Changes in barotolerance, thermotolerance, and cellular morphology throughout the life cycle of Listeria monocytogenes

Chlorine dioxide is a broad-spectrum disinfectant against Shiga toxin-producing Escherichia coli and Listeria monocytogenes in agricultural water

Agricultural water is commonly treated with chlorine-based disinfectants, which are impacted by water quality. Understanding how water quality influences disinfectants such as chlorine dioxide (ClO2) against pathogenic bacteria is important for creating efficacious sanitation regimens. In this study, the minimum inhibitory concentration (MIC) of ClO2 needed to achieve a 3-Log reduction against Shiga toxin-producing Escherichia coli (STEC) and Listeria monocytogenes was compared across agricultural water samples. Sterile ddH2O served as a control to compare with environmental samples from Salinas Valley, CA, and laboratory standards. To test different dosages and water qualities, stock ClO2 was diluted in 24-well plates with target concentrations of 10, 5, 2.5, and 1.25 mg/L. Well plates were inoculated with pathogens and treated with sanitizer for 5 min. Following treatment, surviving pathogens were enumerated using viable cell counts. The results demonstrate that groundwater samples had the highest water quality of the environmental samples and required the lowest concentration of disinfectant to achieve 3-Log reduction against both bacteria, with MIC between 1.4 and 2.0 mg/L. Open-source samples had lower water quality and required a higher concentration of ClO2 for 3-Log reduction, with MIC between 2.8 and 5.8 mg/L for both pathogens. There was no correlation between pH, turbidity, or conductivity/TDS and reduction for either STEC or L. monocytogenes, suggesting no individual water metric was driving reduction. A lower dosage was required to achieve 3-Log reduction against STEC, while L. monocytogenes required greater concentrations to achieve the same level of reduction. Overall, these results help guide growers in using ClO2 as a broad-spectrum disinfectant and demonstrate its efficacy in reaching 3-Log reduction across agricultural water samples.

Lack of correlation between growth, stress, and virulence phenotypes in strains of Salmonella enterica serovar Enteritidis, S. Typhimurium DT104, S. 4,12, b:- and S. Liverpool

Strains of Salmonella Enteritidis (SEnt, n = 10) and S. Typhimurium (STm, n = 11), representing clones with high impact on human health, and strains of S. 4,12: b:- (S412B n = 11) and S. Liverpool (SLiv, n = 4), representing clones with minor impact on human health were characterized for 16 growth, stress, and virulence phenotypes to investigate whether systematic differences exist in their performance in these phenotypes and whether there was correlation between performance in different phenotypes. The term serotype was not found to be predictive of a certain type of performance in any phenotype, and surprisingly, on average, strains of SEnt and STm were not significantly better in adhering to and invading cultured intestinal cells than the less pathogenic types. Forest analysis identified desiccation tolerance and the ability to grow at 42°C with high salt as the characters that separated serovars with low human health impact (S412B/SLiv) from serovars with high human health impact (SEnt/STm). The study showed that variation in phenotypes was high even within serovars and correlation between phenotypes was low, i.e. the way that a strain performed phenotypically in one of the tested conditions had a low predictive value for the performance of the strain in other conditions.

Aspects of high hydrostatic pressure food processing: Perspectives on technology and food safety

The last two decades saw a steady increase of high hydrostatic pressure (HHP) used for treatment of foods. Although the science of biomaterials exposed to high pressure started more than a century ago, there still seem to be a number of unanswered questions regarding safety of foods processed using HHP. This review gives an overview on historical development and fundamental aspects of HHP, as well as on potential risks associated with HHP food applications based on available literature. Beside the combination of pressure and temperature, as major factors impacting inactivation of vegetative bacterial cells, bacterial endospores, viruses, and parasites, factors, such as food matrix, water content, presence of dissolved substances, and pH value, also have significant influence on their inactivation by pressure. As a result, pressure treatment of foods should be considered for specific food groups and in accordance with their specific chemical and physical properties. The pressure necessary for inactivation of viruses is in many instances slightly lower than that for vegetative bacterial cells; however, data for food relevant human virus types are missing due to the lack of methods for determining their infectivity. Parasites can be inactivated by comparatively lower pressure than vegetative bacterial cells. The degrees to which chemical reactions progress under pressure treatments are different to those of conventional thermal processes, for example, HHP leads to lower amounts of acrylamide and furan. Additionally, the formation of new unknown or unexpected substances has not yet been observed. To date, no safety-relevant chemical changes have been described for foods treated by HHP. Based on existing sensitization to non-HHP-treated food, the allergenic potential of HHP-treated food is more likely to be equivalent to untreated food. Initial findings on changes in packaging materials under HHP have not yet been adequately supported by scientific data.

Biofilm Formation by Listeria monocytogenes 15G01, a Persistent Isolate from a Seafood-Processing Plant, Is Influenced by Inactivation of Multiple Genes Belonging to Different Functional Groups

Listeria monocytogenes is a ubiquitous foodborne pathogen that results in a high rate of mortality in sensitive and immunocompromised people. Contamination of food with L. monocytogenes is thought to occur during food processing, most often as a result of the pathogen producing a biofilm that persists in the environment and acting as the source for subsequent dispersal of cells onto food. A survey of seafood-processing plants in New Zealand identified the persistent strain 15G01, which has a high capacity to form biofilms. In this study, a transposon library of L. monocytogenes 15G01 was screened for mutants with altered biofilm formation, assessed by a crystal violet assay, to identify genes involved in biofilm formation. This screen identified 36 transposants that showed a significant change in biofilm formation compared to the wild type. The insertion sites were in 27 genes, 20 of which led to decreased biofilm formation and seven to an increase. Two insertions were in intergenic regions. Annotation of the genes suggested that they are involved in diverse cellular processes, including stress response, autolysis, transporter systems, and cell wall/membrane synthesis. Analysis of the biofilms produced by the transposants using scanning electron microscopy and fluorescence microscopy showed notable differences in the structure of the biofilms compared to the wild type. In particular, inactivation of uvrB and mltD produced coccoid-shaped cells and elongated cells in long chains, respectively, and the mgtB mutant produced a unique biofilm with a sandwich structure which was reversed to the wild-type level upon magnesium addition. The mltD transposant was successfully complemented with the wild-type gene, whereas the phenotypes were not or only partially restored for the remaining mutants.IMPORTANCE The major source of contamination of food with Listeria monocytogenes is thought to be due to biofilm formation and/or persistence in food-processing plants. By establishing as a biofilm, L. monocytogenes cells become harder to eradicate due to their increased resistance to environmental threats. Understanding the genes involved in biofilm formation and their influence on biofilm structure will help identify new ways to eliminate harmful biofilms in food processing environments. To date, multiple genes have been identified as being involved in biofilm formation by L. monocytogenes; however, the exact mechanism remains unclear. This study identified four genes associated with biofilm formation by a persistent strain. Extensive microscopic analysis illustrated the effect of the disruption of mgtB, clsA, uvrB, and mltD and the influence of magnesium on the biofilm structure. The results strongly suggest an involvement in biofilm formation for the four genes and provide a basis for further studies to analyze gene regulation to assess the specific role of these biofilm-associated genes.

Shiga Toxin-Producing Escherichia coli in the Long-Term Survival Phase Exhibit Higher Chlorine Tolerance and Less Sublethal Injury Following Chlorine Treatment of Romaine Lettuce

The extent of chlorine inactivation and sublethal injury of stationary-phase (STAT) and long-term survival-phase (LTS) cells of Shiga toxin-producing Escherichia coli (STEC) in vitro and in a lettuce postharvest wash model was investigated. Four STEC strains were cultured in tryptic soy broth supplemented with 0.6% (w/v) yeast extract (TSBYE; 35°C) for 24 h and 21 d to obtain STAT and LTS cells, respectively. Minimum bactericidal concentration (MBC) and dose-response assays were performed to determine chlorine's antibacterial efficacy against STAT and LTS cells. Chlorine solutions (pH 6.5) and romaine lettuce were each inoculated with STAT and LTS cells to obtain initial populations of ∼7.8 log colony-forming units (CFU)/mL. Survivors in chlorine solutions were determined after 30 s. Inoculated lettuce samples were held at 22°C ± 1°C for 2 h or 20 h and then exposed to chlorine (10-40 ppm) for 60 s. Survivors were enumerated on nonselective and selective agar media following incubation (35°C, 48 h). The MBC for STAT and LTS cells was 0.04 and 0.08 ppm, respectively. Following exposure (30 s) to chlorine at 2.5, 5.0, and 10 ppm, STAT cells were reduced to <1.0 log CFU/mL, whereas LTS survivors were at 5.10 (2.5 ppm), 3.71 (5.0 ppm), and 2.55 (10 ppm) log CFU/mL. At 20 and 40 ppm chlorine, greater log CFU reductions of STAT cells (1.64 and 1.85) were observed compared with LTS cells (0.94 and 0.83) after 2 h of cell contact with lettuce (p < 0.05), but not after 20 h. Sublethal injury in STEC after chlorine (40 ppm) treatment was lower in LTS compared with STAT survivors (p < 0.05). Compared with STAT cells, LTS cells of STEC seem to have higher chlorine tolerance as planktonic cells and as attached cells depending on cell contact time on lettuce. In addition, a higher percentage of LTS cells, compared with STAT cells, survive in a noninjured state after chlorine (40 ppm) treatment of lettuce.

High-pressure processing-induced transcriptome response during recovery of Listeria monocytogenes

Background

High-pressure processing (HPP) is a commonly used technique in the food industry to inactivate pathogens, including L. monocytogenes. It has been shown that L. monocytogenes is able to recover from HPP injuries and can start to grow again during long-term cold storage. To date, the gene expression profiling of L. monocytogenes during HPP damage recovery at cooling temperature has not been studied. In order identify key genes that play a role in recovery of the damage caused by HPP treatment, we performed RNA-sequencing (RNA-seq) for two L. monocytogenes strains (barotolerant RO15 and barosensitive ScottA) at nine selected time points (up to 48 h) after treatment with two pressure levels (200 and 400 MPa).

Results

The results showed that a general stress response was activated by SigB after HPP treatment. In addition, the phosphotransferase system (PTS; mostly fructose-, mannose-, galactitol-, cellobiose-, and ascorbate-specific PTS systems), protein folding, and cobalamin biosynthesis were the most upregulated genes during HPP damage recovery. We observed that cell-division-related genes (divIC, dicIVA, ftsE, and ftsX) were downregulated. By contrast, peptidoglycan-synthesis genes (murG, murC, and pbp2A) were upregulated. This indicates that cell-wall repair occurs as a part of HPP damage recovery. We also observed that prophage genes, including anti-CRISPR genes, were induced by HPP. Interestingly, a large amount of RNA-seq data (up to 85%) was mapped to Rli47, which is a non-coding RNA that is upregulated after HPP. Thus, we predicted that Rli47 plays a role in HPP damage recovery in L. monocytogenes. Moreover, gene-deletion experiments showed that amongst peptidoglycan biosynthesis genes, pbp2A mutants are more sensitive to HPP.

Conclusions

We identified several genes and mechanisms that may play a role in recovery from HPP damage of L. monocytogenes. Our study contributes to new information on pathogen inactivation by HPP.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07407-6.

Mechanisms of Salmonella Attachment and Survival on In-Shell Black Peppercorns, Almonds, and Hazelnuts

Salmonella enterica subspecies I (ssp 1) is the leading cause of hospitalizations and deaths due to known bacterial foodborne pathogens in the United States and is frequently implicated in foodborne disease outbreaks associated with spices and nuts. However, the underlying mechanisms of this association have not been fully elucidated. In this study, we evaluated the influence of storage temperature (4 or 25°C), relative humidity (20 or 60%), and food surface characteristics on the attachment and survival of five individual strains representing S. enterica ssp 1 serovars Typhimurium, Montevideo, Braenderup, Mbandaka, and Enteritidis on raw in-shell black peppercorns, almonds, and hazelnuts. We observed a direct correlation between the food surface roughness and S. enterica ssp 1 attachment, and detected significant inter-strain difference in survival on the shell surface under various storage conditions. A combination of low relative humidity (20%) and ambient storage temperature (25°C) resulted in the most significant reduction of S. enterica on shell surfaces (p < 0.05). To identify genes potentially associated with S. enterica attachment and survival on shell surfaces, we inoculated a library of 120,000 random transposon insertion mutants of an S. Enteritidis strain on almond shells, and screened for mutant survival after 1, 3, 7, and 14 days of storage at 20% relative humidity and 25°C. Mutants in 155 S. Enteritidis genes which are involved in carbohydrate metabolic pathways, aerobic and anaerobic respiration, inner membrane transport, and glutamine synthesis displayed significant selection on almond shells (p < 0.05). Findings of this study suggest that various food attributes, environmental factors, and an unexpectedly complex metabolic and regulatory network in S. enterica ssp 1 collectively contribute to the bacterial attachment and survival on low moisture shell surface, providing new data for the future development of knowledge-based intervention strategies.

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.

Resistance of Listeria monocytogenes to Stress Conditions Encountered in Food and Food Processing Environments

Listeria monocytogenes is a human food-borne facultative intracellular pathogen that is resistant to a wide range of stress conditions. As a consequence, L. monocytogenes is extremely difficult to control along the entire food chain from production to storage and consumption. Frequent and recent outbreaks of L. monocytogenes infections illustrate that current measures of decontamination and preservation are suboptimal to control L. monocytogenes in food. In order to develop efficient measures to prevent contamination during processing and control growth during storage of food it is crucial to understand the mechanisms utilized by L. monocytogenes to tolerate the stress conditions in food matrices and food processing environments. Food-related stress conditions encountered by L. monocytogenes along the food chain are acidity, oxidative and osmotic stress, low or high temperatures, presence of bacteriocins and other preserving additives, and stresses as a consequence of applying alternative decontamination and preservation technologies such high hydrostatic pressure, pulsed and continuous UV light, pulsed electric fields (PEF). This review is aimed at providing a summary of the current knowledge on the response of L. monocytogenes toward these stresses and the mechanisms of stress resistance employed by this important food-borne bacterium. Circumstances when L. monocytogenes cells become more sensitive or more resistant are mentioned and existence of a cross-resistance when multiple stresses are present is pointed out.

Effects of High Pressure Processing and Hot Water Pasteurization of Cooked Sausages on Inactivation of Inoculated Listeria monocytogenes, Natural Populations of Lactic Acid Bacteria, Pseudomonas spp., and Coliforms and Their Recovery during Storage at 4 and 10°C

The study investigated the effects of high pressure processing (HPP; 600 MPa for 3 min) and hot water (HW; 75°C for 15 min) pasteurization on the inactivation of inoculated Listeria monocytogenes, natural populations of lactic acid bacteria, Pseudomonas spp., and coliforms in vacuum-packaged cooked sausages and their recovery during storage at 4 and 10°C for 35 days. Cooking sausages to an internal temperature of 72°C resulted in a >6-log reduction in numbers of inoculated L. monocytogenes. Storage at 4°C resulted in no significant difference ( P > 0.05) in L. monocytogenes numbers in sausages pasteurized by either HPP or HW compared with unpasteurized control. However, at 10°C, L. monocytogenes numbers in unpasteurized control sausages increased to about 7 log CFU/g by day 35, whereas in HPP-pasteurized sausages, numbers remained below the detection limit for up to 21 days and then increased to 4.5 log CFU/g by day 35. HW pasteurization resulted in inhibition of L monocytogenes to below the detection limit throughout the 35-day storage at 10°C. Natural lactic acid bacteria populations were significantly reduced by HPP and HW pasteurization and continued to be significantly lower at the end of the 35-day storage. Unlike most studies that focus on HPP or HW treatment of postcooking surface contamination of meat with Listeria, this study examined the combined effect of cooking, HPP, and HW on raw meat with a high contamination level. This scenario is important in countries where raw meat supply and in-store refrigeration are a challenge. The results suggest that HPP and HW pasteurization could be used to successfully enhance the safety and shelf life of cooked sausages and that HW pasteurization (75°C) was more effective than HPP (600 MPa) to control L. monocytogenes.

Long-Term Survival Phase Cells of Salmonella Typhimurium ATCC 14028 Have Significantly Greater Resistance to Ultraviolet Radiation in 0.85% Saline and Apple Juice

Nonendospore-forming pathogenic bacteria in the long-term survival (LTS) phase can remain viable for months or years and may show reduced susceptibility to various antimicrobial interventions. In the present study, we investigated the response of LTS phase Salmonella enterica serovar Typhimurium (ATCC 14028) to ultraviolet (UV) radiation in 0.85% (w/v) saline and apple juice and the extent of sublethal injury in LTS phase survivors. The LTS-phase Salmonella Typhimurium cells were cultured at 35°C for 14 days in tryptic soy broth with 0.6% (w/v) yeast extract (TSBYE). Exponential- and stationary-phase cells, cultured in TSBYE (35°C) for 2.5 and 18 h, respectively, served as control samples. Cells (10⁷ CFU [colony-forming unit]/mL) from each physiological state were exposed to UV light in saline (80 μW/cm²) and apple juice (1500 μW/cm²). The Salmonella Typhimurium survivors were plated for enumeration on either tryptic soy agar with 0.6% yeast extract or xylose-lysine-tergitol 4 (XLT4) agar and colonies counted after incubation (35°C, 24 h). Of all the growth phases tested, LTS phase cells were consistently impacted the least by UV treatment (p < 0.05). In saline, D-values of exponential, stationary, and LTS Salmonella Typhimurium were 0.35, 0.38, and 0.49 min, respectively. D-values in apple juice at pH 3.63 and pH 5.65 were 2.52, 3.19, and 3.57 min and 3.24, 3.50, and 4.18 min, respectively. UV radiation (80 μW/cm²) of Salmonella Typhimurium in saline for 2.5 min reduced the number of exponential- and stationary-phase cells by ∼7.19 and 6.30 log₁₀ CFU/mL, respectively. In contrast, LTS cells were only reduced by 5.08 log₁₀ CFU/mL. Among the three physiological states, LTS phase cells had the least sublethal injury in the surviving population (p < 0.05). These results indicate that the LTS state cross-protects Salmonella Typhimurium against UV radiation and should be considered in determination of the UV radiation D-value for this pathogen.

Morphological Change and Decreasing Transfer Rate of Biofilm-Featured Listeria monocytogenes EGDe

Listeria monocytogenes , a lethal foodborne pathogen, has the ability to resist the hostile food processing environment and thus frequently contaminates ready-to-eat foods during processing. It is commonly accepted that the tendency of L. monocytogenes ' to generate biofilms on various surfaces enhances its resistance to the harshness of the food processing environment. However, the role of biofilm formation in the transferability of L. monocytogenes EGDe remains controversial. We examined the growth of Listeria biofilms on stainless steel surfaces and their effect on the transferability of L. monocytogenes EGDe. The experiments were a factorial 2 × 2 design with at least three biological replicates. Through scanning electron microscopy, a mature biofilm with intensive aggregates of cells was observed on the surface of stainless steel after 3 or 5 days of incubation, depending on the initial level of inoculation. During biofilm development, L. monocytogenes EGDe carried out binary fission vigorously before a mature biofilm was formed and subsequently changed its cellular morphology from rod shaped to sphere shaped. Furthermore, static biofilm, which was formed after 3 days of incubation at 25°C, significantly inhibited the transfer rate of L. monocytogenes EGDe from stainless steel blades to 15 bologna slices. During 7 days of storage at 4°C, however, bacterial growth rate was not significantly impacted by whether bacteria were transferred from biofilm and the initial concentrations of transferred bacteria on the slice. In conclusion, this study is the first to report a distinct change in morphology of L. monocytogenes EGDe at the late stage of biofilm formation. More importantly, once food is contaminated by L. monocytogenes EGDe, contamination proceeds independently of biofilm development and the initial level of contamination when food is stored at 4°C, even if contamination with L. monocytogenes EGDe was initially undetectable before storage.

Biofilm-Forming Abilities of Listeria monocytogenes Serotypes Isolated from Different Sources

A total of 98 previously characterized and serotyped L. monocytogenes strains, comprising 32 of 1/2a; 20 of 1/2b and 46 of 4b serotype, from clinical and food sources were studied for their capability to form a biofilm. The microtiter plate assay revealed 62 (63.26%) strains as weak, 27 (27.55%) strains as moderate, and 9 (9.18%) strains as strong biofilm formers. Among the strong biofilm formers, 6 strains were of serotype 1/2a and 3 strains were of serotype 1/2b. None of the strain from 4b serotype exhibited strong biofilm formation. No firm correlation (p = 0.015) was noticed between any serotype and respective biofilm formation ability. Electron microscopic studies showed that strong biofilm forming isolates could synthesize a biofilm within 24 h on surfaces important in food industries such as stainless steel, ceramic tiles, high-density polyethylene plastics, polyvinyl chloride pipes, and glass. Cell enumeration of strong, moderate, and weak biofilm was performed to determine if the number of cells correlated with the biofilm-forming capabilities of the isolates. Strong, moderate, and weak biofilm showed 570±127× 10³ cells/cm², 33±26× 10³ cells/cm², 5±3× 10³ cells/cm², respectively, indicating that the number of cells was directly proportional to the strength of the biofilm. The hydrophobicity index (HI) analysis revealed higher hydrophobicity with an increased biofilm formation. Fatty acid methyl esterase analysis revealed the amount of certain fatty acids such as iso-C15:0, anteiso-C15:0, and anteiso-C17:0 fatty acids correlated with the biofilm-forming capability of L. monocytogenes. This study showed that different strains of L. monocytogenes form biofilm of different intensities which did not completely correlate with their serotype; however, it correlated with the number of cells, hydrophobicity, and amount of certain fatty acids.

Infectious causes of cholesteatoma and treatment of infected ossicles prior to reimplantation by hydrostatic high-pressure inactivation

Chronic inflammation, which is caused by recurrent infections, is one of the factors contributing to the pathogenesis of cholesteatoma. If reimplantation of autologous ossicles after a surgical intervention is intended, inactivation of planktonic bacteria and biofilms is desirable. High hydrostatic pressure treatment is a procedure, which has been used to inactivate cholesteatoma cells on ossicles. Here we discuss the potential inactivating effect of high hydrostatic pressure on microbial pathogens including biofilms. Recent experimental data suggest an incomplete inactivation at a pressure level, which is tolerable for the bone substance of ossicles and results at least in a considerable reduction of pathogen load. Further studies are necessary to access how far this quantitative reduction of pathogens is sufficient to prevent ongoing chronic infections, for example, due to forming of biofilms.

Use of used vs. fresh cheese brines and the effect of pH and salt concentration on the survival of Listeria monocytogenes

The aim of the study was to investigate how the use of fresh cheese brines compared with used brines and various combinations of pH and NaCl concentrations affected the survival of Listeria monocytogenes. Cheese brines from five Norwegian small scale cheese producers were analysed and showed great variations in pH (4·54–6·01) and NaCl concentrations (14·1–26·9 %). The survival of five strains of List. monocytogenes (two clinical isolates, two food isolates and one animal isolate) in four different cheese brines (three used and one fresh) was investigated. Results showed significant differences in survival both depending on the strains and the brines. Strains of human outbreak listeriosis cases showed greater ability to survive in the brines compared with food isolates and a List. monocytogenes reference strain (1–2 log10 difference after 200 d). All strains showed highest survival in the freshly prepared brine compared with the used brines. Molecular typing by multiple locus variable number tandem repeats analysis (MLVA) showed that there were no detectable alterations in the examined variable number tandem repeats of the genome in five strains after 200 d storage in any of the salt brines. Combined effects of pH (4·5, 5·25 and 6·0) and NaCl (15, 20 and 25 %) in fresh, filter sterilised brines on the survival of List. monocytogenes were examined and results showed that pathogen populations decreased over time in all brines. Death rates at any given NaCl concentration were highest at low pH (4·5) and death rates at any given pH were highest at low NaCl concentrations (15 %). In conclusion, the use of used brines reduced the survival of List. monocytogenes and a combination of low pH (4·5) and low salt concentrations (15 %) decreased the risk of List. monocytogenes survival compared with higher pH (5·25 or 6·0) and higher NaCl concentrations (20 or 25 %).

Listeria monocytogenes responds to cell density as it transitions to the long-term-survival phase

Listeria monocytogenes was recently found to enter a long-term-survival (LTS) phase, which may help explain its persistence in natural environments and within food processing plants. The purpose of this study was to investigate the effects of initial cell density, initial pH and type of broth (fresh vs. spent) on the transition of L. monocytogenes to the LTS phase and model the change in viable population density with time. Initial cell density (~10(6)-~10(10)CFU/ml) and initial pH (5.36-6.85) both significantly affected the transition of L. monocytogenes to the LTS phase (P<0.001) with initial cell density being the main determining factor. In contrast, type of broth did not significantly affect cell density change during the transition of stationary-phase cells at high initial density to the LTS phase (P>0.05). After 30-d incubation no significant differences in cell densities were observed between either type of broth or between any of the initial cell density/pH treatment combinations (P>0.05), where the mean viable cell density was 4.3±1.1×10(8)CFU/ml. L. monocytogenes responded to viable cell density in accordance with the logistic equation during transition to the LTS phase. The Agr quorum-sensing system does not appear to play a role in the transition to the LTS phase. Further research is needed to better understand the control mechanisms utilized by L. monocytogenes as it transitions to a coccoid, resistant and stable density state in the LTS phase.

Inactivation of human pathogens during phase II composting of manure-based mushroom growth substrate

Commercial production of white button mushrooms (Agaricus bisporus) requires a specialized growth substrate prepared from composted agricultural by-products. Because horse and poultry manures are widely used in substrate formulations, there is a need to determine the extent to which the composting process is capable of eliminating human pathogens. In this study, partially composted substrate was inoculated with a pathogen cocktail (log 10⁶ to 10⁸ CFU/g) containing Listeria monocytogenes, Escherichia coli O157:H7, and Salmonella. Pathogen and indicator-organism reductions were followed at temperatures that typically occurred during a standard 6-day phase II pasteurization and conditioning procedure. Controlled-temperature water bath studies at 48.8, 54.4, and 60°C demonstrated complete destruction of the three pathogens after 36.0, 8.0, and 0.5 h, respectively. Destruction of L. monocytogenes and E. coli O157:H7 at 54.4°C occurred more slowly than E. coli, total coliforms, Enterobacteriaceae, and Salmonella. Microbial reductions that occurred during a standard 6-day phase II pasteurization and conditioning treatment were studied in a small-scale mushroom production research facility. After phase II composting, E. coli, coliforms, and Enterobacteriaceae were below detectable levels, and inoculated pathogens were not detected by direct plating or by enrichment. The results of this study show that a phase II composting process can be an effective control measure for eliminating risks associated with the use of composted animal manures during mushroom production. Growers are encouraged to validate and verify their own composting processes through periodic microbial testing for pathogens and to conduct studies to assure uniform distribution of substrate temperatures during phase II.

Increased water activity reduces the thermal resistance of Salmonella enterica in peanut butter

Increased water activity in peanut butter significantly (P < 0.05) reduced the heat resistance of desiccation-stressed Salmonella enterica serotypes treated at 90 °C. The difference in thermal resistance was less notable when strains were treated at 126 °C. Using scanning electron microscopy, we observed minor morphological changes of S. enterica cells resulting from desiccation and rehydration processes in peanut oil.

Growth advantage in stationary-phase (GASP) phenotype in long-term survival strains of Geobacter sulfurreducens

Geobacter sulfurreducens exists in the subsurface and has been identified in sites contaminated with radioactive metals, consistent with its ability to reduce metals under anaerobic conditions. The natural state of organisms in the environment is one that lacks access to high concentrations of nutrients, namely electron donors and terminal electron acceptors (TEAs). Most studies have investigated G. sulfurreducens under high-nutrient conditions or have enriched for it in environmental systems via acetate amendments. We replicated the starvation state through long-term batch culture of G. sulfurreducens, where both electron donor and TEA were scarce. The growth curve revealed lag, log, stationary, death, and survival phases using acetate as electron donor and either fumarate or iron(III) citrate as TEA. In survival phase, G. sulfurreducens persisted at a constant cell count for as long as 23 months without replenishment of growth medium. Geobacter sulfurreducens demonstrated an ability to acquire a growth advantage in stationary-phase phenotype (GASP), with strains derived from subpopulations from death- or survival phase being able to out-compete mid-log-phase populations when co-cultured. The molecular basis for GASP was not because of any detectable mutation in the rpoS gene (GSU1525) nor because of a mutation in a putative homolog to Escherichia coli lrp, GSU3370.

Effect of lactose monolaurate on pathogenic and nonpathogenic bacteria

The antimicrobial activities of sucrose monolaurate and a novel ester, lactose monolaurate (LML), were tested. Gram-positive bacteria were more susceptible than Gram-negative bacteria to both esters. The minimal bactericidal concentrations of LML were 5 to 9.5 mM for Listeria monocytogenes isolates and 0.2 to 2 mM for Mycobacterium isolates.

Effects of high hydrostatic pressure on bacterial growth on human ossicles explanted from cholesteatoma patients

BACKGROUND

High hydrostatic pressure (HHP) treatment can eliminate cholesteatoma cells from explanted human ossicles prior to re-insertion. We analyzed the effects of HHP treatment on the microbial flora on ossicles and on the planktonic and biofilm states of selected isolates.

METHODOLOGY

Twenty-six ossicles were explanted from cholesteatoma patients. Five ossicles were directly analyzed for microbial growth without further treatment. Fifteen ossicles were cut into two pieces. One piece was exposed to HHP of 350 MPa for 10 minutes. Both the treated and untreated (control) pieces were then assessed semi-quantitatively. Three ossicles were cut into two pieces and exposed to identical pressure conditions with or without the addition of one of two different combinations of antibiotics to the medium. Differential effects of 10-minute in vitro exposure of planktonic and biofilm bacteria to pressures of 100 MPa, 250 MPa, 400 MPa and 540 MPa in isotonic and hypotonic media were analyzed using two patient isolates of Staphylococcus epidermidis and Neisseria subflava. Bacterial cell inactivation and biofilm destruction were assessed by colony counting and electron microscopy.

PRINCIPAL FINDINGS

A variety of microorganisms were isolated from the ossicles. Irrespective of the medium, HHP treatment at 350 MPa for 10 minutes led to satisfying but incomplete inactivation especially of gram-negative bacteria. The addition of antibiotics increased the efficacy of elimination. A comparison of HHP treatment of planktonic and biofilm cells showed that the effects of HPP were reduced by about one decadic logarithmic unit when HPP was applied to biofilms. High hydrostatic pressure conditions that are suitable to inactivate cholesteatoma cells fail to completely sterilize ossicles even if antibiotics are added. As a result of the reduced microbial load and the viability loss of surviving bacteria, however, there is a lower risk of re-infection after re-insertion.

Transcriptomic response of Listeria monocytogenes during the transition to the long-term-survival phase

Listeria monocytogenes can change its cellular morphology from bacilli to cocci during the transition to the long-term-survival (LTS) phase. The LTS cells demonstrated increased baro- and thermotolerance compared to their vegetative counterparts. So far, the underlying mechanisms that trigger this morphological and physiological transition remain largely unknown. In this study, we compared the transcriptomic profiles of L. monocytogenes serotype 4b strain F2365 at different growth stages in tryptic soy broth with yeast extract (TSBYE) using a whole-genome DNA chip approach. We identified a total of 225 differentially expressed genes (≥4-fold; P < 0.05) during the transition to the LTS phase in TSBYE. Genes related to cell envelope structure, energy metabolism, and transport were most significantly upregulated in the LTS phase. The upregulation of compatible solute transporters may lead to the accumulation of cellular solutes, lowering intracellular water activity and thus increasing bacterial stress resistance during the transition to the LTS phase. The downregulation of genes associated with protein synthesis may indicate a status of metabolic dormancy of the LTS cells. The transcriptomic profiles of resuscitated LTS cells in fresh TSBYE resembled those of log-phase cells (r=0.94), as the LTS cells rapidly resume metabolic activities and transit back to log phase with decreased baro- and thermotolerance.

Changes in barotolerance, thermotolerance, and cellular morphology throughout the life cycle of Listeria monocytogenes

Changes in barotolerance, thermotolerance, and cellular morphology throughout the life cycle of Listeria monocytogenes were investigated. For part 1 of this analysis, L. monocytogenes ATCC 19115 was grown to log, stationary, death, and long-term-survival phases at 35 degrees C in tryptic soy broth with yeast extract (TSBYE). Cells were diluted in whole milk that had been subjected to ultrahigh temperatures (UHT whole milk) and then high-pressure processed (HPP) at 400 MPa for 180 s or thermally processed at 62.8 degrees C for 30 s. As cells transitioned from the log to the long-term-survival phase, the D(400 MPa) and D(62.8 degrees C) values increased 10- and 19-fold, respectively. Cells decreased in size as they transitioned from the log to the long-term-survival phase. Rod-shaped cells transitioned to cocci as they entered the late-death and long-term-survival phases. L. monocytogenes strains F5069 and Scott A showed similar results. For part 2 of the analysis, cells in long-term-survival phase were centrifuged, suspended in fresh TSBYE, and incubated at 35 degrees C. As cells transitioned from the long-term-survival phase to log and the stationary phase, they increased in size and log reductions increased following HPP or heat treatment. In part 3 of this analysis, cells in long-term-survival phase were centrifuged, suspended in UHT whole milk, and incubated at 4 degrees C. After HPP or heat treatment, similar results were observed as for part 2. We hypothesize that cells of L. monocytogenes enter a dormant, long-term-survival phase and become more barotolerant and thermotolerant due to cytoplasmic condensation when they transition from rods to cocci. Further research is needed to test this hypothesis and to determine the practical significance of these findings.