Thirty years of viable but nonculturable state research: unsolved molecular mechanisms

Biofilms in Chronic Wounds: Pathogenesis and Diagnosis

Chronic non-healing wounds have become a major worldwide healthcare burden. The impact of biofilms on chronic wound infection is well established. Despite increasing understanding of the underlying mechanism of biofilm formation in chronic wounds, current strategies for biofilm diagnosis in chronic wounds are still far from ideal. In this review, we briefly summarize the mechanism of biofilm formation and focus on current diagnostic approaches of chronic wound biofilms based on morphology, microbiology, and molecular assays. Innovative biotechnological approaches, such as wound blotting and transcriptomic analysis, may further shed light on this unmet clinical need. The continuous development of these sophisticated diagnostic approaches can markedly contribute to the future implementation of point-of-care biofilm detection in chronic wound care.

Sublethal Injury and Viable but Non-culturable (VBNC) State in Microorganisms During Preservation of Food and Biological Materials by Non-thermal Processes

The viable but non-culturable (VBNC) state, as well as sublethal injury of microorganisms pose a distinct threat to food safety, as the use of traditional, culture-based microbiological analyses might lead to an underestimation or a misinterpretation of the product's microbial status and recovery phenomena of microorganisms may occur. For thermal treatments, a large amount of data and experience is available and processes are designed accordingly. In case of innovative inactivation treatments, however, there are still several open points with relevance for the investigation of inactivation mechanisms as well as for the application and validation of the preservation processes. Thus, this paper presents a comprehensive compilation of non-thermal preservation technologies, i.e., high hydrostatic pressure (HHP), pulsed electric fields (PEFs), pulsed light (PL), and ultraviolet (UV) radiation, as well as cold plasma (CP) treatments. The basic technological principles and the cellular and molecular mechanisms of action are described. Based on this, appropriate analytical methods are outlined, i.e., direct viable count, staining, and molecular biological methods, in order to enable the differentiation between viable and dead cells, as well as the possible occurrence of an intermediate state. Finally, further research needs are outlined.

A comprehensive review on the use of second generation TiO2 photocatalysts: Microorganism inactivation

Photocatalytic disinfection practices have been applied for decades and attract current interest along with the developments in synthesis of novel photocatalysts. A survey based investigation was performed for elucidation of photocatalytic treatment details as well as disinfection mechanism of microorganisms. The present work brings significant information on the utilization of second generation TiO₂ photocatalysts for inactivation of microorganisms typically using E. coli as the model microorganism. Special interest was devoted to the role of organic matrix either generated during treatment or as a natural component. Studies on photocatalytic disinfection were extensively reviewed and evaluated with respect to basic operational parameters related to photocatalysis, and types and properties of microorganisms investigated. Degradation mechanism and behavior of microorganisms towards reactive oxygen species during disinfection and organic matrix effects were also addressed. For successful utilization and effective assessment of visible light active photocatalysts, standard protocols for disinfection activity testing have to be set. Further improvement of the efficiency of these materials would be promising for future applications in water treatment processes.

Internal sample process control improves cultivation-independent quantification of thermotolerant Campylobacter

Quantification of Campylobacter is challenging and one major reason is the fact that bacteria lose cultivability due to cold or oxygen stress during storage at retail. Alternative live/dead discriminatory qPCR currently lacks standardization and might overestimate live cells in the presence of dead cells. In this study an internal sample process control (ISPC) was developed. The ISPC consists of a specified number of peroxide-killed C. sputorum cells to be added to each sample in order to monitor (i) the level of reduction of the signal from dead cells and (ii) DNA losses during sample processing. A species-specific fragment of the 16S rRNA gene of C. sputorum was selected as real-time PCR target, based on its similar size and gene copy number compared to the C. jejuni/coli/lari target and confirmed in an exclusivity study. Extension of the amplification oligonucleotides for the target of thermotolerant Campylobacter improved real-time PCR efficiency, rendering the method suitable for quantification according to international standards. Concordant PCR signal variation of both C. jejuni and C. sputorum targets in co-inoculated chicken rinses verified the suitability of the ISPC. This provides a crucial step towards implementation of cultivation-independent quantification for improved food safety of fastidious bacteria.

Starved viable but non-culturable (VBNC) Legionella strains can infect and replicate in amoebae and human macrophages

Legionella infections are among the most important waterborne infections with constantly increasing numbers of cases in industrialized countries, as a result of aging populations, rising numbers of immunocompromised individuals and increased need for conditioned water due to climate change. Surveillance of water systems is based on microbiological culture-based techniques; however, it has been shown that high percentages of the Legionella populations in water systems are not culturable. In the past two decades, the relevance of such viable but non-culturable (VBNC) legionellae has been controversially discussed, and whether VBNC legionellae can directly infect human macrophages, the primary targets of Legionella infections, remains unclear. In this study, it was demonstrated for the first time that several starved VBNC Legionella strains (four L. pneumophila serogroup 1 strains, a serogroup 6 strain and a L. micdadei strain) can directly infect different types of human macrophages and amoebae even after one year of starvation in ultrapure water. However, under these conditions, the strains caused infection with reduced efficacy, as represented by the lower percentages of infected cells, prolonged time in co-culture and higher multiplicities of infection required. Interestingly, the VBNC cells remained mostly non-culturable even after multiplication within the host cells. Amoebal infection by starved VBNC Legionella, which likely occurs in oligotrophic biofilms, would result in an increase in the bacterial concentration in drinking-water systems. If cells remain in the VBNC state, the real number of active legionellae will be underestimated by the use of culture-based standard techniques. Thus, further quantitative research is needed in order to determine, whether and how many starved VBNC Legionella cells are able to cause disease in humans.

Helicobacter pylori: A foodborne pathogen?

Helicobacter pylori (H. pylori) is an organism that is widespread in the human population and is sometimes responsible for some of the most common chronic clinical disorders of the upper gastrointestinal tract in humans, such as chronic-active gastritis, duodenal and gastric ulcer disease, low-grade B-cell mucosa associated lymphoid tissue lymphoma of the stomach, and gastric adenocarcinoma, which is the third leading cause of cancer death worldwide. The routes of infection have not yet been firmly established, and different routes of transmission have been suggested, although the most commonly accepted hypothesis is that infection takes place through the faecal-oral route and that contaminated water and foods might play an important role in transmission of the microorganism to humans. Furthermore, several authors have considered H. pylori to be a foodborne pathogen because of some of its microbiological and epidemiological characteristics. H. pylori has been detected in drinking water, seawater, vegetables and foods of animal origin. H. pylori survives in complex foodstuffs such as milk, vegetables and ready-to-eat foods. This review article presents an overview of the present knowledge on the microbiological aspects in terms of phenotypic characteristics and growth requirements of H. pylori, focusing on the potential role that foodstuffs and water may play in the transmission of the pathogen to humans and the methods successfully used for the detection of this microorganism in foodstuffs and water.

Resuscitation of functional bacterial community for enhancing biodegradation of phenol under high salinity conditions based on Rpf

This study assumed that key degraders of functional bacterial community were prone to enter into the viable but non-culturable (VBNC) state under high saline phenolic conditions, and resuscitation-promoting factor (Rpf) could strengthen these degraders for better performances. Based on these assumptions, Rpf was used to enhance salt-tolerant phenol-degrading capability of functional populations in activated sludge. Results suggested that Rpf accelerated the start-up process during sludge domestication, and significantly enhanced salt-tolerant phenol-degrading capability. High-throughput sequencing showed that the resuscitation and stimulation functions of Rpf linked mainly to the genus Corynebacterium within the phylum Actinobacteria, and the genera Proteiniphilum and Petrimonas within the phylum Bacteroidete. These key functional populations contributed to better phenol-degrading capabilities under high salinity conditions. This study indicated that Rpf is a promising additive for improving biological treatment performance of saline phenolic wastewater.

Induction of a viable but non-culturable state in Salmonella Typhimurium is correlated with free radicals generated by thermosonication

The viable but non-culturable (VBNC) state in bacteria is gaining more and more attention around the world, mainly due to it is not only an important strategy for adaption to a stressful environment but also may possibly pose a threat to food safety and public health. It was found that a small subpopulation of Salmonella Typhimurium in pure culture was induced into a VBNC state during thermosonication (TS) processing in our previous study, though few known about the situation in real food and how bacteria were induced into that special state. Base on the speculation that free radicals generated during TS affected induction of VBNC, the relationship between them was investigated preliminarily. It was observed that higher intensity of TS treatment, such as higher power, elevated temperature and prolonged duration resulted in more viable S. Typhimurium cells in carrot juice been inducted into VBNC state. The observed results showed that VBNC incidence indexes were 1000 as TS treatments at 57-62 °C and 380 W for 6 min, and 53 °C and 380 W for 8-10 min were applied, indicating a 100% VBNC state of viable cells in those cases. The ESR spectra revealed three kinds of free radicals, including carbon centered (ethanol) radicals, hydroxyl radical and hydrogen protons were generated in carrot juice during TS processing. The intensity of free radicals was tied to the TS processing parameters and also influenced the occurrence of VBNC. A nonlinear sigmoidal curve of the intensity of free radicals VS the VBNC incidence index in three stages, including a slow phase (with free radicals intensity of 0-0.10), a rapid growth phase (with free radicals intensity ranged 0.10-0.14) and a final equilibrium phase (with free radicals intensity greater than 0.14), was observed and well fitted with the Boltzmann model. Moreover, the significance of free radicals generated during TS processing for induction of VBNC state was verified and confirmed with 0-200 mM sodium pyruvate. The obtained results may contribute to understand the complicated phenomenon and guide the application of TS as a decontamination technique.

Presence and Persistence of Salmonella in Water: The Impact on Microbial Quality of Water and Food Safety

Salmonella ranks high among the pathogens causing foodborne disease outbreaks. According to the Centers for Disease Control and Prevention, Salmonella contributed to about 53.4% of all foodborne disease outbreaks from 2006 to 2017, and approximately 32.7% of these foodborne Salmonella outbreaks were associated with consumption of produce. Trace-back investigations have suggested that irrigation water may be a source of Salmonella contamination of produce and a vehicle for transmission. Presence and persistence of Salmonella have been reported in surface waters such as rivers, lakes, and ponds, while ground water in general offers better microbial quality for irrigation. To date, culture methods are still the gold standard for detection, isolation and identification of Salmonella in foods and water. In addition to culture, other methods for the detection of Salmonella in water include most probable number, immunoassay, and PCR. The U.S. Food and Drug Administration (FDA) issued the Produce Safety Rule (PSR) in January 2013 based on the Food Safety Modernization Act (FSMA), which calls for more efforts toward enhancing and improving approaches for the prevention of foodborne outbreaks. In the PSR, agricultural water is defined as water used for in a way that is intended to, or likely to, contact covered produce, such as spray, wash, or irrigation. In summary, Salmonella is frequently present in surface water, an important source of water for irrigation. An increasing evidence indicates irrigation water as a source (or a vehicle) for transmission of Salmonella. This pathogen can survive in aquatic environments by a number of mechanisms, including entry into the viable but nonculturable (VBNC) state and/or residing within free-living protozoa. As such, assurance of microbial quality of irrigation water is critical to curtail the produce-related foodborne outbreaks and thus enhance the food safety. In this review, we will discuss the presence and persistence of Salmonella in water and the mechanisms Salmonella uses to persist in the aquatic environment, particularly irrigation water, to better understand the impact on the microbial quality of water and food safety due to the presence of Salmonella in the water environment.

Persistence in Phytopathogenic Bacteria: Do We Know Enough?

Phytopathogenic bacteria affect a wide range of crops worldwide and have a negative impact in agriculture due to their associated economic losses and environmental impacts. Together with other biotic and abiotic stress factors, they pose a threat to global food production. Therefore, understanding bacterial survival strategies is an essential step toward the development of new strategies to control plant diseases. One mechanism used by bacteria to survive under stress conditions is the formation of persister cells. Persisters are a small fraction of phenotypic variants within an isogenic population that exhibits multidrug tolerance without undergoing genetic changes. They are dormant cells that survive treatment with antimicrobials by inactivating the metabolic functions that are disrupted by these compounds. They are thus responsible for the recalcitrance of many human diseases, and in the same way, they are thought to contribute to the survival of bacterial phytopathogens under a range of stresses they face in the environment. It is believed that persister cells of bacterial phytopathogens may lead to the reoccurrence of disease by recovering growth and recolonizing the host plant after the end of stress. However, compared to human pathogens, little is known about persister cells in phytopathogens, especially about their genetic regulation. In this review, we describe the overall knowledge on persister cells and their regulation in bacterial phytopathogens, focusing on their ability to survive stress conditions, to recover from dormancy and to maintain virulence.

Detection of Clavibacter michiganensis subsp. michiganensis in viable but nonculturable state from tomato seed using improved qPCR

Clavibacter michiganensis subsp. michiganensis (Cmm) is a seed-borne pathogen that causes bacterial canker disease of tomato. Cmm is typically detected in tomato seeds using quantitative real-time polymerase chain reaction (qPCR) combined with culture-based isolation. The viable but nonculturable (VBNC) state of Cmm may result in the underestimation or false negative detection of the pathogen. In the present study, propidium monoazide (PMA) and its improved structure PMAxx were used to pretreat Cmm prior to DNA extraction, followed by qPCR. Both PMA and PMAxx could bind to the chromosomal DNA of dead bacterial cells and therefore block DNA amplification by PCR. This effect, however, does not occur in living bacterial cells, as the chemicals cannot penetrate through the undamaged cell membrane. Both viable and dead Cmm cells were treated with PMA and PMAxx at various concentrations. With this treatment, the range of the cell population was determined for effective detection. PMAxx showed a better discrimination effect than PMA on the viable and dead cells of Cmm and was therefore used throughout the present study. VBNC cells of Cmm (108 CFU mL-1) was induced by 50 μM copper sulfate, which was detected at different sampling times up to a month by using both PMAxx-qPCR and flow cytometry assays. The optimal PMAxx concentration was 20 μM for detecting membrane-intact Cmm cells. High specificity and sensitivity were obtained at Cmm concentrations ranging from 103 to 107 CFU mL-1. The accurate and robust results of PMAxx-qPCR were confirmed by flow cytometry method to detect viable Cmm cells. Furthermore, the PMAxx-qPCR assay was successfully used in detecting VBNC Cmm cells in tomato seeds with as few as 10 seeds per set.

Monitoring Viable Cells of the Biological Control Agent Lactobacillus plantarum PM411 in Aerial Plant Surfaces by Means of a Strain-Specific Viability Quantitative PCR Method

A viability quantitative PCR (v-qPCR) assay was developed for the unambiguous detection and quantification of Lactobacillus plantarum PM411 viable cells in aerial plant surfaces. A 972-bp region of a PM411 predicted prophage with mosaic architecture enabled the identification of a PM411 strain-specific molecular marker. Three primer sets with different amplicon lengths (92, 188, and 317 bp) and one TaqMan probe were designed. All the qPCR assays showed good linearity over a 4-log range and good efficiencies but differed in sensitivity. The nucleic acid-binding dye PEMAX was used to selectively detect and enumerate viable bacteria by v-qPCR. The primer set amplifying a 188-bp DNA fragment was selected as the most suitable for v-qPCR. The performance of the method was assessed on apple blossoms, pear, strawberry, and kiwifruit leaves in potted plants under controlled environmental conditions, as well as pear and apple blossoms under field conditions, by comparing v-qPCR population estimations to those obtained by qPCR and specific plate counting on de Man-Rogosa-Sharpe (MRS)-rifampin. The population estimation did not differ significantly between methods when conditions were conducive to bacterial survival. However, under stressful conditions, differences between methods were observed due to cell death or viable-but-nonculturable state induction. While qPCR overestimated the population level, plate counting underestimated this value in comparison to v-qPCR. PM411 attained stable population levels of viable cells on the flower environment under high relative humidity. However, the unfavorable conditions on the leaf surface and the relatively dryness in the field caused an important decrease in the viable population.

IMPORTANCE The v-qPCR method in combination with plate counting and qPCR is a powerful tool for studies of colonization and survival under field conditions, to improve formulations and delivery strategies of PM411, and to optimize the dose and timing of spray schedules. It is expected that PEMAX v-qPCR could also be developed for monitoring other strains on plant surfaces not only as biological control agents but also beneficial bacteria useful in the sustainable management of crop production.

Transcriptional Response of Staphylococcus aureus to Sunlight in Oxic and Anoxic Conditions

The transcriptional response of Staphylococcus aureus strain Newman to sunlight exposure was investigated under both oxic and anoxic conditions using RNA sequencing to gain insight into potential mechanisms of inactivation. S. aureus is a pathogenic bacterium detected at recreational beaches which can cause gastrointestinal illness and skin infections, and is of increasing public health concern. To investigate the S. aureus photostress response in oligotrophic seawater, S. aureus cultures were suspended in seawater and exposed to full spectrum simulated sunlight. Experiments were performed under oxic or anoxic conditions to gain insight into the effects of oxygen-mediated and non-oxygen-mediated inactivation mechanisms. Transcript abundance was measured after 6 h of sunlight exposure using RNA sequencing and was compared to transcript abundance in paired dark control experiments. Culturable S. aureus decayed following biphasic inactivation kinetics with initial decay rate constants of 0.1 and 0.03 m² kJ⁻¹ in oxic and anoxic conditions, respectively. RNA sequencing revealed that 71 genes had different transcript abundance in the oxic sunlit experiments compared to dark controls, and 18 genes had different transcript abundance in the anoxic sunlit experiments compared to dark controls. The majority of genes showed reduced transcript abundance in the sunlit experiments under both conditions. Three genes (ebpS, NWMN_0867, and NWMN_1608) were found to have the same transcriptional response to sunlight between both oxic and anoxic conditions. In the oxic condition, transcripts associated with porphyrin metabolism, nitrate metabolism, and membrane transport functions were increased in abundance during sunlight exposure. Results suggest that S. aureus responds differently to oxygen-dependent and oxygen-independent photostress, and that endogenous photosensitizers play an important role during oxygen-dependent indirect photoinactivation.

Rapid assessment of viable but non-culturable Bacillus coagulans MTCC 5856 in commercial formulations using Flow cytometry

Accurate enumeration of bacterial count in probiotic formulation is imperative to ensure that the product adheres to regulatory standards and citation in consumer product label. Standard methods like plate count, can enumerate only replicating bacterial population under selected culture conditions. Viable but non culturable bacteria (VBNC) retain characteristics of living cells and can regain cultivability by a process known as resuscitation. This is a protective mechanism adapted by bacteria to evade stressful environmental conditions. B. coagulans MTCC 5856(LactoSpore®) is a probiotic endospore which can survive for decades in hostile environments without dividing. In the present study, we explored the use of flow cytometry to enumerate the viable count of B. coagulans MTCC 5856 under acidic and alkaline conditions, high temperature and in commercial formulations like compressed tablets and capsules. Flow cytometry (FCM) was comparable to plate count method when the spores were counted at physiological conditions. We show that VBNC state is induced in B. coagulans MTCC 5856by high temperature and acidic pH. The cells get resuscitated under physiological conditions and FCM was sensitive to detect the VBNC spores. Flow cytometry showed excellent ability to assess the viable spore count in commercial probiotic formulations of B. coagulans MTCC 5856. The results establish Flow cytometry as a reliable method to count viable bacteria in commercial probiotic preparations. Sporulation as well as existence as VBNC could contribute to the extreme stability of B. coagulans MTCC 5856.

Analysis of Staphylococcus aureus cell viability, sublethal injury and death induced by synergistic combination of ultrasound and mild heat

This study was designed to investigate the combined effects of ultrasound and mild heat on the viability of S. aureus in association with the cell membrane integrity and intracellular enzyme activity. Cells were treated by ultrasound under 55°C for 3, 5, 7, 10, and 15min. The dynamic changes of S. aureus cell viability, sublethal injury and death were evaluated using flow cytometric assay. Microscopies were applied to identify the morphological appearance, ultrastructure and topography of S. aureus. The results showed the membrane damage was synchronous with esterase inhibition during the exposure to sonication, leading to the immediate lethal effect. On the other hand, bacteria under the mild heat at 55°C were inactivated via a sublethal injury process. The different lethal modes were observed between sonication and mild heat treatments, which could synergistically inactivate S. aureus. The antibacterial value of thermo-sonication was greater than the sum of the individual treatments. The thermo-sonication combination synergistically reduced the number of sublethal cells and also resulted in severe cell damage.

Physicochemical Factors Influence the Abundance and Culturability of Human Enteric Pathogens and Fecal Indicator Organisms in Estuarine Water and Sediment

To assess fecal pollution in coastal waters, current monitoring is reliant on culture-based enumeration of bacterial indicators, which does not account for the presence of viable but non-culturable or sediment-associated micro-organisms, preventing effective quantitative microbial risk assessment (QMRA). Seasonal variability in viable but non-culturable or sediment-associated bacteria challenge the use of fecal indicator organisms (FIOs) for water monitoring. We evaluated seasonal changes in FIOs and human enteric pathogen abundance in water and sediments from the Ribble and Conwy estuaries in the UK. Sediments possessed greater bacterial abundance than the overlying water column, however, key pathogenic species (Shigella spp., Campylobacter jejuni, Salmonella spp., hepatitis A virus, hepatitis E virus and norovirus GI and GII) were not detected in sediments. Salmonella was detected in low levels in the Conwy water in spring/summer and norovirus GII was detected in the Ribble water in winter. The abundance of E. coli and Enterococcus spp. quantified by culture-based methods, rarely matched the abundance of these species when measured by qPCR. The discrepancy between these methods was greatest in winter at both estuaries, due to low CFU's, coupled with higher gene copies (GC). Temperature accounted for 60% the variability in bacterial abundance in water in autumn, whilst in winter salinity explained 15% of the variance. Relationships between bacterial indicators/pathogens and physicochemical variables were inconsistent in sediments, no single indicator adequately described occurrence of all bacterial indicators/pathogens. However, important variables included grain size, porosity, clay content and concentrations of Zn, K, and Al. Sediments with greater organic matter content and lower porosity harbored a greater proportion of non-culturable bacteria (including dead cells and extracellular DNA) in winter. Here, we show the link between physicochemical variables and season which govern culturability of human enteric pathogens and FIOs. Therefore, knowledge of these factors is critical for accurate microbial risk assessment. Future water quality management strategies could be improved through monitoring sediment-associated bacteria and non-culturable bacteria. This could facilitate source apportionment of human enteric pathogens and FIOs and direct remedial action to improve water quality.

Synergetic effects of ultrasound and slightly acidic electrolyzed water against Staphylococcus aureus evaluated by flow cytometry and electron microscopy

This study evaluated the synergetic effects of ultrasound and slightly acidic electrolyzed water (SAEW) on the inactivation of Staphylococcus aureus using flow cytometry and electron microscopy. The individual ultrasound treatment for 10min only resulted in 0.36logCFU/mL reductions of S. aureus, while the SAEW treatment alone for 10min resulted in 3.06logCFU/mL reductions. The log reductions caused by combined treatment were enhanced to 3.68logCFU/mL, which were greater than the sum of individual treatments. This phenomenon was referred to as synergistic effects. FCM analysis distinguished live and dead cells as well as revealed dynamic changes in the physiological states of S. aureus after different treatments. The combined treatment greatly reduced the number of viable but nonculturable (VBNC) bacteria to 0.07%; in contrast, a single ultrasound treatment for 10min induced the formation of VBNC cells to 45.75%. Scanning and transmission electron microscopy analysis revealed that greater damage to the appearance and ultrastructure of S. aureus were achieved after combined ultrasound-SAEW treatment compared to either treatment alone. These results indicated that combining ultrasound with SAEW is a promising sterilization technology with potential uses for environmental remediation and food preservation.

Flow cytometric characterization of bacterial abundance and physiological status in a nitrifying-denitrifying activated sludge system treating landfill leachate

Flow cytometry has recently been presented as a research tool in the assessment of the viability/activity of activated sludge from municipal wastewater treatment plants, but it has not put in practice for industrial biotreatments yet. In this study, for the first time ever, the reliability and significance of the multiparameter flow cytometry applied to the biological nitrification-denitrification treatment of leachate have been evaluated. Using a double staining procedure (cFDA/PI), the viable, damaged, and dead subpopulations were determined, and the results were compared to those obtained with conventional methods, such as nitrogen and oxygen uptake rates or plate counting. Flow cytometry showed that viable cells represented approximately 47% of the total population, whereas active cells accounted for 90%. For both sludge from nitrification and denitrification processes, with less than 1% of them being also culturable in plate. Either flow cytometry or uptake rates revealed that health status of sludge remained constant throughout the biotreatment, which is consistent with the high recirculation rates. Under anaerobic starvation conditions, physiological status of sludge remained constant as well as specific oxygen and denitrification rates. Nevertheless, both the culturability in plate and the nitrification rate significantly decreased. These findings proved that multiparameter flow cytometry is a useful tool for the assessment of the viability and activity of sludge from a nitrification-denitrification biotreatment process. These results gathered all the bacterial communities in the sludge, so the decay in minority populations, such as nitrifying bacteria, requires the use of a complementary technique to evaluate specific activities.

Toxin-Antitoxin Systems: Implications for Plant Disease

Toxin-antitoxin (TA) systems are gene modules that are ubiquitous in free-living prokaryotes. Diverse in structure, cellular function, and fitness roles, TA systems are defined by the presence of a toxin gene that suppresses bacterial growth and a toxin-neutralizing antitoxin gene, usually encoded in a single operon. Originally viewed as DNA maintenance modules, TA systems are now thought to function in many roles, including bacterial stress tolerance, virulence, phage defense, and biofilm formation. However, very few studies have investigated the significance of TA systems in the context of plant-microbe interactions. This review discusses the potential impact and application of TA systems in plant-associated bacteria, guided by insights gained from animal-pathogenic model systems.

The Importance of Bacterial Culture to Food Microbiology in the Age of Genomics

Culture-based and genomics methods provide different insights into the nature and behavior of bacteria. Maximizing the usefulness of both approaches requires recognizing their limitations and employing them appropriately. Genomic analysis excels at identifying bacteria and establishing the relatedness of isolates. Culture-based methods remain necessary for detection and enumeration, to determine viability, and to validate phenotype predictions made on the bias of genomic analysis. The purpose of this short paper is to discuss the application of culture-based analysis and genomics to the questions food microbiologists routinely need to ask regarding bacteria to ensure the safety of food and its economic production and distribution. To address these issues appropriate tools are required for the detection and enumeration of specific bacterial populations and the characterization of isolates for, identification, phylogenetics, and phenotype prediction.

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.

Physiological and Transcriptome Response of the Polycyclic Aromatic Hydrocarbon Degrading Novosphingobium sp. LH128 after Inoculation in Soil

Soil bioaugmentation involves the inoculation of pollutant-degrading bacteria to accelerate pollutant degradation. Often the inoculum shows a dramatic decrease in Colony Forming Units (CFU) upon soil inoculation but this behavior is not well-understood. In this study, the physiology and transcriptomic response of a GFP tagged variant of Novosphingobium sp. LH128 was examined after inoculation into phenanthrene spiked soil. Four hours after inoculation, strain LH128-GFP showed about 99% reduction in CFU while microscopic counts of GFP-expressing cells were identical to the expected initial cell density, indicating that the reduction in CFU number is explained by cells entering into a Viable But Non-Culturable (VBNC)-like state and not by cell death. Transcriptome analysis showed a remarkably higher expression of phenanthrene degradation genes 4 h after inoculation, compared to the inoculum suspension concomitant with an increased expression of genes involved in stress response. This indicates that the cells were active in phenanthrene degradation while experiencing stress. Between 4 h and 10 days, CFU numbers increased to numbers comparable to the inoculated cell density. Our results suggest that strain LH128-GFP enters a VBNC-like state upon inoculation into soil but is metabolically active and that VBNC cells should be taken into account in evaluating bioaugmentation approaches.

Survival strategies of Escherichia coli and Vibrio spp.: contribution of the viable but nonculturable phenotype to their stress-resistance and persistence in adverse environments

In their natural ecosystems, bacteria are continuously exposed to changing environmental factors including physicochemical parameters (e.g. temperature, pH, etc.), availability of nutrients as well as interaction(s) with other organisms. To increase their tolerance and survival under adverse conditions, bacteria trigger a number of adaptation mechanisms. One of the well-known adaptation responses of the non-spore-forming bacteria is the acquisition of the viable but non-culturable (VBNC) state. This phenotype is induced by different stress factors (e.g. low temperature) and is characterized by the temporal loss of culturability, which can potentially be restored. Moreover, this response can be combined with the bust and boom strategy, which implies the death of the main population of the stressed cells (or their entry into the VBNC state) upon stress, thus enabling the remaining cells (i.e. residual culturable population) to subsist at the expense of the dead or/and VBNC cells. In this review, we discuss the characteristics of the VBNC state, its biological significance and contribution to bacterial survival.

Evaluation of Molecular Methods for the Detection and Quantification of Pathogen-Derived Nucleic Acids in Sediment

The accurate detection of pathogens in environmental matrices, such as sediment, is critical in understanding pathogen fate and behavior in the environment. In this study, we assessed the usefulness of methods for the detection and quantification of Vibrio spp. and norovirus (NoV) nucleic acids in sediment. For bacteria, a commonly used direct method using hexadecyltrimethylammonium bromide (CTAB) and phenol-chloroform-isoamyl alcohol (PCI) extraction was optimized, whereas for NoV, direct and indirect (virus elution-concentration) methods were evaluated. For quantification, commercially available quantitative PCR (qPCR) and reverse transcription qPCR (RT-qPCR) kits were tested alongside a digital PCR (dPCR) approach. CTAB-based extraction combined with 16 h polyethylene glycol 6000 (PEG6000) precipitation was found to be suitable for the direct extraction of high abundance bacterial and viral nucleic acids. For the indirect extraction of viral RNA, beef extract-based elution followed by PEG6000 precipitation and extraction using the NucliSENS® MiniMag® Nucleic Acid Purification System and the PowerViral® Environmental RNA/DNA Isolation Kit and qRT-PCR resulted in 83-112 and 63-69% recoveries of NoV, respectively. dPCR resulted in lower viral recoveries (47 and 9%) and ~4 orders of magnitude lower Vibrio concentrations (3.6-4.6 log10 gc/100 g sediment) than was observed using qPCR. The use of internal controls during viral quantification revealed that the RT step was more affected by inhibitors than the amplification. The methods described here are suitable for the enumeration of viral and/or bacterial pathogens in sediment, however the use of internal controls to assess efficiency is recommended.

Polymicrobial Ventilator-Associated Pneumonia: Fighting In Vitro Candida albicans-Pseudomonas aeruginosa Biofilms with Antifungal-Antibacterial Combination Therapy

The polymicrobial nature of ventilator-associated pneumonia (VAP) is now evident, with mixed bacterial-fungal biofilms colonizing the VAP endotracheal tube (ETT) surface. The microbial interplay within this infection may contribute for enhanced pathogenesis and exert impact towards antimicrobial therapy. Consequently, the high mortality/morbidity rates associated to VAP and the worldwide increase in antibiotic resistance has promoted the search for novel therapeutic strategies to fight VAP polymicrobial infections. Under this scope, this work aimed to assess the activity of mono- vs combinational antimicrobial therapy using one antibiotic (Polymyxin B; PolyB) and one antifungal (Amphotericin B; AmB) agent against polymicrobial biofilms of Pseudomonas aeruginosa and Candida albicans. The action of isolated antimicrobials was firstly evaluated in single- and polymicrobial cultures, with AmB being more effective against C. albicans and PolyB against P. aeruginosa. Mixed planktonic cultures required equal or higher antimicrobial concentrations. In biofilms, only PolyB at relatively high concentrations could reduce P. aeruginosa in both monospecies and polymicrobial populations, with C. albicans displaying only punctual disturbances. PolyB and AmB exhibited a synergistic effect against P. aeruginosa and C. albicans mixed planktonic cultures, but only high doses (256 mg L^-1) of PolyB were able to eradicate polymicrobial biofilms, with P. aeruginosa showing loss of cultivability (but not viability) at 2 h post-treatment, whilst C. albicans only started to be inhibited after 14 h. In conclusion, combination therapy involving an antibiotic and an antifungal agent holds an attractive therapeutic option to treat severe bacterial-fungal polymicrobial infections. Nevertheless, optimization of antimicrobial doses and further clinical pharmacokinetics/pharmacodynamics and toxicodynamics studies underpinning the optimal use of these drugs are urgently required to improve therapy effectiveness and avoid reinfection.

Promoting resuscitation of viable but nonculturable cells of Vibrio harveyi by a resuscitation-promoting factor-like protein YeaZ

AIMS

To demonstrate the resuscitation-promoting activities of recombinant YeaZ from Vibrio harveyi SF-1.

METHODS AND RESULTS

The gene of resuscitation-promoting factor YeaZ was cloned from genomic DNA of V. harveyi SF-1. The gene was expressed in Escherichia coli, and the expressed protein was purified by Ni²⁺ -affinity chromatography. A yeaZ mutant was constructed by using the suicide plasmid pNQ705 with homologous recombination. Disruption of yeaZ did not affect cell growth significantly in 2216 E broth at 28°C. The wild-type and mutant viable but nonculturable (VBNC) cells could be resuscitated by temperature upshift method. In addition, the recombinant YeaZ increased the culturable counts from 1·27 × 10⁴  CFU per ml and 1·99 × 10⁴ CFU per ml to 2·88 × 10⁵  CFU per ml and 4·59 × 10⁵ CFU per ml, respectively. After the VBNC cells of wild-type and mutant cells were maintained at 4°C for 120 days, no resuscitation was obtained by temperature upshift method, but addition of the recombinant YeaZ promoted the resuscitation of the wild-type and mutant cells, with the culturable cell counts of 1·13 × 10³ and 1·44 × 10³ CFU per ml, respectively. Disruption of yeaZ decreased the virulence of V. harveyi in zebrafish. The lethal dose 50% of the yeaZ null mutant was more than 10-fold higher than that of the wild-type cells.

CONCLUSIONS

The recombinant YeaZ could efficiently promote resuscitation of the wild-type and mutant cells of V. harveyi from VBNC to culturable state. The protein also promoted resuscitation of the VBNC wild-type and mutant cells, which were maintained at 4°C for 120 days and not recovered by temperature upshift method. Disruption of yeaZ decreased the virulence of V. harveyi in zebrafish.

SIGNIFICANCE AND IMPACT OF THE STUDY

Here, we show clear evidence of a resuscitation-promoting factor YeaZ of V. harveyi and the roles in resuscitation of the VBNC cells and its pathogenicity.

Abundance and Distribution of Enteric Bacteria and Viruses in Coastal and Estuarine Sediments-a Review

The long term survival of fecal indicator organisms (FIOs) and human pathogenic microorganisms in sediments is important from a water quality, human health and ecological perspective. Typically, both bacteria and viruses strongly associate with particulate matter present in freshwater, estuarine and marine environments. This association tends to be stronger in finer textured sediments and is strongly influenced by the type and quantity of clay minerals and organic matter present. Binding to particle surfaces promotes the persistence of bacteria in the environment by offering physical and chemical protection from biotic and abiotic stresses. How bacterial and viral viability and pathogenicity is influenced by surface attachment requires further study. Typically, long-term association with surfaces including sediments induces bacteria to enter a viable-but-non-culturable (VBNC) state. Inherent methodological challenges of quantifying VBNC bacteria may lead to the frequent under-reporting of their abundance in sediments. The implications of this in a quantitative risk assessment context remain unclear. Similarly, sediments can harbor significant amounts of enteric viruses, however, the factors regulating their persistence remains poorly understood. Quantification of viruses in sediment remains problematic due to our poor ability to recover intact viral particles from sediment surfaces (typically <10%), our inability to distinguish between infective and damaged (non-infective) viral particles, aggregation of viral particles, and inhibition during qPCR. This suggests that the true viral titre in sediments may be being vastly underestimated. In turn, this is limiting our ability to understand the fate and transport of viruses in sediments. Model systems (e.g., human cell culture) are also lacking for some key viruses, preventing our ability to evaluate the infectivity of viruses recovered from sediments (e.g., norovirus). The release of particle-bound bacteria and viruses into the water column during sediment resuspension also represents a risk to water quality. In conclusion, our poor process level understanding of viral/bacterial-sediment interactions combined with methodological challenges is limiting the accurate source apportionment and quantitative microbial risk assessment for pathogenic organisms associated with sediments in aquatic environments.

Surface Colonization and Activity of the 2,6-Dichlorobenzamide (BAM) Degrading Aminobacter sp. Strain MSH1 at Macro- and Micropollutant BAM Concentrations

Aminobacter sp. MSH1 uses the groundwater micropollutant 2,6-dichlorobenzamide (BAM) as a C and N source and is a potential catalyst for biotreatment of BAM-contaminated groundwater in filtration units of drinking water treatment plants (DWTPs). The oligotrophic environment of DWTPs including trace pollutant concentrations, and the high flow rates impose challenges for micropollutant biodegradation in DWTPs. To understand how trace BAM concentrations affect MSH1 surface colonization and BAM degrading activity, MSH1 was cultivated in flow channels fed continuously with BAM macro- and microconcentrations in a N- and C-limiting medium. At all BAM concentrations, MSH1 colonized the flow channel. BAM degradation efficiencies were concentration-dependent, ranging between 70 and 95%. Similarly, BAM concentration affected surface colonization, but at 100 μg/L BAM and lower, colonization was similar to that in systems without BAM, suggesting that assimilable organic carbon and nitrogen other than those supplied by BAM sustained colonization at BAM microconcentrations. Comparison of specific BAM degradation rates in flow channels and in cultures of suspended freshly grown cells indicated that starvation conditions in flow channels receiving BAM microconcentrations resulted into MSH1 biomasses with 10-100-times reduced BAM degrading activity and provided a kinetic model for predicting BAM degradation under continuous C and N starvation.

Persisters-as elusive as ever

Persisters—a drug-tolerant sub-population in an isogenic bacterial culture—have been featured throughout the last decade due to their important role in recurrent bacterial infections. Numerous investigations detail the mechanisms responsible for the formation of persisters and suggest exciting strategies for their eradication. In this review, we argue that the very term “persistence” is currently used to describe a large and heterogeneous set of physiological phenomena that are functions of bacterial species, strains, growth conditions, and antibiotics used in the experiments. We caution against the oversimplification of the mechanisms of persistence and urge for a more rigorous validation of the applicability of these mechanisms in each case.

Microbial spoilage, quality and safety within the context of meat sustainability

Meat is a nutrient-dense food that provides ideal conditions for microbes to grow and defines its perishable nature. Some organisms simply spoil it while others are a threat to our health. In either case, meat must be discarded from the food chain and, being wasted and consequently an environmental burden. Worldwide, more than 20% of the meat produced is either lost or wasted. Hence, coordinated efforts from farm to table are required to improve microbial control as part of our effort towards global sustainability. Also, new antimicrobial systems and technologies arise to better fulfill consumer trends and demands, new lifestyles and markets, but for them to be used to their full extent, it is imperative to understand how they work at the molecular level. Undetected survivors, either as injured, dormant, persister or viable but non-culturable (VBNC) cells, undermine proper risk evaluation and management.

Determination of viable legionellae in engineered water systems: Do we find what we are looking for?

In developed countries, legionellae are one of the most important water-based bacterial pathogens caused by management failure of engineered water systems. For routine surveillance of legionellae in engineered water systems and outbreak investigations, cultivation-based standard techniques are currently applied. However, in many cases culture-negative results are obtained despite the presence of viable legionellae, and clinical cases of legionellosis cannot be traced back to their respective contaminated water source. Among the various explanations for these discrepancies, the presence of viable but non-culturable (VBNC) Legionella cells has received increased attention in recent discussions and scientific literature. Alternative culture-independent methods to detect and quantify legionellae have been proposed in order to complement or even substitute the culture method in the future. Such methods should detect VBNC Legionella cells and provide a more comprehensive picture of the presence of legionellae in engineered water systems. However, it is still unclear whether and to what extent these VBNC legionellae are hazardous to human health. Current risk assessment models to predict the risk of legionellosis from Legionella concentrations in the investigated water systems contain many uncertainties and are mainly based on culture-based enumeration. If VBNC legionellae should be considered in future standard analysis, quantitative risk assessment models including VBNC legionellae must be proven to result in better estimates of human health risk than models based on cultivation alone. This review critically evaluates current methods to determine legionellae in the VBNC state, their potential to complement the standard culture-based method in the near future, and summarizes current knowledge on the threat that VBNC legionellae may pose to human health.

Growth Phase, Oxygen, Temperature, and Starvation Affect the Development of Viable but Non-culturable State of Vibrio cholerae

Vibrio cholerae can enter into a viable but non-culturable (VBNC) state in order to survive in unfavorable environments. In this study, we studied the roles of five physicochemical and microbiological factors or states, namely, different strains, growth phases, oxygen, temperature, and starvation, on the development of VBNC of V. cholerae in artificial sea water (ASW). Different strains of the organism, the growth phase, and oxygen levels affected the progress of VBNC development. It was found that the VBNC state was induced faster in V. cholerae serogroup O1 classical biotype strain O395 than in O1 El Tor biotype strains C6706 and N16961. When cells in different growth phases were used for VBNC induction, stationary-phase cells lost their culturability more quickly than exponential-phase cells, while induction of a totally non-culturable state took longer to achieve for stationary-phase cells in all three strains, suggesting that heterogeneity of cells should be considered. Aeration strongly accelerated the loss of culturability. During the development of the VBNC state, the culturable cell count under aeration conditions was almost 10(6)-fold lower than under oxygen-limited conditions for all three strains. The other two factors, temperature and nutrients-rich environment, may prevent the induction of VBNC cells. At 22 or 37°C in ASW, most of the cells rapidly died and the culturable cell count reduced from about 10(8) to 10(6)-10(5) CFU/mL. The total cell counts showed that cells that lost viability were decomposed, and the viable cell counts were the same as culturable cell counts, indicating that the cells did not reach the VBNC state. VBNC state development was blocked when ASW was supplied with Luria-Bertani broth (LB), but it was not affected in ASW with M9, suggesting that specific nutrients in LB may prevent the development of VBNC state. These results revealed that the five factors evaluated in this study had different roles during the progress of VBNC induction. Changing a single factor could influence and even block the development of the VBNC state. These findings provide new insight to help design further studies to better understand the mechanisms which trigger the development and regulation of the VBNC state.

Correlations between cyanobacterial density and bacterial transformation to the viable but nonculturable (VBNC) state in four freshwater water bodies

Nutrient concentrations, phytoplankton density and community composition, and the viable but nonculturable (VBNC) state of heterotrophic bacteria were investigated in three connected reservoirs and a small isolated lake in South China to study the relationship between biotic and abiotic factors and the VBNC state in bacteria. Nutrient concentrations in the reservoirs increased in the direction of water flow, whereas Wenshan Lake was more eutrophic. Cyanobacterial blooms occurred in all four water bodies, with differing seasonal trends and dominant species. In Xili and Tiegang Reservoirs, the VBNC ratio (percent of VBNC state bacteria over total viable bacteria) was high for most of the year and negatively correlated with cyanobacterial density. Laboratory co-culture experiments were performed with four heterotrophic bacterial species isolated from Wenshan Lake (Escherichia coli, Klebsiella peneumoniae, Bacillus megaterium and Bacillus cereus) and the dominant cyanobacterial species (Microcystis aeruginosa). For the first three bacterial species, the presence of M. aeruginosa induced the VBNC state and the VBNC ratio was positively correlated with M. aeruginosa density. However, B. cereus inhibited M. aeruginosa growth. These results demonstrate that cyanobacteria could potentially regulate the transformation to the VBNC state of waterborne bacteria, and suggest a role for bacteria in cyanobacterial bloom initiation and termination.

Pseudomonas aeruginosa Biofilm Response and Resistance to Cold Atmospheric Pressure Plasma Is Linked to the Redox-Active Molecule Phenazine

Pseudomonas aeruginosa is an important opportunistic pathogen displaying high antibiotic resistance. Its resistance is in part due to its outstanding ability to form biofilms on a range of biotic and abiotic surfaces leading to difficult-to-treat, often long-term infections. Cold atmospheric plasma (CAP) is a new, promising antibacterial treatment to combat antibiotic-resistant bacteria. Plasma is ionized gas that has antibacterial properties through the generation of a mix of reactive oxygen and nitrogen species (RONS), excited molecules, charged particles and UV photons. Our results show the efficient removal of P. aeruginosa biofilms using a plasma jet (kINPen med), with no viable cells detected after 5 min treatment and no attached biofilm cells visible with confocal microscopy after 10 min plasma treatment. Because of its multi-factorial action, it is widely presumed that the development of bacterial resistance to plasma is unlikely. However, our results indicate that a short plasma treatment (3 min) may lead to the emergence of a small number of surviving cells exhibiting enhanced resistance to subsequent plasma exposure. Interestingly, these cells also exhibited a higher degree of resistance to hydrogen peroxide. Whole genome comparison between surviving cells and control cells revealed 10 distinct polymorphic regions, including four belonging to the redox active, antibiotic pigment phenazine. Subsequently, the interaction between phenazine production and CAP resistance was demonstrated in biofilms of transposon mutants disrupted in different phenazine pathway genes which exhibited significantly altered sensitivity to CAP.

Environmental (Saprozoic) Pathogens of Engineered Water Systems: Understanding Their Ecology for Risk Assessment and Management

Major waterborne (enteric) pathogens are relatively well understood and treatment controls are effective when well managed. However, water-based, saprozoic pathogens that grow within engineered water systems (primarily within biofilms/sediments) cannot be controlled by water treatment alone prior to entry into water distribution and other engineered water systems. Growth within biofilms or as in the case of Legionella pneumophila, primarily within free-living protozoa feeding on biofilms, results from competitive advantage. Meaning, to understand how to manage water-based pathogen diseases (a sub-set of saprozoses) we need to understand the microbial ecology of biofilms; with key factors including biofilm bacterial diversity that influence amoebae hosts and members antagonistic to water-based pathogens, along with impacts from biofilm substratum, water temperature, flow conditions and disinfectant residual—all control variables. Major saprozoic pathogens covering viruses, bacteria, fungi and free-living protozoa are listed, yet today most of the recognized health burden from drinking waters is driven by legionellae, non-tuberculous mycobacteria (NTM) and, to a lesser extent, Pseudomonas aeruginosa. In developing best management practices for engineered water systems based on hazard analysis critical control point (HACCP) or water safety plan (WSP) approaches, multi-factor control strategies, based on quantitative microbial risk assessments need to be developed, to reduce disease from largely opportunistic, water-based pathogens.

Enumeration of viable non-culturable Vibrio cholerae using propidium monoazide combined with quantitative PCR

The well-known human pathogenic bacterium, Vibrio cholerae, can enter a physiologically viable but non-culturable (VBNC) state under stress conditions. The differentiation of VBNC cells and nonviable cells is essential for both disease prevention and basic research. Among all the methods for detecting viability, propidium monoazide (PMA) combined with real-time PCR is popular because of its specificity, sensitivity, and speed. However, the effect of PMA treatment is not consistent and varies among different species and conditions. In this study, with an initial cell concentration of 1×10(8) CFU/ml, time and dose-effect relationships of different PMA treatments were evaluated via quantitative real-time PCR using live cell suspensions, dead cell suspensions and VBNC cell suspensions of V. cholerae O1 El Tor strain C6706. The results suggested that a PMA treatment of 20 μM PMA for 20 min was optimal under our conditions. This treatment maximized the suppression of the PCR signal from membrane-compromised dead cells but had little effect on the signal from membrane-intact live cells. In addition to the characteristics of PMA treatment itself, the initial concentration of the targeted bacteria showed a significant negative influence on the stability of PMA-PCR assay in this study. We developed a strategy that mimicked a 1×10(8) CFU/ml cell concentration with dead bacteria of a different bacterial species, the DNA of which cannot be amplified using the real time PCR primers. With this strategy, our optimal approach successfully overcame the impact of low cell density and generated stable and reliable results for counting viable cells of V. cholerae in the VBNC state.

A factor converting viable but nonculturable Vibrio cholerae to a culturable state in eukaryotic cells is a human catalase

In our previous work, we demonstrated that viable but nonculturable (VBNC) Vibrio cholerae O1 and O139 were converted to culturable by coculture with eukaryotic cells. Furthermore, we isolated a factor converting VBNC V. cholerae to culturable (FCVC) from a eukaryotic cell line, HT-29. In this study, we purified FCVC by successive column chromatographies comprising UNO Q-6 anion exchange, Bio-Scale CHT2-1 hydroxyapatite, and Superdex 200 10/300 GL. Homogeneity of the purified FCVC was demonstrated by SDS-PAGE. Nano-LC MS/MS analysis showed that the purified FCVC was a human catalase. An experiment of RNAi knockdown of catalase mRNA from HT-29 cells and treatment of the purified FCVC with a catalase inhibitor, 3-amino-1,2,4-triazole confirmed that the FCVC was a catalase. A possible role of the catalase in converting a VBNC V. cholerae to a culturable state in the human intestine is discussed.