Avirulent viable but non culturable cells of Listeria monocytogenes need the presence of an embryo to be recovered in egg yolk and regain virulence after recovery

Aquatic environment drives the emergence of cell wall-deficient dormant forms in Listeria

Stressed bacteria can enter a dormant viable but non-culturable (VBNC) state. VBNC pathogens pose an increased health risk as they are undetectable by growth-based techniques and can wake up back into a virulent state. Although widespread in bacteria, the mechanisms governing this phenotypic switch remain elusive. Here, we investigate the VBNC state transition in the human pathogen Listeria monocytogenes. We show that bacteria starved in mineral water become VBNC by converting into osmotically stable cell wall-deficient coccoid forms, a phenomenon that occurs in other Listeria species. We reveal the bacterial stress response regulator SigB and the autolysin NamA as major actors of VBNC state transition. We lastly show that VBNC Listeria revert to a walled and virulent state after passage in chicken embryos. Our study provides more detail on the VBNC state transition mechanisms, revealing wall-free bacteria naturally arising in aquatic environments as a potential survival strategy in hypoosmotic and oligotrophic conditions.

Exploring the Antimicrobial Efficacy of Low-Cost Commercial Disinfectants Utilized in the Agro-Food Industry Wash Tanks: Towards Enhanced Hygiene Practices

The increase in vegetable consumption has underlined the importance of minimizing the risks associated with microbiological contamination of fresh produce. The critical stage of the vegetable washing process has proven to be a key point for cross-contamination and the persistence of pathogens. In this context, the agri-food industry has widely adopted the use of disinfectants to reduce the bacterial load in the wash water. Therefore, we conducted laboratory-scale experiments in order to demonstrate the antimicrobial activity of disinfectants used in the wash tank of agro-food industries. Different wash water matrices of shredded lettuce, shredded cabbage, diced onion, and baby spinach were treated with sodium hypochlorite (NaClO), chlorine dioxide (ClO2), and per-oxyacetic acid (PAA) at recommended concentrations. To simulate the presence of pathogenic bacteria, a cocktail of E. coli O157:H7 was inoculated into the process water samples (PWW) to determine whether concentrations of disinfectants inhibit the pathogen or bring it to a viable non-culturable state (VBNC). Hereby, we used quantitative qPCR combined with different photo-reactive dyes such as ethidium monoazide (EMA) and propidium monoazide (PMA). The results indicated that concentrations superior to 20 ppm NaClO inhibit the pathogen E. coli O157:H7 artificially inoculated in the process water. Concentrations between 10-20 ppm ClO2 fail to induce the pathogen to the VBNC state. At concentrations of 80 ppm PAA, levels of culturable bacteria and VBNC of E. coli O157:H7 were detected in all PWWs regardless of the matrix. Subsequently, this indicates that the recommended concentrations of ClO2 and PAA for use in the fresh produce industry wash tank do not inhibit the levels of E. coli O157:H7 present in the wash water.

Susceptibility of Campylobacter jejuni to Stressors in Agrifood Systems and Induction of a Viable-but-Nonculturable State

Many bacteria can become viable but nonculturable (VBNC) in response to stressors commonly identified in agrifood systems. Campylobacter is able to enter the VBNC state to evade unfavorable environmental conditions, but how food processing can induce Campylobacter jejuni to enter this state and the potential role of foods in inducing the VBNC state in C. jejuni remains largely unknown. In this study, the culturability and viability of C. jejuni cells were investigated under chlorine treatment (25 ppm), aerobic stress (atmospheric condition), and low-temperature (4°C) conditions that mimicked food processing. In addition, the behaviors of C. jejuni cells in ultrahigh-temperature (UHT) and pasteurized milk were also monitored during refrigerated storage. The numbers of viable and culturable C. jejuni cells in both the pure bacterial culture and food matrices were separately determined by propidium monoazide (PMA)-quantitative PCR (qPCR) and plating assay. The C. jejuni cells lost their culturability but partially retained their viability (1% to 10%) once mixed with chlorine. In comparison, ~10% of C. jejuni cells were induced to enter the VBNC state after 24 h and 20 days under aerobic and low-temperature conditions, respectively. The viability of the C. jejuni cells remained stable during the induction process in UHT (>10%) and pasteurized (>10%) milk. The number of culturable C. jejuni cells decreased quickly in pasteurized milk, but culturable cells could still be detected in the end (day 21). In contrast, the number of culturable C. jejuni cells slowly decreased, and they became undetectable after >42 days in UHT milk. The C. jejuni cells responded differently to various stress conditions and survived in high numbers in the VBNC state in agrifood systems. IMPORTANCE The VBNC state of pathogens can pose risks to food safety and public health because the pathogens cannot be detected using conventional microbiological culture-based methods but can resuscitate under favorable conditions to develop virulence. As a leading cause of human gastroenteritis worldwide, C. jejuni can enter the VBNC state to survive in the environment and food-processing chain with high prevalence. In this study, the effect of food-processing conditions and food products on the development of VBNC state in C. jejuni was investigated, providing a better understanding of the interaction between C. jejuni and the agroecosystem. The knowledge elicited from this study can aid in developing novel intervention strategies to reduce the food safety risks associated with this microbe.

Wake Up! Resuscitation of Viable but Nonculturable Bacteria: Mechanism and Potential Application

The viable but nonculturable (VBNC) state is a survival strategy for bacteria when encountered with unfavorable conditions. Under favorable environments such as nutrient supplementation, external stress elimination, or supplementation with resuscitation-promoting substances, bacteria will recover from the VBNC state, which is termed "resuscitation". The resuscitation phenomenon is necessary for proof of VBNC existence, which has been confirmed in different ways to exclude the possibility of culturable-cell regrowth. The resuscitation of VBNC cells has been widely studied for the purpose of risk control of recovered pathogenic or spoilage bacteria. From another aspect, the resuscitation of functional bacteria can also be considered a promising field to explore. To support this point, the resuscitation mechanisms were comprehensively reviewed, which could provide the theoretical foundations for the application of resuscitated VBNC cells. In addition, the proposed applications, as well as the prospects for further applications of resuscitated VBNC bacteria in the food industry are discussed in this review.

The Viable But Non-Culturable State of Listeria monocytogenes in the One-Health Continuum

Many bacterial species, including several pathogens, can enter a so-called “viable but non-culturable” (VBNC) state when subjected to stress. Bacteria in the VBNC state are metabolically active but have lost their ability to grow on standard culture media, which compromises their detection by conventional techniques based on bacterial division. Under certain conditions, VBNC bacteria can regain their growth capacity and, for pathogens, their virulence potential, through a process called resuscitation. Here, we review the current state of knowledge of the VBNC state of Listeria monocytogenes (Lm), a Gram-positive pathogenic bacterium responsible for listeriosis, one of the most dangerous foodborne zoonosis. After a brief summary of characteristics of VBNC bacteria, we highlight work on VBNC Lm in the environment and in agricultural and food industry settings, with particular emphasis on the impact of antimicrobial treatments. We subsequently discuss recent data suggesting that Lm can enter the VBNC state in the host, raising the possibility that VBNC forms contribute to the asymptomatic carriage of this pathogen in wildlife, livestock and even humans. We also consider the resuscitation and virulence potential of VBNC Lm and the danger posed by these bacteria to at-risk individuals, particularly pregnant women. Overall, we put forth the hypothesis that VBNC forms contribute to adaptation, persistence, and transmission of Lm between different ecological niches in the One-Health continuum, and suggest that screening for healthy carriers, using alternative techniques to culture-based enrichment methods, should better prevent listeriosis risks.

The Impact of Protease during Recovery from Viable but Non-Culturable (VBNC) State in Vibrio cholerae

Vibrio cholerae can survive cold stress by entering into a viable but non-culturable (VBNC) state, and resuscitation can be induced either by temperature upshift only or the addition of an anti-dormancy stimulant such as resuscitation-promoting factors (Rpfs) at suitable temperature. In this study, the role of proteinase K was analyzed as an Rpf in V. cholerae. A VBNC state was induced in V. cholerae AN59 in artificial seawater (ASW) media at 4 °C, and recovery could be achieved in filtered VBNC microcosm, called spent ASW media, merely by a temperature upshift to 37 °C. The resuscitation ability of spent ASW was further enhanced by the addition of proteinase K. The mode of action of proteinase K was investigated by comparing its effect on the growth of the VBNC and culturable state of V. cholerae in ASW and spent ASW media. The presence of proteinase K allowed culturable cells to grow faster in ASW by reducing the generation time. However, this effect of proteinase K was more pronounced in stressed VBNC cells. Moreover, proteinase K-supplemented spent ASW could also accelerate the transition of VBNC into recovered cells followed by rapid growth. Additionally, we found that dead bacterial cells were the substrate on which proteinase K acts to support high growth in spent ASW. So, the conclusion is that the proteinase K could efficiently promote the recovery and growth of dormant VBNC cells at higher temperatures by decreasing the duration of the initial lag phase required for transitioning from the VBNC to recovery state and increasing the growth rate of these recovered cells.

Viable but nonculturable bacteria and their resuscitation: implications for cultivating uncultured marine microorganisms

Culturing has been the cornerstone of microbiology since Robert Koch first successfully cultured bacteria in the late nineteenth century. However, even today, the majority of microorganisms in the marine environment remain uncultivated. There are various explanations for the inability to culture bacteria in the laboratory, including lack of essential nutrients, osmotic support or incubation conditions, low growth rate, development of micro-colonies, and the presence of senescent or viable but nonculturable (VBNC) cells. In the marine environment, many bacteria have been associated with dormancy, as typified by the VBNC state. VBNC refers to a state where bacteria are metabolically active, but are no longer culturable on routine growth media. It is apparently a unique survival strategy that has been adopted by many microorganisms in response to harsh environmental conditions and the bacterial cells in the VBNC state may regain culturability under favorable conditions. The resuscitation of VBNC cells may well be an important way to cultivate the otherwise uncultured microorganisms in marine environments. Many resuscitation stimuli that promote the restoration of culturability have so far been identified; these include sodium pyruvate, quorum sensing autoinducers, resuscitation-promoting factors Rpfs and YeaZ, and catalase. In this review, we focus on the issues associated with bacterial culturability, the diversity of bacteria entering the VBNC state, mechanisms of induction into the VBNC state, resuscitation factors of VBNC cells and implications of VBNC resuscitation stimuli for cultivating these otherwise uncultured microorganisms. Bringing important microorganisms into culture is still important in the era of high-throughput sequencing as their ecological functions in the marine environment can often only be known through isolation and cultivation.

Optimization of resuscitation-promoting broths for the revival of Vibrio parahaemolyticus from a viable but nonculturable state

This study was conducted to examine the effect of formulated resuscitation-promoting broths on the revival of viable but nonculturable Vibrio parahaemolyticus induced by cold and starvation stresses. Vibrio parahaemolyticus was incubated in artificial sea water at 4 °C for more than 8 months until this bacterium became undetectable, while retaining its intact cell count of more than 10⁵ CFU/field over time. On day 250, V. parahaemolyticus was collected and enriched in tryptic soy broth supplemented with 3% NaCl, 10,000 U/mg catalase, 2% sodium pyruvate, 20 mM MgSO₄, 5 mM EDTA, and a cell-free supernatant taken from V. parahaemolyticus ATCC 17802 in the stationary phase (pH 8). V. parahaemolyticus returned partially to a culturable state with a maximal cell density of 7.91 log CFU/mL in this formulated medium following 7 days of enrichment at 25 °C. In contrast, no V. parahaemolyticus was resuscitated when enriched in alkaline peptone water and tryptic soy broth.

Detection and Potential Virulence of Viable but Non-Culturable (VBNC) Listeria monocytogenes: A Review

The detection, enumeration, and virulence potential of viable but non-culturable (VBNC) pathogens continues to be a topic of discussion. While there is a lack of definitive evidence that VBNC Listeria monocytogenes (Lm) pose a public health risk, recent studies suggest that Lm in its VBNC state remains virulent. VBNC bacteria cannot be enumerated by traditional plating methods, so the results from routine Lm testing may not demonstrate a sample's true hazard to public health. We suggest that supplementing routine Lm testing methods with methods designed to enumerate VBNC cells may more accurately represent the true level of risk. This review summarizes five methods for enumerating VNBC Lm: Live/Dead BacLight^TM staining, ethidium monoazide and propidium monoazide-stained real-time polymerase chain reaction (EMA- and PMA-PCR), direct viable count (DVC), 5-cyano-2,3-ditolyl tetrazolium chloride-4',6-diamidino-2-phenylindole (CTC-DAPI) double staining, and carboxy-fluorescein diacetate (CDFA) staining. Of these five supplementary methods, the Live/Dead BacLight^TM staining and CFDA-DVC staining currently appear to be the most accurate for VBNC Lm enumeration. In addition, the impact of the VBNC state on the virulence of Lm is reviewed. Widespread use of these supplemental methods would provide supporting data to identify the conditions under which Lm can revert from its VBNC state into an actively multiplying state and help identify the environmental triggers that can cause Lm to become virulent. Highlights: Rationale for testing for all viable Listeria (Lm) is presented. Routine environmental sampling and plating methods may miss viable Lm cells. An overview and comparison of available VBNC testing methods is given. There is a need for resuscitation techniques to recover Lm from VBNC. A review of testing results for post VBNC virulence is compared.

Study on the Viable but Non-culturable (VBNC) State Formation of Staphylococcus aureus and Its Control in Food System

A Viable but non-culturable (VBNC) state is a bacterial survival strategy under reverse conditions. It poses a significant challenge for public health and food safety. In this study, the effect of external environmental conditions including acid, nutrition, and salt concentrations on the formation of S. aureus VBNC states at low temperatures were investigated. Different acidity and nutritional conditions were then applied to food products to control the VBNC state formation. Four different concentration levels of each factor (acid, nutrition, and salt) were selected in a total of 16 experimental groups. Nutrition showed the highest influence on the VBNC state formation S. aureus, followed by acid and salt. The addition of 1% acetic acid could directly kill S. aureus cells and inhibit the formation of the VBNC state with a nutrition concentration of 25, 50, and 100%. A propidium monoazide-polymerase chain reaction (PMA-PCR) assay was applied and considered as a rapid and sensitive method to detect S. aureus in VBNC state with the detection limit of 104 CFU/mL.

Viable but Nonculturable Escherichia coli O157:H7 and Salmonella enterica in Fresh Produce: Rapid Determination by Loop-Mediated Isothermal Amplification Coupled with a Propidium Monoazide Treatment

Escherichia coli O157:H7 and Salmonella enterica are leading causes of foodborne outbreaks linked to fresh produce. Both species can enter the "viable but nonculturable" (VBNC) state that precludes detection using conventional culture-based or molecular methods. In this study, we assessed propidium monoazide-quantitative PCR (PMA-qPCR) assays and novel methods combining PMA and loop-mediated isothermal amplification (LAMP) for the detection and quantification of VBNC E. coli O157:H7 and S. enterica in fresh produce. The performance of PMA-LAMP assays targeting the wzy gene of E. coli O157:H7 and the agfA gene of S. enterica and the performance of PMA-qPCR assays were compared in pure culture and spiked tomato, lettuce, and spinach. No cross-reaction was observed in the specificity tests. The values representing the limit of detection (LOD) seen with PMA-LAMP were 9.0 CFU/reaction for E. coli O157:H7 and 4.6 CFU/reaction for S. enterica in pure culture and were 5.13 × 103 or 5.13 × 104 CFU/g for VBNC E. coli O157:H7 and 1.05 × 104 or 1.05 × 105 CFU/g for VBNC S. enterica in fresh produce, representing results comparable to those obtained by PMA-qPCR. Standard curves showed correlation coefficients ranging from 0.925 to 0.996, indicating a good quantitative capacity of PMA-LAMP for determining populations of both bacterial species in the VBNC state. The PMA-LAMP assay was completed with considerable economy of time (30 min versus 1 h) and achieved sensitivity and quantitative capacity comparable to those seen with a PMA-qPCR assay. PMA-LAMP is a rapid, sensitive, and robust method for the detection and quantification of VBNC E. coli O157:H7 and S. enterica in fresh produce.IMPORTANCE VBNC pathogenic bacteria pose a potential risk to the food industry because they do not multiply on routine microbiological media and thus can evade detection in conventional plating assays. Both E. coli O157:H7 and S. enterica have been reported to enter the VBNC state under a range of environmental stress conditions and to resuscitate under favorable conditions and are a potential cause of human infections. PMA-LAMP methods developed in this study provide a rapid, sensitive, and specific way to determine levels of VBNC E. coli O157:H7 and S. enterica in fresh produce, which potentially decreases the risks related to the consumption of fresh produce contaminated by enteric pathogens in this state. PMA-LAMP can be further applied in the field study to enhance our understanding of the fate of VBNC pathogens in the preharvest and postharvest stages of fresh produce.

Virulence genes expression in viable but non-culturable state of Listeria monocytogenes in fish meat

This study aimed to evaluate the fate of Listeria monocytogenes in water microcosm and rainbow trout fillet under salinity stress of 0% and 30% NaCl at refrigerator temperature (4 ± 2 ℃). Bacterial culturability was studied by standard culture and colony count method. Reverse transcription-PCR (RT-PCR) of 16 S rRNA gene was used to detect viability of non-culturable bacteria. Also, the qualitative expression of pathogenic genes (hly and inlA) was studied using RT-PCR. The results showed that bacteria in water microcosm lost their culturability at 13 days under 0% salinity (starvation or distilled water) and at 27 days under 30% salinity; however, bacteria in rainbow trout fillet remained culturable under 0% and 30% NaCl. RT-PCR of 16 S rRNA gene was positive for all treatments during the period of this study, indicating the entering of L. monocytogenes into the viable but non-culturable state in water microcosm under 0% and 30% NaCl. Also, viable but non-culturable L. monocytogenes retained the expression of hly and inlA genes. So, it could be concluded that L. monocytogenes in viable but non-culturable state can cause serious health problems and further investigation is necessary to elucidate the effects of other processing and storage conditions (light, dark, smoking, etc.) on behavior of L. monocytogenes in smoked and salted fish.

Plasma-Treated Air and Water-Assessment of Synergistic Antimicrobial Effects for Sanitation of Food Processing Surfaces and Environment

The synergistic antimicrobial effects of plasma-processed air (PPA) and plasma-treated water (PTW), which are indirectly generated by a microwave-induced non-atmospheric pressure plasma, were investigated with the aid of proliferation assays. For this purpose, microorganisms (Listeria monocytogenes, Escherichia coli, Pectobacterium carotovorum, sporulated Bacillus atrophaeus) were cultivated as monocultures on specimens with polymeric surface structures. Both the distinct and synergistic antimicrobial potential of PPA and PTW were governed by the plasma-on time (5⁻50 s) and the treatment time of the specimens with PPA/PTW (1⁻5 min). In single PTW treatment of the bacteria, an elevation of the reduction factor with increasing treatment time could be observed (e.g., reduction factor of 2.4 to 3.0 for P. carotovorum). In comparison, the combination of PTW and subsequent PPA treatment leads to synergistic effects that are clearly not induced by longer treatment times. These findings have been valid for all bacteria (L. monocytogenes > P. carotovorum = E. coli). Controversially, the effect is reversed for endospores of B. atrophaeus. With pure PPA treatment, a strong inactivation at 50 s plasma-on time is detectable, whereas single PTW treatment shows no effect even with increasing treatment parameters. The use of synergistic effects of PTW for cleaning and PPA for drying shows a clear alternative for currently used sanitation methods in production plants. Highlights: Non-thermal atmospheric pressure microwave plasma source used indirect in two different modes-gaseous and liquid; Measurement of short and long-living nitrite and nitrate in corrosive gas PPA (plasma-processed air) and complex liquid PTW (plasma-treated water); Application of PTW and PPA in single and combined use for biological decontamination of different microorganisms.

Induction of the viable but non-culturable state in bacterial pathogens by household cleaners and inorganic salts

Effective monitoring of microbial pathogens is essential for a successful preventive food safety and hygiene strategy. However, as most monitoring strategies are growth-based, these tests fail to detect pathogenic bacteria that have entered the viable but non-culturable (VBNC) state. The present study reports the induction of the VBNC state in five human pathogens by commercially available household cleaners in combination with inorganic salts. We determined that non-ionic surfactants, a common ingredient in household cleaners, can induce the VBNC state, when combined with salts. A screening study with 630 surfactant/salt combinations indicates a correlation between the hydrophobicity of the surfactant and VBNC induction in L. monocytogenes, E. coli, S. enterica serovar Typhimurium, S. aureus and toxin-producing enteropathogenic E. coli. Cells that were exposed to combinations of surfactants and salts for 5 min and up to 1 h lost their culturability on standard growth media while retaining their ATP production, fermentation of sugars and membrane integrity, which suggests intact and active metabolism. Screening also revealed major differences between Gram-negative and Gram-positive bacteria; the latter being more susceptible to VBNC induction. Combinations of such detergents and salts are found in many different environments and reflect realistic conditions in industrial and domestic surroundings. VBNC cells present in industrial environments, food-processing plants and even our daily routine represent a serious health risk due to possible resuscitation, unknown spreading, production of toxins and especially their invisibility to routine detection methods, which rely on culturability of cells and fail to detect VBNC pathogens.

Viable-but-Nonculturable Listeria monocytogenes and Salmonella enterica Serovar Thompson Induced by Chlorine Stress Remain Infectious

The microbiological safety of fresh produce is monitored almost exclusively by culture-based detection methods. However, bacterial food-borne pathogens are known to enter a viable-but-nonculturable (VBNC) state in response to environmental stresses such as chlorine, which is commonly used for fresh produce decontamination. Here, complete VBNC induction of green fluorescent protein-tagged Listeria monocytogenes and Salmonella enterica serovar Thompson was achieved by exposure to 12 and 3 ppm chlorine, respectively. The pathogens were subjected to chlorine washing following incubation on spinach leaves. Culture data revealed that total viable L. monocytogenes and Salmonella Thompson populations became VBNC by 50 and 100 ppm chlorine, respectively, while enumeration by direct viable counting found that chlorine caused a <1-log reduction in viability. The pathogenicity of chlorine-induced VBNC L. monocytogenes and Salmonella Thompson was assessed by using Caenorhabditis elegans Ingestion of VBNC pathogens by C. elegans resulted in a significant life span reduction (P = 0.0064 and P < 0.0001), and no significant difference between the life span reductions caused by the VBNC and culturable L. monocytogenes treatments was observed. L. monocytogenes was visualized beyond the nematode intestinal lumen, indicating resuscitation and cell invasion. These data emphasize the risk that VBNC food-borne pathogens could pose to public health should they continue to go undetected.IMPORTANCE Many bacteria are known to enter a viable-but-nonculturable (VBNC) state in response to environmental stresses. VBNC cells cannot be detected by standard laboratory culture techniques, presenting a problem for the food industry, which uses these techniques to detect pathogen contaminants. This study found that chlorine, a sanitizer commonly used for fresh produce, induces a VBNC state in the food-borne pathogens Listeria monocytogenes and Salmonella enterica It was also found that chlorine is ineffective at killing total populations of the pathogens. A life span reduction was observed in Caenorhabditis elegans that ingested these VBNC pathogens, with VBNC L. monocytogenes as infectious as its culturable counterpart. These data show that VBNC food-borne pathogens can both be generated and avoid detection by industrial practices while potentially retaining the ability to cause disease.

Preharvest Farming Practices Impacting Fresh Produce Safety

Advancements in agriculture and food processing techniques have been instrumental in the development of modern human societies. Vast improvements in agronomic practices, handling, and processing have allowed us to produce and preserve mass quantities of food. Yet despite all these innovations and potentially as a consequence of these mass production practices, more and more outbreaks of human pathogens linked to raw and processed foods are identified every year. It is evident that our increased capacity for microbial detection has contributed to the greater number of outbreaks detected. However, our understanding of how these events originate and what agronomic, packaging, and environmental factors influence the survival, persistence, and proliferation of human pathogens remains of scientific debate. This review seeks to identify those past and current challenges to the safety of fresh produce and focuses on production practices and how those impact produce safety. It reflects on 20 years of research, industry guidelines, and federal standards and how they have evolved to our current understanding of fresh produce safety. This document is not intended to summarize and describe all fruit and vegetable farming practices across the United States and the rest of the world. We understand the significant differences in production practices that exist across regions. This review highlights those general farming practices that significantly impact past and current food safety issues. It focuses on current and future research needs and on preharvest food safety control measures in fresh-produce safety that could provide insight into the mechanisms of pathogen contamination, survival, and inactivation under field and packinghouse conditions.

Differential Effect of Food Sanitizers on Formation of Viable but Nonculturable Salmonella enterica in Poultry

A method for microscopic enumeration of viable Salmonella enterica in meat samples was developed by using the LIVE/DEAD BacLight kit technology. A two-step centrifugation and wash process was developed to clean the samples from food and chemical impurities that might otherwise interfere with the appropriate staining reactions. The accuracy of the BacLight kit-based viability assessments was confirmed with various validation tests that were conducted by following the manufacturer's instructions. For the biocide challenge tests, chicken parts each bearing around 8.5 log of S. enterica were sprayed with common food sanitizers such as 1,3-dibromo-5,5-dimethylhydantoin (DBDMH), lactic acid (LA), and peracetic acid (PAA). The log reduction (LR) of S. enterica for each test biocide was evaluated by microscopic and conventional culture plate methods. The results show that both LA and PAA treatments generated a greater number of microscopic counts compared with the corresponding plate counts with differences being around half a log. This discrepancy is believed to occur when cells enter a so-called viable but nonculturable (VBNC) state, and to our knowledge, this is the first report documenting the presence of VBNC in PAA- and LA-treated food samples. In contrast, the BacLight-based viable counts were comparable to the culture-based enumerations of all DBDMH-treated samples. Therefore, we concluded that DBDMH-treated meat did not contain significant VBNC populations of S. enterica. A detailed description of our spray system, the dye validation, and the treatment reproducibility are also provided in this work.

Further Evidence of How Unbuffered Starvation at 4°C Influences Listeria monocytogenes EGD-e, HCC23, F2365, and Scott A

The soilborne pathogen Listeria monocytogenes frequently contaminates food products and food processing environments and is able to survive desiccation, high osmotic pressures, and starvation. However, little is known about how this pathogen survives starvation at 4°C. This study provides evidence that L. monocytogenes is able to survive total nutrient starvation for 4 weeks. L. monocytogenes strains EGD-e, Scott A, F2365, and HCC23 were starved individually in sterile water. Colony counts declined over 4 weeks, with Scott A declining the most rapidly. Transmission electron microscopy images revealed degradation of starving cell membranes and altered cytosols. Starving cells were subjected to the metabolic inhibitors fluoride, arsenite, 2,4-dinitrophenol, iodoacetate, and cyanide individually. Iodoacetate, which inhibits glyceraldehyde-3-phosphate dehydrogenase, completely reduced cultivable counts below the level of detection compared with the control starving cells; 2,4-dinitrophenol, which dissipates proton motive force, almost completely reduced cultivable counts. These results suggest that L. monocytogenes strains EGD-e, Scott A, F2365, and HCC23 are actively using part of the glycolysis pathway while starving. These results suggest that starving L. monocytogenes cells retain aspects of active metabolism.

Improved sample preparation for direct quantitative detection of Escherichia coli O157 in soil using qPCR without pre-enrichment

The prominence of fresh produce as a vehicle for foodborne pathogens such as enterohaemorrhagic Escherichia coli (EHEC) O157 is rising, where disease cases can cause hospitalization and in some cases death. This rise emphasises the necessity for accurate and sensitive methods for detection of pathogens in soil, potential sources of contamination of fresh produce. The complexity of the soil matrix has previously proven prohibitive to pathogen detection via molecular methods without the use of a culture enrichment step, thereby excluding the detection of viable but non-culturable cells. Here, a sample preparation procedure to facilitate a direct qPCR assay is developed for the detection of E. coli O157 in soil, bypassing culture steps in favour of sample separation through pulsification release and filtration. In sand and peat-based compost, the method is sensitive to 10 CFU g^-1 soil. When testing soils from agricultural sites, it was found that several were qPCR positive for E. coli O157 while being culture-negative, with peat-based compost possessing a concentration of 200 tir gene copies per gram. This procedure offers a rapid, quantitative assessment of the potential presence of E. coli O157 in soils which can act as a prescreen of their suitability to grow fresh produce safely.

Current Perspectives on Viable but Non-culturable State in Foodborne Pathogens

The viable but non-culturable (VBNC) state, a unique state in which a number of bacteria respond to adverse circumstances, was first discovered in 1982. Unfortunately, it has been reported that many foodborne pathogens can be induced to enter the VBNC state by the limiting environmental conditions during food processing and preservation, such as extreme temperatures, drying, irradiation, pulsed electric field, and high pressure stress, as well as the addition of preservatives and disinfectants. After entering the VBNC state, foodborne pathogens will introduce a serious crisis to food safety and public health because they cannot be detected using conventional plate counting techniques. This review provides an overview of the various features of the VBNC state, including the biological characteristics, induction and resuscitation factors, formation and resuscitation mechanisms, detection methods, and relationship to food safety.

Diffusible substances from lactic acid bacterial cultures exert strong inhibitory effects on Listeria monocytogenes and Salmonella enterica serovar enteritidis in a co-culture model

Background

Food-borne infections cause huge economic and human life losses. Listeria monocytogenes and Salmonella enterica serovar Enteritidis are among the top ranking pathogens causing such losses. Control of such infections is hampered by persistent contamination of foods and food-processing environments, resistance of pathogens to sanitizing agents, existence of heterogeneous populations of pathogens (including culturable and viable but non-culturable cells) within the same food items, and inability to detect all such pathogens by culture-based methods. Modern methods such as flow cytometry allow analyses of cells at the single cell level within a short time and enable better and faster detection of such pathogens and distinctions between live and dead cells. Such methods should be complemented by control strategies including the use of beneficial bacteria that produce metabolites capable of inhibiting food-borne pathogens. In this study, broth cultures of lactic acid bacteria (LAB) isolated from fermented milk were tested for production of substances capable of inhibiting L. monocytogenes and S. Enteritidis in co-culture with LAB by assessment of colony-forming units (CFU) and live:dead cell populations by flow cytometry.

Results

The LAB isolates belonged to the species Lactococcus lactis, Enterococcus faecalis and Enterococcus faecium. Some LAB were effective in inhibition. Plating indicated up to 99% reduction in CFU from co-cultures compared to control cultures. Most of the bacteria in both cultures were in the viable but non-culturable state. The flow data showed that there were significantly higher dead cell numbers in co-cultures than in control cultures, indicating that such killing was caused by diffusible substances produced by the LAB cultures.

Conclusion

This study showed that metabolites from selected local LAB species can be used to significantly reduce pathogen load. However, conditions of use and application need to be further investigated and optimized for large-scale utilization.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-017-0944-3) contains supplementary material, which is available to authorized users.

Impact of environmental factors on the culturability and viability of Listeria monocytogenes under conditions encountered in food processing plants

The ability of Listeria monocytogenes to adhere to and persist on surfaces for months or even years may be responsible for its transmission from contaminated surfaces to food products. Hence the necessity to find effective means to prevent the establishment of L. monocytogenes in food processing environments. The aim of this study was to assess, through a fractional experimental design, the environmental factors that could affect the survival of L. monocytogenes cells on surfaces to thereby prevent the persistence of this pathogen in conditions mimicking those encountered in food processing plants: culture with smoked salmon juice or meat exudate, use of two materials with different hygiene status, biofilm of L. monocytogenes in pure-culture or dual-culture with a Pseudomonas fluorescens strain, application of a drying step after cleaning and disinfection (C&D) and comparison of two strains of L. monocytogenes. Bacterial survival was assessed by culture, qPCR to quantify total cells, and propidium monoazide coupled with qPCR to quantify viable cells and highlight viable but non-culturable (VBNC) cells. Our results showed that failure to apply C&D causes cell persistence on surfaces. Moreover, the sanitation procedure leads only to a loss of culturability and appearance of VBNC populations. However, an additional daily drying step after C&D optimises the effectiveness of these procedures to reduce culturable populations. Our results reinforce the importance to use molecular tools to monitor viable pathogens in food processing plants to avoid underestimating the amounts of cells using only methods based on cell culture.

Fate of Viable but Non-culturable Listeria monocytogenes in Pig Manure Microcosms

The fate of two strains of Listeria monocytogenes and their ability to become viable but non-culturable (VBNC) was investigated in microcosms containing piggery effluents (two raw manures and two biologically treated manures) stored for 2 months at 8 and 20°C. Levels of L. monocytogenes were estimated using the culture method, qPCR, and propidium monoazide treatment combined with qPCR (qPCRPMA). The chemical composition and the microbial community structure of the manures were also analyzed. The strains showed similar decline rates and persisted up to 63 days. At day zero, the percentage of VBNC cells among viable cells was higher in raw manures (81.5-94.8%) than in treated manures (67.8-79.2%). The changes in their proportion over time depended on the temperature and on the type of effluent: the biggest increase was observed in treated manures at 20°C and the smallest increase in raw manures at 8°C. The chemical parameters had no influence on the behavior of the strains, but decrease of the persistence of viable cells was associated with an increase in the microbial richness of the manures. This study demonstrated that storing manure altered the culturability of L. monocytogenes, which rapidly entered the VBNC state, and underlines the importance of including VBNC cells when estimating the persistence of the pathogens in farm effluents.

Effect of a VietnameseCinnamomum cassiaessential oil and its major componenttrans-cinnamaldehyde on the cell viability, membrane integrity, membrane fluidity, and proton motive force ofListeria innocua

The antibacterial mechanism of a Cinnamomum cassia essential oil from Vietnam and of its main component (trans-cinnamaldehyde, 90% (m/m) of C. cassia essential oil) against a Listeria innocua strain was investigated to estimate their potential for food preservation. In the presence of C. cassia essential oil or trans-cinnamaldehyde at their minimal bactericidal concentration (2700 μg·mL–1), L. innocua cells fluoresced green after staining with Syto9® and propidium iodide, as observed by epifluorescence microscopy, suggesting that the perturbation of membrane did not cause large pore formation and cell lysis but may have introduced the presence of viable but nonculturable bacteria. Moreover, the fluidity, potential, and intracellular pH of the cytoplasmic membrane were perturbed in the presence of the essential oil or trans-cinnamaldehyde. However, these membrane perturbations were less severe in the presence of trans-cinnamaldehyde than in the presence of multicomponent C. cassia essential oil. This indicates that in addition to trans-cinnamaldehyde, other minor C. cassia essential oil components play a major role in its antibacterial activity against L. innocua cells.

Current Perspectives on Viable but Non-Culturable (VBNC) Pathogenic Bacteria

Under stress conditions, many species of bacteria enter into starvation mode of metabolism or a physiologically viable but non-culturable (VBNC) state. Several human pathogenic bacteria have been reported to enter into the VBNC state under these conditions. The pathogenic VBNC bacteria cannot be grown using conventional culture media, although they continue to retain their viability and express their virulence. Though there have been debates on the VBNC concept in the past, several molecular studies have shown that not only can the VBNC state be induced under in vitro conditions but also that resuscitation from this state is possible under appropriate conditions. The most notable advance in resuscitating VBNC bacteria is the discovery of resuscitation-promoting factor (Rpf), which is a bacterial cytokines found in both Gram-positive and Gram-negative organisms. VBNC state is a survival strategy adopted by the bacteria, which has important implication in several fields, including environmental monitoring, food technology, and infectious disease management; and hence it is important to investigate the association of bacterial pathogens under VBNC state and the water/foodborne outbreaks. In this review, we describe various aspects of VBNC bacteria, which include their proteomic and genetic profiles under the VBNC state, conditions of resuscitation, methods of detection, antibiotic resistance, and observations on Rpf.

Listeria monocytogenes can form biofilms in tap water and enter into the viable but non-cultivable state

Listeria monocytogenes is a foodborne pathogen that can be transmitted through contaminated raw food or by ready-to-eat products that have been in contact with contaminated surfaces. Tap water (TW) is used to wash produce, as a processed food constituent and to wash processing surfaces and floors. The main aim of this work was to investigate the formation and survival of L. monocytogenes biofilms on stainless steel (SS) coupons in TW at 4, 22, 30 and 37 °C. For that, coupons with biofilm were visualised in situ while other coupons were scraped to quantify total cells by SYTO 9, cultivable numbers by plating onto brain heart infusion agar and viable numbers by the direct viable count method. Results showed that L. monocytogenes can form biofilms on SS surfaces in TW at any temperature, including at 4 °C. The number of total cells was similar for all the conditions tested while cultivable numbers varied between the level of detection (<8.3 CFU cm(-2)) and 3.5 × 10(5) CFU cm(-2), meaning between 7.0 × 10(4) and 1.1 × 10(7) cells cm(-2) have entered the viable but non-cultivable (VBNC) state. This work clearly demonstrates that L. monocytogenes can form biofilms in TW and that sessile cells can remain viable and cultivable in some conditions for at least the 48 h investigated. On the other hand, VBNC adaptation suggests that the pathogen can remain undetectable using traditional culture recovery techniques, which may give a false indication of processing surface hygiene status, leading to potential cross-contamination of food products.

Viable but nonculturable bacteria: food safety and public health perspective

The viable but nonculturable (VBNC) state is a unique survival strategy of many bacteria in the environment in response to adverse environmental conditions. VBNC bacteria cannot be cultured on routine microbiological media, but they remain viable and retain virulence. The VBNC bacteria can be resuscitated when provided with appropriate conditions. A good number of bacteria including many human pathogens have been reported to enter the VBNC state. Though there have been disputes on the existence of VBNC in the past, extensive molecular studies have resolved most of them, and VBNC has been accepted as a distinct survival state. VBNC pathogenic bacteria are considered a threat to public health and food safety due to their nondetectability through conventional food and water testing methods. A number of disease outbreaks have been reported where VBNC bacteria have been implicated as the causative agent. Further molecular and combinatorial research is needed to tackle the threat posed by VBNC bacteria with regard to public health and food safety.

Can sludge dewatering reactivate microorganisms in mesophilically digested anaerobic sludge? Case of belt filter versus centrifuge

The anaerobic digestion process that successfully reduces the organic content of sludge is one of the most common alternatives to meet pathogen reduction requirements for particular classes of biosolids. However, recently it was reported that, much higher densities of indicator bacteria were measured in dewatered cake samples compared to samples collected after anaerobic digestion. Additionally, this increase was commonly observed after centrifugation but not after belt filter dewatering. Several hypotheses were tested to explain this occurrence; however, much of the attention was given to the reactivation of the indicator bacteria which might enter a viable but non-culturable state (VBNC) during digestion. The objective of this research is to examine sludge samples from 5 different full-scale treatment plants in order to observe the effect of dewatering processes on the reactivation potential of indicator bacteria. The bacterial enumerations were performed by both Standard Culturing Methods (SCM) and quantitative polymerase chain (qPCR) on samples collected after digestion and dewatering. Results obtained by SCM indicated that in two investigated treatment plants operating belt filter dewatering, an average 0.6 log decrease was observed after the dewatering process. However, 0.7-1.4 log increases were observed immediately after centrifuge dewatering for the other three treatment plants. On the other hand, qPCR results gave 0.1-1.9 log higher numbers compared to SCM. Comparative evaluation of results obtained by two analytical methods for five treatment plants indicates that the differences observed might be originating from both reactivation of VBNC bacteria and amplification of DNA from dead cells found in the sludge.

Critical issues in detecting viable Listeria monocytogenes cells by real-time reverse transcriptase PCR

Rapid and specific detection of viable Listeria monocytogenes cells, particularly in processed foods, is a major challenge in the food industry. To assess the suitability of using RNA-based detection methods to detect viable cells, several sets of PCR primers and florescent probes were designed targeting the 16S rRNA, internalin A, and ribosomal protein L4 genes. One-step real-time reverse transcriptase (RT) PCR assays were conducted using RNAs extracted from control and heat-treated L. monocytogenes samples. The cycle threshold values were significantly higher in heat-treated cells than in controls. However, real-time RT-PCR amplification signals were still detected even in samples stored at room temperature for 24 h after lethal treatments, and the intensity of the signals was correlated with the cell population. The 16S rRNA molecules were the most stable of the three targets evaluated, and the impact on detection efficacy of the relative positions of the PCR primers within the target genes was limited under the experimental conditions. These results suggest that real-time RT-PCR assays have advantages over conventional PCR assays for assessing viable L. monocytogenes cells, but the results are affected by the stability of the RNA molecules targeted. These findings could have a major impact on interpretation of RNA-based detection data and gene expression studies.

Nonmultiplying bacteria are profoundly tolerant to antibiotics

Bacteria survive treatments with antimicrobial agents; they achieve this in two ways. Firstly, bacteria quickly become tolerant to these agents. This tolerance is temporary, reversible, and associated with slowing of the multiplication rate. Secondly, bacteria can undergo genetic mutations leading to permanent clonal resistance to antimicrobial agents. In patients with infections, nonmultiplying bacteria, some of which may be viable but nonculturable, exist side by side with multiplying bacteria. Current antibiotics capable of killing actively multiplying bacteria have very limited or no effect against nonmultiplying bacteria. Treatment of such infections requires a regimen of multiple antimicrobial agents in order to control nonmultiplying persistent bacteria. This is especially important in tuberculosis where there is co-existence of slowly multiplying tolerant bacteria with fast growing sensitive organisms. For this reason, a prolonged length of chemotherapy, lasting 6 months, is necessary to achieve cure. This long duration of treatment is due to the slow, inadequate effect of antibiotics on nonmultiplying persistent bacteria. Similar problems with eradication of persistent bacteria are evident in the treatment of biofilms. These bacteria serve as a pool for recurrent infections. Extended courses of antibiotics increase the likelihood of genetic resistance, raise the cost of treatments, and lead to more side effects.

Variability of Listeria monocytogenes virulence: a result of the evolution between saprophytism and virulence?

The genus Listeria consists of eight species but only two are pathogenic. Human listeriosis due to Listeria monocytogenes is a foodborne disease. L. monocytogenes is widespread in the environment living as a saprophyte, but is also capable of making the transition into a pathogen following its ingestion by susceptible humans or animals. It is now known that many distinct strains of L. monocytogenes differ in their virulence and epidemic potential. Unfortunately, there is currently no standard definition of virulence levels and no complete comprehensive overview of the evolution of Listeria species and L. monocytogenes strains taking into account the presence of both epidemic and low-virulence strains. This article focuses on the methods and genes allowing us to determine the pathogenic potential of Listeria strains, and the evolution of Listeria virulence. The presence of variable levels of virulence within L. monocytogenes has important consequences on detection of Listeria strains and risk analysis but also on our comprehension of how certain pathogens will behave in a population over evolutionary time.

Defining established and emerging microbial risks in the aquatic environment: current knowledge, implications, and outlooks

This timely review primarily addresses important but presently undefined microbial risks to public health and to the natural environment. It specifically focuses on current knowledge, future outlooks and offers some potential alleviation strategies that may reduce or eliminate the risk of problematic microbes in their viable but nonculturable (VBNC) state and Cryptosporidium oocysts in the aquatic environment. As emphasis is placed on water quality, particularly surrounding efficacy of decontamination at the wastewater treatment plant level, this review also touches upon other related emerging issues, namely, the fate and potential ecotoxicological impact of untreated antibiotics and other pharmaceutically active compounds in water. Deciphering best published data has elucidated gaps between science and policy that will help stakeholders work towards the European Union's Water Framework Directive (2000/60/EC), which provides an ambitious legislative framework for water quality improvements within its region and seeks to restore all water bodies to "good ecological status" by 2015. Future effective risk-based assessment and management, post definition of the plethora of dynamic inter-related factors governing the occurrence, persistence and/or control of these presently undefined hazards in water will also demand exploiting and harnessing tangential advances in allied disciplines such as mathematical and computer modeling that will permit efficient data generation and transparent reporting to be undertaken by well-balanced consortia of stakeholders.

Physiological aspects of Listeria monocytogenes during inactivation accelerated by mild temperatures and otherwise non-growth permissive acidic and hyperosmotic conditions

Application of simultaneous low pH (pH 3.5) and low water activity (a(w)=0.9; 2.5M NaCl) conditions to Listeria monocytogenes strains ScottA and FW03/0035, and growth permissive temperatures from 25 degrees C up to 45 degrees C result in increasingly accelerated inactivation rates. This phenomenon was related to i) increased cell permeability as suggested by ethidium homodimer-1 uptake and ii) de-energization as indicated by rapidly reduced ATP basal levels. Enrichment-based recovery experiments indicated that the stress conditions eventually lead to complete loss of reproductive capacity, possibly corresponding to an irreversible collapse of pH homeostasis. Transcriptomic analyses were used to obtain further insights into the physiology of the inactivation process occurring at 25 degrees C where inactivation times were more prolonged. QPCR, mRNA decay and microarray experiments revealed transcripts of tufA and other genes become substantially more stable during inactivation resulting from exposure to combined low pH/a(w) and from non-growth permissive temperature exposure. Genes that appear to be important for initial survival of combined low pH/a(w) were delineated by K-means clustering of expression data and included an overrepresentation of SigB-activated genes, the overall response of which fades with increasing time of inactivation exposure.

Recent findings on the viable but nonculturable state in pathogenic bacteria

Many bacteria, including a variety of important human pathogens, are known to respond to various environmental stresses by entry into a novel physiological state, where the cells remain viable, but are no longer culturable on standard laboratory media. On resuscitation from this 'viable but nonculturable' (VBNC) state, the cells regain culturability and the renewed ability to cause infection. It is likely that the VBNC state is a survival strategy, although several interesting alternative explanations have been suggested. This review describes the VBNC state, the various chemical and physical factors known to induce cells into this state, the cellular traits and gene expression exhibited by VBNC cells, their antibiotic resistance, retention of virulence and ability to attach and persist in the environment, and factors that have been found to allow resuscitation of VBNC cells. Along with simple reversal of the inducing stresses, a variety of interesting chemical and biological factors have been shown to allow resuscitation, including extracellular resuscitation-promoting proteins, a novel quorum-sensing system (AI-3) and interactions with amoeba. Finally, the central role of catalase in the VBNC response of some bacteria, including its genetic regulation, is described.

The ability to enter into an avirulent viable but non-culturable (VBNC) form is widespread among Listeria monocytogenes isolates from salmon, patients and environment

Media-based bacteriological testing will fail to detect non-culturable organisms and the risk of consuming viable but non-culturable (VBNC) Listeria monocytogenes is unknown. We have here studied whether L. monocytogenes obtained from seafoods, processing environment and clinical cases enter the VBNC state and assessed the virulence of the non-culturable forms of the bacteria. A number of 16 L. monocytogenes strains were starved in microcosm water at 4 °C until loss of culturability. Metabolic activity in the VBNC form was measured as ATP generation using a luciferase assay and membrane integrity was examined using the LIVE/DEAD BacLight assay. All tested L. monocytogenes strains entered the VBNC state after starvation in microcosm water. Ongoing mRNA synthesis of hly in VBNC L. monocytogenes cells re-incubated in culture medium indicated a potential virulence of these forms. Sodium pyruvate and replenishment of nutrient were used in attempts to resuscitate VBNC cells. However, VBNC L. monocytogenes were not resuscitated under these conditions. VBNC L. monocytogenes were tested for virulence in a cell plaque assay and by intraperitoneally inoculation in immunodeficient RAG1^−/− mice. Inoculation of VBNC L. monocytogenes in immunodeficient mice did not cause morbidity, and plaque assay on HT-29 cells in culture indicated that the VBNC cells were avirulent. The results indicate that the risk of non-culturable L. monocytogenes in foods, when the VBNC state is induced by starvation, is negligible.

Resistance of Listeria monocytogenes, Escherichia coli O157:H7 and Campylobacter jejuni after exposure to repetitive cycles of mild bactericidal treatments

While maintaining nutritional and sensorial attributes of fresh foods mild processing technologies generally deliver microbiologically perishable food products. Currently little information exists on possible increase in the resistance of pathogens after repetitive exposure to mild (sub-lethal) treatments. Multiple strain-cocktails of Listeria monocytogenes, Escherichia coli O157:H7 and Campylobacter jejuni were exposed to 20 consecutive cycles of sub-lethal inactivation by three different techniques. Used techniques comprised inactivation with lactic acid (LA), chlorine dioxide (ClO(2)) and intense light pulses (ILP). Results showed that the selection of resistant cells was both species and technique dependent. While repetitive cycles of ClO(2) treatment did not result in increased resistance, repetitive inactivation with LA yielded L. monocytogenes culture of higher resistance in comparison to the parental culture. The increased resistance, expressed as decreased level of reduction in bacterial counts in subsequent inactivation cycles, was also observed with ILP for both L. monocytogenes and E. coli O157:H7 strains. Visual trend observations were confirmed through statistical linear regression analysis. No such effects were noted for C. jejuni which became undetectable after first 2-5 cycles. Current findings indicate the ability of foodborne pathogens to adapt to mild bactericidal treatments creating new challenges in risk assessment and more specifically in hazard analysis.

Nonculturable response of animal enteropathogens in the agricultural environment and implications for food safety

Concerns about animal enteropathogen contamination of fresh horticultural products have,increased worldwide and are mainly due to the ability of bacteria to survive under stress conditions in the agricultural environment and during raw-vegetable processing. This review challenges the idea that the viable but nonculturable phenomenon that has been proven to occur in plant-associated environments contributes to human pathogen survival and might be correlated with foodborne infection. Factors associated with the nonculturable response of bacteria in the field and during postharvest processing and distribution are discussed, specifically for the most common animal enteropathogens linked with the consumption of raw products: Escherichia coli O157:H7, Salmonella, Listeria monocytogenes, and Shigella spp. The accurate detection of live bacterial populations is essential for pathogen screening in food and environmental safety control and in epidemiological analysis and may have to be considered for identification of critical control points at the time of food inspection.

Stress- and growth rate-related differences between plate count and real-time PCR data during growth of Listeria monocytogenes

To assess the overestimation of bacterial cell counts in real-time PCR in relation to stress and growth phase, four different strains of L. monocytogenes were exposed to combinations of osmotic stress (0.5 to 8% [vol/vol] NaCl) and acid stress (pH 5 to 7) in a culture model at a growth temperature of 10 degrees C or were grown under optimal conditions. Growth curves obtained from real-time PCR, optical density, and viable count data were compared. As expected, optical density data revealed entirely different growth curves. Good to moderate growth conditions yielded good correlation of real-time PCR data and plate count data (r(2) = 0.96 and 0.99) with similar cell counts. When growth conditions became worse, the numbers of CFU decreased during the stationary phase, whereas real-time-PCR-derived bacterial cell equivalents differed in this regard; the correlation worsened (r(2) = 0.84). However, fitted growth curves revealed that maximum growth rates calculated from real-time PCR data were not significantly different from those derived from plate count data. The overestimation of bacterial cell counts by real-time PCR observed in the stationary phase under higher-stress conditions might be explained by the accumulation of viable but nonculturable bacteria or dead bacteria and extracellular DNA. Considering these results, real-time PCR data collected from naturally contaminated samples should be viewed with caution.

Evidence of lethal and sublethal injury in food-borne bacterial pathogens exposed to high-intensity pulsed-plasma gas discharges

AIMS

To apply scanning electron microscopy, image analysis and a fluorescent viability stain to assess lethal and sublethal injury in food-borne bacteria exposed to pulsed-plasma gas discharges (PPGD).

METHODS AND RESULTS

The fluorescent redox probe 5-cyano-2,3-ditolyl tetrazolium chloride (CTC) was used for enumerating actively respiring cells of Campylobacter jejuni, Escherichia coli, Listeria monocytogenes, Staphylococcus aureus and Salmonella enterica serovar Typhimurium that were suspended in sterile water at 4 degrees C and exposed to separate PPGD and heat treatments. While there was good agreement between use of respiratory staining (RS) and direct-selective agar plate counting (PC) for enumerating untreated bacteria, there were c. 1 and 3 log-unit differences in surviving cell numbers per millilitre for test organisms subjected to PPGD and heat treatments respectively, when enumerated by these different viability indicators. PPGD-treated bacteria were markedly altered at the cellular level when examined by scanning electron microscopy.

CONCLUSIONS

Use of this RS method revealed that substantial subpopulations of test bacteria rendered incapable of forming colonies by separate PPGD and heat treatments may remain metabolically active.

SIGNIFICANCE AND IMPACT OF THE STUDY

Use of this RS method offers interesting perspectives on assessing established and novel microbial inactivation methods, and may also provide a better understanding of mechanisms involved in microbial inactivation induced by high-intensity PPGD treatments.