderate and strong biofilm-producers, respectively. The capability for adhesion/biofilm formation on stainless steel surfaces under the experimental conditions studied varied among the strains studied, and distinct secondary models were obtained to describe the behavior of the five S. enterica tested. All strains showed adhesion at pH 4 up to 4% of NaCl and at 20 °C and 35 °C. The probability of adhesion decreased when NaCl concentrations were >8% and at 8 °C, as well as in pH values ≤ 5 and NaCl concentrations > 6%, for all tested strains. At pH 7 and 6, biofilm formation for S. Enteritidis, S. Infantis, S. Typhimurium, S. Heidelberg was observed up to 6% of NaCl at 35 °C and 20 °C. The predicted boundaries for adhesion were pH values < 5 and NaCl ≥ 4% and at temperatures <20 °C. For biofilm formation, the predicted boundaries were pH values < 5 and NaCl concentrations ≥ 2% and at temperatures <20 °C for all strains. The secondary models obtained describe the variability in boundaries of adhesion and biofilm formation on stainless steel by five strains belonging to different S. enterica serovars. The boundary models can be used to predict adhesion and biofilm formation ability on stainless steel by S. enterica as affected by pH, NaCl and temperature.

Preliminary Transcriptome Analysis of Mature Biofilm and Planktonic Cells of Salmonella Enteritidis Exposure to Acid Stress

Salmonella has emerged as a well-recognized food-borne pathogen, with many strains able to form biofilms and thus cause cross-contamination in food processing environments where acid-based disinfectants are widely encountered. In the present study, RNA sequencing was employed to establish complete transcriptome profiles of Salmonella Enteritidis in the forms of planktonic and biofilm-associated cells cultured in Tryptic Soytone Broth (TSB) and acidic TSB (aTSB). The gene expression patterns of S. Enteritidis significantly differed between biofilm-associated and planktonic cells cultivated under the same conditions. The assembled transcriptome of S. Enteritidis in this study contained 5,442 assembled transcripts, including 3,877 differentially expressed genes (DEGs) identified in biofilm and planktonic cells. These DEGs were enriched in terms such as regulation of biological process, metabolic process, macromolecular complex, binding and transferase activity, which may play crucial roles in the biofilm formation of S. Enteritidis cultivated in aTSB. Three significant pathways were observed to be enriched under acidic conditions: bacterial chemotaxis, porphyrin-chlorophyll metabolism and sulfur metabolism. In addition, 15 differentially expressed novel non-coding small RNAs (sRNAs) were identified, and only one was found to be up-regulated in mature biofilms. This preliminary study of the S. Enteritidis transcriptome serves as a basis for future investigations examining the complex network systems that regulate Salmonella biofilm in acidic environments, which provide information on biofilm formation and acid stress interaction that may facilitate the development of novel disinfection procedures in the food processing industry.

The Antitoxin Protein of a Toxin-Antitoxin System from Xylella fastidiosa Is Secreted via Outer Membrane Vesicles

The Xylella fastidiosa subsp pauca strain 9a5c is a Gram-negative, xylem-limited bacterium that is able to form a biofilm and affects citrus crops in Brazil. Some genes are considered to be involved in biofilm formation, but the specific mechanisms involved in this process remain unknown. This limited understanding of how some bacteria form biofilms is a major barrier to our comprehension of the progression of diseases caused by biofilm-producing bacteria. Several investigations have shown that the toxin-antitoxin (TA) operon is related to biofilm formation. This operon is composed of a toxin with RNAse activity and its cognate antitoxin. Previous reports have indicated that the antitoxin is able to inhibit toxin activity and modulate the expression of the operon as well as other target genes involved in oxidative stress and mobility. In this study, we characterize a toxin-antitoxin system consisting of XfMqsR and XfYgiT, respectively, from X. fastidiosa subsp. pauca strain 9a5c. These proteins display a high similarity to their homologs in X. fastidiosa strain Temecula and a predicted tridimensional structure that is similar to MqsR-YgiT from Escherichia coli. The characterization was performed using in vitro assays such as analytical ultracentrifugation (AUC), size exclusion chromatography, isothermal titration calorimetry, and Western blotting. Using a fluorometric assay to detect RNAses, we demonstrated that XfMqsR is thermostable and can degrade RNA. XfMqsR is inhibited by XfYgiT, which interacts with its own promoter. XfYgiT is known to be localized in the intracellular compartment; however, we provide strong evidence that X. fastidiosa secretes wild-type XfYgiT into the extracellular environment via outer membrane vesicles, as confirmed by Western blotting and specific immunofluorescence labeling visualized by fluorescence microscopy. Taken together, our results characterize the TA system from X. fastidiosa strain 9a5c, and we also discuss the possible influence of wild-type XfYgiT in the cell.

New Weapons to Fight Old Enemies: Novel Strategies for the (Bio)control of Bacterial Biofilms in the Food Industry

Biofilms are microbial communities characterized by their adhesion to solid surfaces and the production of a matrix of exopolymeric substances, consisting of polysaccharides, proteins, DNA and lipids, which surround the microorganisms lending structural integrity and a unique biochemical profile to the biofilm. Biofilm formation enhances the ability of the producer/s to persist in a given environment. Pathogenic and spoilage bacterial species capable of forming biofilms are a significant problem for the healthcare and food industries, as their biofilm-forming ability protects them from common cleaning processes and allows them to remain in the environment post-sanitation. In the food industry, persistent bacteria colonize the inside of mixing tanks, vats and tubing, compromising food safety and quality. Strategies to overcome bacterial persistence through inhibition of biofilm formation or removal of mature biofilms are therefore necessary. Current biofilm control strategies employed in the food industry (cleaning and disinfection, material selection and surface preconditioning, plasma treatment, ultrasonication, etc.), although effective to a certain point, fall short of biofilm control. Efforts have been explored, mainly with a view to their application in pharmaceutical and healthcare settings, which focus on targeting molecular determinants regulating biofilm formation. Their application to the food industry would greatly aid efforts to eradicate undesirable bacteria from food processing environments and, ultimately, from food products. These approaches, in contrast to bactericidal approaches, exert less selective pressure which in turn would reduce the likelihood of resistance development. A particularly interesting strategy targets quorum sensing systems, which regulate gene expression in response to fluctuations in cell-population density governing essential cellular processes including biofilm formation. This review article discusses the problems associated with bacterial biofilms in the food industry and summarizes the recent strategies explored to inhibit biofilm formation, with special focus on those targeting quorum sensing.

Biofilm formation, phenotypic production of cellulose and gene expression in Salmonella enterica decrease under anaerobic conditions

Salmonella enterica subsp. enterica is one of the main food-borne pathogens. This microorganism combines an aerobic life outside the host with an anaerobic life within the host. One of the main concerns related to S. enterica is biofilm formation and cellulose production. In this study, biofilm formation, morphotype, cellulose production and transcription of biofilm and quorum sensing-related genes of 11 S. enterica strains were tested under three different conditions: aerobiosis, microaerobiosis, and anaerobiosis. The results showed an influence of oxygen levels on biofilm production. Biofilm formation was significantly higher (P<0.05) in aerobiosis than in microaerobiosis and anaerobiosis. Cellulose production and RDAR (red, dry, and rough) were expressed only in aerobiosis. In microaerobiosis, the strains expressed the SAW (smooth and white) morphotype, while in anaerobiosis the colonies appeared small and red. The expression of genes involved in cellulose synthesis (csgD and adrA) and quorum sensing (sdiA and luxS) was reduced in microaerobiosis and anaerobiosis in all S. enterica strains tested. This gene expression levels were less reduced in S. Typhimurium and S. Enteritidis compared to the tested serotypes. There was a relationship between the expression of biofilm and quorum sensing-related genes. Thus, the results from this study indicate that biofilm formation and cellulose production are highly influenced by atmospheric conditions. This must be taken into account as contamination with these bacteria can occur during food processing under vacuum or modified atmospheres.

The pulsed light inactivation of veterinary relevant microbial biofilms and the use of a RTPCR assay to detect parasite species within biofilm structures

The presence of pathogenic organisms namely parasite species and bacteria in biofilms in veterinary settings, is a public health concern in relation to human and animal exposure. Veterinary clinics represent a significant risk factor for the transfer of pathogens from housed animals to humans, especially in cases of wound infection and the shedding of faecal matter. This study aims to provide a means of detecting veterinary relevant parasite species in bacterial biofilms, and to provide a means of disinfecting these biofilms. A real time PCR assay was utilized to detect parasite DNA in Bacillus cereus biofilms on stainless steel and PVC surfaces. Results show that both Cryptosporidium and Giardia attach to biofilms in large numbers (100-1000 oo/cysts) in as little as 72 hours. Pulsed light successfully inactivated all test species (Listeria, Salmonella, Bacillus, Escherichia) in planktonic and biofilm form with an increase in inactivation for every increase in UV dose.