Recovery of hydrogen peroxide-sensitive culturable cells of Vibrio vulnificus gives the appearance of resuscitation from a viable but nonculturable state

Clinical protective effects of polyhexamethylene biguanide hydrochloride (PHMB) against Vibrio parahaemolyticus causing translucent post-larvae disease (VpTPD) in Penaeus vannamei

A new emerging disease called "translucent post-larvae disease" (TPD) in Penaeus vannamei, caused by a novel type of highly lethal Vibro parahaemolyticus (VpTPD), has become an urgent threat to the shrimp farming industry in China. In order to develop an effective disinfectant for the prevention and control of the VpTPD, the clinical protective effects of polyhexamethylene biguanide hydrochloride (PHMB) against VpTPD in Penaeus vannamei were investigated by carrying out an acute toxicity test of PHMB on post-larvae of P. vannamei and its effect of treatment test on VpTPD infection. The results showed that the median lethal concentration of disinfectant (LC50) values of PHMB to post-larvae of P. vannamei after treatment for 24 h, 48 h, 72 h, 96 h were 16.13 mg/L (14.18-18.57), 10.77 mg/L (9.93-11.72), 9.68 mg/L (8.53-11.64), 9.14 mg/L (7.70-10.99), respectively. In addition, a clinical trial showed that 1 mg/L PHMB showed a strong protective effect on the post-larvae of shrimp challenged with 101-104 CFU/ml of VpTPD. The relative percentage survival (RPS) of 1 mg/L PHMB on post-larvae of P. vannamei challenged with VpTPD at 101, 102, 103 and 104 CFU/ml were 63.65 %±6.81, 62.96 %±5.56, 60.00 %±3.75 and 66.67 %±3.75 at 96 hours post infection. The results highlight the clinical protective effects of the PHMB and therefor PHMB can be used as a preventive measure to control early TPD infection in shrimp culture. This study also provides valuable information for the prevention of other bacterial diseases in shrimp culture.

Reaching unreachables: Obstacles and successes of microbial cultivation and their reasons

In terms of the number and diversity of living units, the prokaryotic empire is the most represented form of life on Earth, and yet it is still to a significant degree shrouded in darkness. This microbial "dark matter" hides a great deal of potential in terms of phylogenetically or metabolically diverse microorganisms, and thus it is important to acquire them in pure culture. However, do we know what microorganisms really need for their growth, and what the obstacles are to the cultivation of previously unidentified taxa? Here we review common and sometimes unexpected requirements of environmental microorganisms, especially soil-harbored bacteria, needed for their replication and cultivation. These requirements include resuscitation stimuli, physical and chemical factors aiding cultivation, growth factors, and co-cultivation in a laboratory and natural microbial neighborhood.

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.

CAMPYAIR, a New Selective, Differential Medium for Campylobacter spp. Isolation without the Need for Microaerobic Atmosphere

Campylobacter spp. are considered the most frequent bacterial cause of acute gastroenteritis worldwide. Although the diarrhea produced by these bacteria is self-limiting, the pathogen has been associated with severe long-term sequelae following acute signs and symptoms of the illness. However, research on Campylobacter is hampered by costs and technical requirements for isolating and culturing the bacterium, especially in low and middle-income countries. Therefore, attempts have been made to simplify these culture methods and to reduce costs associated with conducting research on Campylobacter. Recently, a liquid medium which allows selective enrichment of Campylobacter using aerobic incubation has been described. However, a solid medium is also needed for the isolation of pure colonies, enumeration of bacterial populations, and other studies on the pathogen. Therefore, a new medium (CAMPYAIR) was developed, based on the formulation of the liquid medium. CAMPYAIR is a solid chromogenic medium that supports the growth of Campylobacter isolates within 48 h of incubation in aerobic atmospheres. Moreover, CAMPYAIR contains antibiotic supplements with an enhanced ability to recover Campylobacter from environmental samples that may also contain non-campylobacter bacteria. The addition of the indicator 2,3,5-triphenyltetrazolium (TTC) to the medium differentiates Campylobacter from other bacteria growing on the media. The findings from studies on CAMPYAIR suggest that the utilization of the new selective, differential medium could help to reduce the costs, equipment, and technical training required for Campylobacter isolation from clinical and environmental samples.

Generation of Distinct Differentially Culturable Forms of Burkholderia following Starvation at Low Temperature

Bacteria have developed unique mechanisms to adapt to environmental stresses and challenges of the immune system. Here, we report that Burkholderia pseudomallei, the causative agent of melioidosis, and its laboratory surrogate, Burkholderia thailandensis, utilize distinct mechanisms for surviving starvation at different incubation temperatures. At 21°C, Burkholderia are present as short rods which can rapidly reactivate and form colonies on solid media. At 4°C, Burkholderia convert into coccoid forms that cannot be cultured on solid agar but can be resuscitated in liquid media supplemented with supernatant obtained from logarithmic phase cultures of B. thailandensis, or catalase and Tween 80, thus displaying characteristics of differentially culturable bacteria (DCB). These DCB have low intensity fluorescence when stained with SYTO 9, have an intact cell membrane (propidium iodide negative), and contain 16S rRNA at levels comparable with growing cells. We also present evidence that lytic transglycosylases, a family of peptidoglycan-remodeling enzymes, are involved in the generation of coccoid forms and their resuscitation to actively growing cells. A B. pseudomallei ΔltgGCFD mutant with four ltg genes deleted did not produce coccoid forms at 4°C and could not be resuscitated in the liquid media evaluated. Our findings provide insights into the adaptation of Burkholderia to nutrient limitation and the generation of differentially culturable bacteria. IMPORTANCE Bacterial pathogens exhibit physiologically distinct forms that enable their survival in an infected host, the environment and following exposure to antimicrobial agents. B. pseudomallei causes the disease melioidosis, which has a high mortality rate and is difficult to treat with antibiotics. The bacterium is endemic to several countries and detected in high abundance in the environment. Here, we report that during starvation at low temperature, B. pseudomallei produces coccoid forms that cannot grow in standard media and which, therefore, can be challenging to detect using common tools. We provide evidence that the formation of these cocci is mediated by cell wall-specialized enzymes and lytic transglycosylases, and that resuscitation of these forms occurs following the addition of catalase and Tween 80. Our findings have important implications for the disease control and detection of B. pseudomallei, an agent of both public health and defense interest.

Assessing Viability and Stress Tolerance of Probiotics—A Review

The interest in probiotics has increased rapidly the latest years together with the global market for probiotic products. Consequently, establishing reliable microbiological methods for assuring the presence of a certain number of viable microorganisms in probiotic products has become increasingly important. To assure adequate numbers of viable cells, authorities are enquiring for information on viability rates within a certain shelf-life in colony forming units (CFU). This information is obtained from plate count enumeration, a method that enables detection of bacterial cells based on their ability to replicate. Although performing plate count enumeration is one manner of assessing viability, cells can still be viable without possessing the ability to replicate. Thus, to properly assess probiotic viability, further analysis of a broader group of characteristics using several types of methods is proposed. In addition to viability, it is crucial to identify how well the cells in a probiotic product can survive in the gastrointestinal tract (GIT) and thus be able to mediate the desired health benefit while passing through the human body. A broad spectrum of different assay designs for assessing probiotic gastric tolerance have been used in research and quality control. However, the absence of any consensus on how to assess these qualities makes it difficult to compare between laboratories and to translate the results into in vivo tolerance. This review presents and discusses the complexity of assuring that a probiotic is suitable for beneficial consumption. It summarizes the information that can be subtracted from the currently available methods for assessment of viability and stress tolerance of a probiotic, hereby altogether defined as “activity.” Strengths and limitations of the different methods are presented together with favorable method combinations. Finally, the importance of choosing a set of analyses that reveals the necessary aspects of probiotic activity for a certain product or application is emphasized.

'Viable but non-culturable cells' are dead

Most bacteria lead lives of quiet desperation, so they sleep. By sleeping, bacteria survive ubiquitous stress, such as antibiotics, and can resuscitate to reconstitute infections. As for other nearly universal and highly regulated processes such as biofilm formation, in persistence, a small population of cells have an elegantly-regulated pathway to become dormant. By inactivating their ribosomes, persister cells sleep through stress and resuscitate once (i) the stress is removed, (ii) nutrients are presented and (iii) ribosome content reaches a threshold. During stress, cells often become spheroid and die, becoming hollow, membrane-enclosed vessels. How cellular content is lost is unclear, but it is obvious that these 'cell shells' are dead; i.e., 'There's no there there'. Critically, due to their intact membranes, the shells appear with membrane-impenetrant stains as 'viable' particles. Unfortunately, the microbiology field of 'viable but non-culturable cells' (VBNCs), though important for demonstrating the existence of dormant bacteria as a result of myriad stress states, has often mistaken these non-viable shells as viable particles that mysteriously may be reborn, when an appropriate incantation is made. We argue here, based on experimental data, that if resuscitation occurs, it is the persister (always-viable) cell population that revives, rather than the cell husks, which are dead.

Induction of Escherichia coli into a VBNC state through chlorination/chloramination and differences in characteristics of the bacterium between states

Many pathogens can enter into a viable but nonculturable (VBNC) state in response to harsh environmental stresses. Bacteria in this state can retain certain features of viable cells, such as cellular integrity, metabolic activity, or virulence and may present health risks associated with drinking water. In this study, we investigated the ability of chlorination and chloramination, which are widely used methods to disinfect drinking water, to induce Escherichia coli into a VBNC state. After treatment with chlorine and chloramine at concentrations of 1, 2, 3, and 4 mg/L, the counts of culturable E. coli cells decreased from 10⁶ CFU/mL to 0 CFU/mL at 5-60 min post treatment. Meanwhile, viable cell counts were still approximately 10³-10⁵ cells/mL. These viable E. coli cells may be induced into a VBNC state by chlorination and chloramination. Scanning electron microscopy and laser confocal microscopy showed that some bacteria maintained cellular integrity, but the average length of VBNC cells was less than that of culturable cells. Respiratory activity of VBNC cells decreased approximately 50% relative to that of culturable cells. We also used heavy water (D₂O) combined with Raman microspectroscopy to show that E. coli in a VBNC state retained metabolic activity involving water (e.g. condensation reactions) at the single-cell level. Furthermore, soxR, gadA, and katG genes remained highly expressed, suggesting that VBNC cells were physiologically active. Finally, resuscitation of VBNC cells induced by chlorine in Luria-Bertani (LB) broth was identified by calculating the generation time. Results of this study will facilitate a better understanding of the health risks associated with VBNC bacteria and the development of more effective strategies for drinking water disinfection.

Deletion of oxyR in Legionella pneumophila causes growth defect on agar

The intracellular pathogen Legionella pneumophila (Lp) is a strict aerobe, surviving and replicating in environments where it frequently encounters reactive oxygen species (ROS), such as the nutrient-poor water environment and its replicative niche inside host cells. In many proteobacteria, the LysR-type regulator OxyR controls the oxidative stress response; however, the importance of the OxyR homologue in Lp is still unclear. Therefore, we undertook the characterization of phenotypes associated with the deletion of oxyR in Lp. Contrary to the wild type, the oxyR deletion mutant exhibits a severe growth defect on charcoal - yeast extract (CYE) agar lacking α-ketoglutarate supplementation. Growth in AYE broth (CYE without agar and charcoal), in amoeba and in human cultured macrophages, and survival in water is unaffected by the deletion. Supplementing CYE agar with antioxidants that neutralize ROS or introducing the oxyR gene in trans rescues the observed growth defect. Moreover, the mutant grows as well as the wild type on CYE plates made with agarose instead of agar, suggesting that a compound present in the latter is responsible for the growth defect phenotype.

Stimulation of Vibrio vulnificus Pyruvate Kinase in the Presence of Glucose to Cope With H2O2 Stress Generated by Its Competitors

The bacterial phosphoenolpyruvate (PEP):carbohydrate phosphotransferase system (PTS) regulates a variety of cellular processes in addition to catalyzing the coupled transport and phosphorylation of carbohydrates. We recently reported that, in the presence of glucose, HPr of the PTS is dephosphorylated and interacts with pyruvate kinase A (PykA) catalyzing the conversion of PEP to pyruvate in Vibrio vulnificus. Here, we show that this interaction enables V. vulnificus to survive H₂O₂ stress by increasing pyruvate production. A pykA deletion mutant was more susceptible to H₂O₂ stress than wild-type V. vulnificus without any decrease in the expression level of catalase, and this sensitivity was rescued by the addition of pyruvate. The H₂O₂ sensitivity difference between wild-type and pykA mutant strains becomes more apparent in the presence of glucose. Fungi isolated from the natural habitat of V. vulnificus retarded the growth of the pykA mutant more severely than the wild-type strain in the presence of glucose by glucose oxidase-dependent generation of H₂O₂. These data suggest that V. vulnificus has evolved to resist the killing action of its fungal competitors by increasing pyruvate production in the presence of glucose.

Improved measurements of scant hydrogen peroxide enable experiments that define its threshold of toxicity for Escherichia coli

Escherichia coli is a model organism that has been exploited to reveal key details of hydrogen peroxide stress: the biomolecules that H₂O₂ most rapidly damages and the defensive tactics that organisms use to fend it off. Much less clear is the amount of exogenous H₂O₂ that is sufficient to injure the bacterium and/or to trigger its stress response. To fill this gap, we need to study the behavior of cells when they are exposed to defined amounts of H₂O₂ on an hours-long time scale. Such experiments are difficult because bacteria rapidly consume H₂O₂ that is added to test cultures. Further, lab media itself can generate H₂O₂, and media components interfere with the quantification of H₂O₂ levels. In this study we describe mechanisms by which media components interfere with H₂O₂ determinations, and we identify simple ways to minimize and correct for this interference. Using these techniques, it was shown that standard media generate so much H₂O₂ that most intracellular H₂O₂ derives from the medium rather than from endogenous metabolism. Indeed, bacteria spread on plates must induce their stress response or else perish. Finally, two straightforward methods were used to sustain low-micromolar steady-state concentrations of H₂O₂. In this way we determined that > 2 μM extracellular H₂O₂ is sufficient to trigger the intracellular OxyR stress response, and > 5 μM begins to impair cell growth in a minimal medium. These concentrations are orders of magnitude lower than the doses that have typically been used in lab experiments. The new approaches should enable workers to study how various organisms cope with natural levels of H₂O₂ stress.

Mimicking lichens: incorporation of yeast strains together with sucrose-secreting cyanobacteria improves survival, growth, ROS removal, and lipid production in a stable mutualistic co-culture production platform

BACKGROUND

The feasibility of heterotrophic-phototrophic symbioses was tested via pairing of yeast strains Cryptococcus curvatus, Rhodotorula glutinis, or Saccharomyces cerevisiae with a sucrose-secreting cyanobacterium Synechococcus elongatus.

RESULTS

The phototroph S. elongatus showed no growth in standard BG-11 medium with yeast extract, but grew well in BG-11 medium alone or supplemented with yeast nitrogen base without amino acids (YNB w/o aa). Among three yeast species, C. curvatus and R. glutinis adapted well to the BG-11 medium supplemented with YNB w/o aa, sucrose, and various concentrations of NaCl needed to maintain sucrose secretion from S. elongatus, while growth of S. cerevisiae was highly dependent on sucrose levels. R. glutinis and C. curvatus grew efficiently and utilized sucrose produced by the partner in co-culture. Co-cultures of S. elongatus and R. glutinis were sustained over 1 month in both batch and in semi-continuous culture, with the final biomass and overall lipid yields in the batch co-culture 40 to 60% higher compared to batch mono-cultures of S. elongatus. The co-cultures showed enhanced levels of palmitoleic and linoleic acids. Furthermore, cyanobacterial growth in co-culture with R. glutinis was significantly superior to axenic growth, as S. elongatus was unable to grow in the absence of the yeast partner when cultivated at lower densities in liquid medium. Accumulated reactive oxygen species was observed to severely inhibit axenic growth of cyanobacteria, which was efficiently alleviated through catalase supply and even more effectively with co-cultures of R. glutinis.

CONCLUSIONS

The pairing of a cyanobacterium and eukaryotic heterotroph in the artificial lichen of this study demonstrates the importance of mutual interactions between phototrophs and heterotrophs, e.g., phototrophs provide a carbon source to heterotrophs, and heterotrophs assist phototrophic growth and survival by removing/eliminating oxidative stress. Our results establish a potential stable production platform that combines the metabolic capability of photoautotrophs to capture inorganic carbon with the channeling of the resulting organic carbon directly to a robust heterotroph partner for producing biofuel and other chemical precursors.

Some Like It Hot: Heat Resistance of Escherichia coli in Food

Heat treatment and cooking are common interventions for reducing the numbers of vegetative cells and eliminating pathogenic microorganisms in food. Current cooking method requires the internal temperature of beef patties to reach 71°C. However, some pathogenic Escherichia coli such as the beef isolate E. coli AW 1.7 are extremely heat resistant, questioning its inactivation by current heat interventions in beef processing. To optimize the conditions of heat treatment for effective decontaminations of pathogenic E. coli strains, sufficient estimations, and explanations are necessary on mechanisms of heat resistance of target strains. The heat resistance of E. coli depends on the variability of strains and properties of food formulations including salt and water activity. Heat induces alterations of E. coli cells including membrane, cytoplasm, ribosome and DNA, particularly on proteins including protein misfolding and aggregations. Resistant systems of E. coli act against these alterations, mainly through gene regulations of heat response including EvgA, heat shock proteins, σE and σS, to re-fold of misfolded proteins, and achieve antagonism to heat stress. Heat resistance can also be increased by expression of key proteins of membrane and stabilization of membrane fluidity. In addition to the contributions of the outer membrane porin NmpC and overcome of osmotic stress from compatible solutes, the new identified genomic island locus of heat resistant performs a critical role to these highly heat resistant strains. This review aims to provide an overview of current knowledge on heat resistance of E. coli, to better understand its related mechanisms and explore more effective applications of heat interventions in food industry.

Bacteriological Monitoring of Water Reservoirs in Oxygen Humidifiers: Safety of Prolonged and Multipatient Use of Prefilled Disposable Oxygen Humidifier Bottles

Bacterial colony counts in water specimens from oxygen humidifiers that used reusable water reservoirs were compared with counts in water specimens from humidifiers that used prefilled disposable reservoir bottles to evaluate the effects of prolonged and multipatient use of humidifiers. Bacteria were detected after 1 week of operation in water specimens collected from many humidifiers with reusable reservoirs, but no bacteria were detected in water specimens from disposable bottles for up to 12 weeks during use of the humidifier by multiple patients.

Stepwise changes in viable but nonculturable Vibrio cholerae cells

Many bacterial species are known to become viable but nonculturable (VBNC) under conditions that are unsuitable for growth. In this study, the requirements for resuscitation of VBNC-state Vibrio cholerae cells were found to change over time. Although VBNC cells could initially be converted to culturable by treatment with catalase or HT-29 cell extract, they subsequently entered a state that was not convertible to culturable by these factors. However, fluorescence microscopy revealed the presence of live cells in this state, from which VBNC cells were resuscitated by co-cultivation with HT-29 human colon adenocarcinoma cells. Ultimately, all cells entered a state from which they could not be resuscitated, even by co-cultivation with HT-29. These characteristic changes in VBNC-state cells were a common feature of strains in both V. cholerae O1 and O139 serogroups. Thus, the VBNC state of V. cholerae is not a single property but continues to change over time.

Regulation of oxidative stress resistance in Campylobacter jejuni, a microaerophilic foodborne pathogen

Campylobacter jejuni is one of the leading bacterial causes of human gastroenteritis. Due to the increasing rates of human campylobacteriosis, C. jejuni is considered as a serious public health concern worldwide. C. jejuni is a microaerophilic, fastidious bacterium. C. jejuni must overcome a wide range of stress conditions during foodborne transmission to humans, such as food preservation and processing conditions, and even in infection of the gastrointestinal tracts of humans. Particularly, this microaerophilic foodborne pathogen must survive in the atmospheric conditions prior to the initiation of infection. C. jejuni possesses unique regulatory mechanisms for oxidative stress resistance. Lacking OxyR and SoxRS that are highly conserved in other Gram-negative foodborne pathogens, C. jejuni modulates the expression of genes involved in oxidative stress resistance mainly via the peroxide resistance regulator and Campylobacter oxidative stress regulator. Based on recent findings of ours and others, in this review, we described how C. jejuni regulates the expression of oxidative stress defense.

Survival, induction and resuscitation of Vibrio cholerae from the viable but non-culturable state in the Southern Caribbean Sea

The causative agent of cholera, Vibrio cholerae, can enter into a viable but non-culturable (VBNC) state in response to unfavorable conditions. The aim of this study was to evaluate the in situ survival of V. cholerae in an aquatic environment of the Southern Caribbean Sea, and its induction and resuscitation from the VBNC state. V. cholerae non-O1, non-O139 was inoculated into diffusion chambers placed at the Cuare Wildlife Refuge, Venezuela, and monitored for plate, total and viable cells counts. At 119 days of exposure to the environment, the colony count was < 10 CFU/mL and a portion of the bacterial population entered the VBNC state. Additionally, the viability decreased two orders of magnitude and morphological changes occurred from rod to coccoid cells. Among the aquatic environmental variables, the salinity had negative correlation with the colony counts in the dry season. Resuscitation studies showed significant recovery of cell cultivability with spent media addition (p < 0.05). These results suggest that V. cholerae can persist in the VBNC state in this Caribbean environment and revert to a cultivable form under favorable conditions. The VBNC state might represent a critical step in cholera transmission in susceptible areas.

A hidden pitfall in the preparation of agar media undermines microorganism cultivability

Microbiologists have been using agar growth medium for over 120 years. It revolutionized microbiology in the 1890s when microbiologists were seeking effective methods to isolate microorganisms, which led to the successful cultivation of microorganisms as single clones. But there has been a disparity between total cell counts and cultivable cell counts on plates, often referred to as the "great plate count anomaly," that has long been a phenomenon that still remains unsolved. Here, we report that a common practice microbiologists have employed to prepare agar medium has a hidden pitfall: when phosphate was autoclaved together with agar to prepare solid growth media (PT medium), total colony counts were remarkably lower than those grown on agar plates in which phosphate and agar were separately autoclaved and mixed right before solidification (PS medium). We used a pure culture of Gemmatimonas aurantiaca T-27(T) and three representative sources of environmental samples, soil, sediment, and water, as inocula and compared colony counts between PT and PS agar plates. There were higher numbers of CFU on PS medium than on PT medium using G. aurantiaca or any of the environmental samples. Chemical analysis of PT agar plates suggested that hydrogen peroxide was contributing to growth inhibition. Comparison of 454 pyrosequences of the environmental samples to the isolates revealed that taxa grown on PS medium were more reflective of the original community structure than those grown on PT medium. Moreover, more hitherto-uncultivated microbes grew on PS than on PT medium.

The importance of the viable but non-culturable state in human bacterial pathogens

Many bacterial species have been found to exist in a viable but non-culturable (VBNC) state since its discovery in 1982. VBNC cells are characterized by a loss of culturability on routine agar, which impairs their detection by conventional plate count techniques. This leads to an underestimation of total viable cells in environmental or clinical samples, and thus poses a risk to public health. In this review, we present recent findings on the VBNC state of human bacterial pathogens. The characteristics of VBNC cells, including the similarities and differences to viable, culturable cells and dead cells, and different detection methods are discussed. Exposure to various stresses can induce the VBNC state, and VBNC cells may be resuscitated back to culturable cells under suitable stimuli. The conditions that trigger the induction of the VBNC state and resuscitation from it are summarized and the mechanisms underlying these two processes are discussed. Last but not least, the significance of VBNC cells and their potential influence on human health are also reviewed.

Continuous hydrogen peroxide production by organic buffers in phytoplankton culture media

We investigated the production of hydrogen peroxide (HOOH) in illuminated seawater media containing a variety of zwitterionic buffers. Production rates varied extensively among buffers, with 4-(2-hydroxyethyl)1-piperazineethanesulfonic acid (HEPES) highest and N-Tris(hydroxymethyl)methyl-3-aminopropanesulfonic acid (TAPS) among the lowest. The rate of HOOH accumulation was remarkably consistent over many days, and increased linearly with buffer concentration, natural seawater concentration, and light level. Concentrations of HEPES commonly used in culture media (1-10 mM) generated enough HOOH to kill the axenic Prochlorococcus strain VOL1 during growth in enriched seawater media at lower, environmentally realistic cell concentrations and/or under high light exposure. We also demonstrated that HEPES can be used experimentally to study the biological effects of chronic exposure to sublethal levels of HOOH such as may be experienced by light-exposed microorganisms.

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

Viable but nonculturable (VBNC) cells were recognized 30 years ago; and despite decades of research on the topic, most results are disperse and apparently incongruous. Since its description, a huge controversy arose regarding the ecological significance of this state: is it a degradation process without real significance for bacterial life cycles or is it an adaptive strategy of bacteria to cope with stressful conditions? In order to solve the molecular mechanisms of VBNC state induction and resuscitation, researchers in the field must be aware and overcome common issues delaying research progress. In this review, we discuss the intrinsic characteristic features of VBNC cells, the first clues on what is behind the VBNC state's induction, the models proposed for their resuscitation and the current methods to prove not only that cells are in VBNC state but also that they are able to resuscitate.

Roles of alkyl hydroperoxide reductase subunit C (AhpC) in viable but nonculturable Vibrio parahaemolyticus

Alkyl hydroperoxide reductase subunit C (AhpC) is the catalytic subunit responsible for the detoxification of reactive oxygen species that form in bacterial cells or are derived from the host; thus, AhpC facilitates the survival of pathogenic bacteria under environmental stresses or during infection. This study investigates the role of AhpC in the induction and maintenance of a viable but nonculturable (VBNC) state in Vibrio parahaemolyticus. In this investigation, ahpC1 (VPA1683) and ahpC2 (VP0580) were identified in chromosomes II and I of this pathogen, respectively. Mutants with deletions of these two ahpC genes and their complementary strains were constructed from the parent strain KX-V231. The growth of these strains was monitored on tryptic soy agar-3% NaCl in the presence of the extrinsic peroxides H(2)O(2) and tert-butyl hydroperoxide (t-BOOH) at different incubation temperatures. The results revealed that both ahpC genes were protective against t-BOOH, while ahpC1 was protective against H(2)O(2). The protective function of ahpC2 at 4°C was higher than that of ahpC1. The times required to induce the VBNC state (4.7 weeks) at 4°C in a modified Morita mineral salt solution with 0.5% NaCl and then to maintain the VBNC state (4.7 weeks) in an ahpC2 mutant and an ahpC1 ahpC2 double mutant were significantly shorter than those for the parent strain (for induction, 6.2 weeks; for maintenance, 7.8 weeks) and the ahpC1 mutant (for induction, 6.0 weeks; for maintenance, 8.0 weeks) (P < 0.03). Complementation with an ahpC2 gene reversed the effects of the ahpC2 mutation in shortening the times for induction and maintenance of the VBNC state. This investigation identified the different functions of the two ahpC genes and confirmed the particular role of ahpC2 in the VBNC state of V. parahaemolyticus.

Induction of viable but nonculturable Escherichia coli O157:H7 by high pressure CO2 and its characteristics

The viable but nonculturable (VBNC) state is a survival strategy adopted by many pathogens when exposed to harsh environmental stresses. In this study, we investigated for the first time that whether high pressure CO2 (HPCD), one of the nonthermal pasteurization techniques, can induce Escherichia coli O157:H7 into the VBNC state. By measuring plate counts, viable cell counts and total cell counts, E. coli O157:H7 in 0.85% NaCl solution (pH 7.0) was able to enter the VBNC state by HPCD treatment at 5 MPa and four temperatures (25°C, 31°C, 34°C and 37°C). Meanwhile, with the improvement of treatment temperature, the time required for E. coli O157:H7 to enter VBNC state would shorten. Enzymatic activities in these VBNC cells were lower than those in the exponential-phase cells by using API ZYM kit, which were also reduced with increasing the treatment temperature, but the mechanical resistance of the VBNC cells to sonication was enhanced. These results further confirmed VBNC state was a self-protection mechanism for some bacteria, which minimized cellular energetic requirements and increased the cell resistance. When incubated in tryptic soy broth at 37°C, the VBNC cells induced by HPCD treatment at 25°C, 31°C and 34°C achieved resuscitation, but their resuscitation capabilities decreased with increasing the treatment temperature. Furthermore, electron microscopy revealed changes in the morphology and interior structure of the VBNC cells and the resuscitated cells. These results demonstrated that HPCD could induce E. coli O157:H7 into the VBNC state. Therefore, it is necessary to detect if there exist VBNC microorganisms in HPCD-treated products by molecular-based methods for food safety.

Escherichia coli K-12 survives anaerobic exposure at pH 2 without RpoS, Gad, or hydrogenases, but shows sensitivity to autoclaved broth products

Escherichia coli and other enteric bacteria survive exposure to extreme acid (pH 2 or lower) in gastric fluid. Aerated cultures survive via regulons expressing glutamate decarboxylase (Gad, activated by RpoS), cyclopropane fatty acid synthase (Cfa) and others. But extreme-acid survival is rarely tested under low oxygen, a condition found in the stomach and the intestinal tract. We observed survival of E. coli K-12 W3110 at pH 1.2–pH 2.0, conducting all manipulations (overnight culture at pH 5.5, extreme-acid exposure, dilution and plating) in a glove box excluding oxygen (10% H₂, 5% CO₂, balance N₂). With dissolved O₂ concentrations maintained below 6 µM, survival at pH 2 required Cfa but did not require GadC, RpoS, or hydrogenases. Extreme-acid survival in broth (containing tryptone and yeast extract) was diminished in media that had been autoclaved compared to media that had been filtered. The effect of autoclaved media on extreme-acid survival was most pronounced when oxygen was excluded. Exposure to H₂O₂ during extreme-acid treatment increased the death rate slightly for W3110 and to a greater extent for the rpoS deletion strain. Survival at pH 2 was increased in strains lacking the anaerobic regulator fnr. During anaerobic growth at pH 5.5, strains deleted for fnr showed enhanced transcription of acid-survival genes gadB, cfa, and hdeA, as well as catalase (katE). We show that E. coli cultured under oxygen exclusion (<6 µM O₂) requires mechanisms different from those of aerated cultures. Extreme acid survival is more sensitive to autoclave products under oxygen exclusion.

Seasonal shifts in population structure of Vibrio vulnificus in an estuarine environment as revealed by partial 16S ribosomal DNA sequencing

Abstract The partial sequence (600 bp) containing the most variable region of Vibrio vulnificus 16S ribosomal DNA (rDNA) was determined for 208 randomly selected V. vulnificus strains isolated from Galveston Bay, TX, USA between June 2000 and June 2001. A comparative analysis of the determined partial 16S rDNA sequences revealed the existence of two different partial 16S rDNA sequences (type A and type B, 1.3% base substitutions) among the 208 V. vulnificus isolates. A higher proportion of 16S rDNA type A strains was isolated in June and July while a considerably higher proportion of type B strains was isolated in September. In addition, after no V. vulnificus strains were detected during the winter months (December-February), only type A strains were isolated during the following months (March-May). The results suggest that the relative abundance of type A and type B V. vulnificus strains in Galveston Bay varies with the season and that the differences between the two 16S rDNA types may affect the viability of these organisms in the natural environment.

Mechanisms for photoinactivation of Enterococcus faecalis in seawater

Field studies in fresh and marine waters consistently show diel fluctuations in concentrations of enterococci, indicators of water quality. We investigated sunlight inactivation of Enterococcus faecalis to gain insight into photoinactivation mechanisms and cellular responses to photostress. E. faecalis bacteria were exposed to natural sunlight in clear, filtered seawater under both oxic and anoxic conditions to test the relative importance of oxygen-mediated and non-oxygen-mediated photoinactivation mechanisms. Multiple methods were used to assess changes in bacterial concentration, including cultivation, quantitative PCR (qPCR), propidium monoazide (PMA)-qPCR, LIVE/DEAD staining using propidium iodide (PI), and cellular activity, including ATP concentrations and expression of the superoxide dismutase-encoding gene, sodA. Photoinactivation, based on numbers of cultivable cells, was faster in oxic than in anoxic microcosms exposed to sunlight, suggesting that oxygen-mediated photoinactivation dominated. There was little change in qPCR signal over the course of the experiment, demonstrating that the nucleic acid targets were not damaged to a significant extent. The PMA-qPCR signal was also fairly stable, consistent with the observation that the fraction of PI-permeable cells was constant. Thus, damage to the membrane was minimal. Microbial ATP concentrations decreased in all microcosms, particularly the sunlit oxic microcosms. The increase in relative expression of the sodA gene in the sunlit oxic microcosms suggests that cells were actively responding to oxidative stress. Dark repair was not observed. This research furthers our understanding of photoinactivation mechanisms and the conditions under which diel fluctuations in enterococci can be expected in natural and engineered systems.

Viability kinetics, induction, resuscitation and quantitative real-time polymerase chain reaction analyses of viable but nonculturable Vibrio cholerae O1 in freshwater microcosm

AIM

To study the induction of a viable but nonculturable (VBNC) state in Vibrio cholerae O1 in freshwater, in response to cold temperatures (4°C) and starvation.

METHODS AND RESULTS

Vibrio cholerae O1 cells were inoculated in freshwater microcosm and incubated at 4°C. The cells became coccoid, rugose and subsequently nonculturable by day 16 on tryptic soy agar (TSA) and by day 23 on TSA-SP, while 87 and 65% of the cells retained their membrane integrity, respectively. Viable cells were observed until day 30 using direct fluorescent antibody-direct viable count method. In vitro resuscitation was demonstrated by temperature upshift. Utilizing 16S rRNA as an endogenous control, the DNA pol II (27·43-fold), fliG (12·44-fold), ABC transporter (27·11-fold), relA (60·76-fold) and flaC (15·29-fold) were significantly up-regulated in VBNC cells, while the expression of fadL-3 was comparable. The expression of DNA pol II, fliG, ABC transporter, relA and flaC was 3·3, 1·1, 5·9, 5·8 and 1·2-fold, respectively, for resuscitated cells. VBNC cells were found to be virulent, as ctxA and tcpA were expressed.

CONCLUSIONS

Vibrio cholerae undergoes both phenotypic alteration and genotypic modulation to protect itself from stress in freshwater.

SIGNIFICANCE AND IMPACT OF THE STUDY

Induction and resuscitation of the VBNC state in freshwater is important for an understanding of the epidemiology of cholera in the freshwater environment.

Year round patchiness of Vibrio vulnificus within a temperate Texas bay

AIMS

To investigate with high geographical resolution the small-scale spatial and temporal distribution of the pathogen Vibrio vulnificus throughout the water column in a temperate Texas bay where numerous V. vulnificus infections had been reported by the regional media the previous summer.

METHODS AND RESULTS

Surface and bottom water samples were collected from 19 sites between April 2005 and October 2006 from Matagorda Bay, TX. Physicochemical parameters were measured and V. vulnificus were analysed using quantitative polymerase chain reaction (Q-PCR) as a means of overcoming constraints of traditional culturing techniques. V. vulnificus was detected through out the year, although its temporal and spatial distribution was patchy. V. vulnificus abundances at individual sites ranged from <10 to >1·1×10(3)cellsml(-1) . No statistically reliable predictive model related to the physicochemical parameters could be developed for this pathogen.

CONCLUSIONS

This study demonstrates that year round detection of V. vulnificus while likely in the viable but nonculturable (VBNC) state during the winter months and emphasizes why physicochemical factors are insufficient metrics for robust regression modelling of this pathogen.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study provides an effective new tool, Q-PCR, to study environmental distribution of V. vulnificus and that in the light of the patchy distribution observed, new reliable approaches and a mechanistic understanding of pathogen ecology need to be considered to effectively model the aquatic distribution of V. vulnificus.

Characterization of the "viable but nonculturable" (VBNC) state in the wine spoilage yeast Brettanomyces

Although the viable but not culturable (VBNC) state has been studied in detail in bacteria, it has been suggested that maintenance of viability with loss of culturability also exists in eukaryotic cells, such as in the wine spoilage yeast Brettanomyces. To provide conclusive evidence for the existence of a VBNC state in this yeast, we investigated its capacity to become viable and nonculturable after sulfite stress, and its ability to recover culturability after stressor removal. Sulfite addition induced loss of culturability but maintenance of viability. Increasing the medium pH to decrease the concentration of toxic SO(2) allowed yeast cells to become culturable again, thus demonstrating the occurrence of a VBNC state in Brettanomyces upon SO(2) exposure. Relative to culturable Brettanomyces, VBNC yeast cells were found to display a 22% decrease in size on the basis of laser granulometry. Assays for 4-ethylguaiacol and 4-ethylphenol, volatile phenols produced by Brettanomyces, indicated that spoilage compound production could persist in VBNC cells. These morphological and physiological changes in VBNC Brettanomyces were coupled to extensive protein pattern modifications, as inferred by comparative two-dimensional electrophoresis and mass spectrometric analyses. Upon identification of 53 proteins out of the 168 spots whose abundance was significantly modified in treated cells relative to control, we propose that the SO(2)-induced VBNC state in Brettanomyces is characterized by a reduced glycolytic flux coupled to changes in redox homeostatis/protein turnover-related processes. This study points out the existence of common mechanisms between yeast and bacteria upon entry to the VBNC state.

Inactivation and sub-lethal injury of salmonella typhi, salmonella typhimurium and vibrio cholerae in copper water storage vessels

Background

This study provides information on the antibacterial effect of copper against the water-borne pathogens Salmonella Typhi, Salmonella Typhimurium and Vibrio cholerae.

Methods

Suspensions of each pathogen were kept in water within a traditional copper vessel at 30°C for 24 h. Samples were withdrawn, diluted and plated onto suitable growth media. Conventional enumeration of healthy (uninjured) bacteria was carried out using standard aerobic incubation conditions. Additionally, reactive oxygen species-neutralised (ROS-n) conditions were achieved by adding the peroxide scavenger sodium pyruvate to the medium with anaerobic incubation, to enumerate uninjured (ROS-insensitive) and injured (ROS-sensitive) bacteria. Differences between log-transformed means of conventional (aerobic) and ROS-n counts were statistically evaluated using t tests.

Results

Overall, all three pathogens were inactivated by storage in copper vessels for 24 h. However, for shorter-term incubation (4-12 h), higher counts were observed under ROS-n conditions than under aerobic conditions, which demonstrate the presence of substantial numbers of sub-lethally injured cells prior to their complete inactivation.

Conclusions

The present study has for the first time confirmed that these bacterial pathogens are inactivated by storage in a copper vessel within 24 h. However, it has also demonstrated that it is necessary to account for short-term sub-lethal injury, manifest as ROS-sensitivity, in order to more fully understand the process. This has important practical implications in terms of the time required to store water within a copper vessel to completely inactivate these bacteria and thereby remove the risk of water-borne disease transmission by this route.

Influence of growth conditions on pressure resistance of Vibrio parahaemolyticus in oysters and the optimization of postpressure treatment recovery conditions

Vibrio parahaemolyticus ATCC 43996 was grown at 15°C for 53 h, 20°C for 24 h, 25°C for 12 h, 30°C for 9 h, 35°C for 9 h, or 40°C for 6 h to early stationary phase. Oyster meats were blended, autoclaved at 121°C for 15 min, inoculated with V. parahaemolyticus, and pressure treated at 250 MPa for 2 and 3 min and at 300 MPa for 1 and 2 min at 21°C. Overall, growth temperatures of 20 and 40°C yielded the greatest pressure resistance in V. parahaemolyticus. The effects of salt concentration and H(2)O(2)-degrading compounds on the recovery of V. parahaemolyticus also were investigated. Sterile oyster meats were inoculated with V. parahaemolyticus and treated at 250 MPa for 1, 2, or 3 min at 21°C. These meats were then blended with 0.1% peptone water supplemented with 0.5 to 1.5% NaCl and plated on tryptic soy agar (TSA) supplemented with 0 to 3.5% NaCl. For recovery of pressure-injured cells, peptone water with 1% NaCl and TSA with 0.5% NaCl were the best diluent and plating medium, respectively. Addition of sodium pyruvate (0.05 to 0.2%) or catalase (8 to 32 U/ml) did not increase the recovery of V. parahaemolyticus after pressure treatment. The effect of incubation temperature and gas atmosphere on the recovery of V. parahaemolyticus after pressure treatment also was determined. Aerobic incubation at 30°C resulted in the highest recovery of V. parahaemolyticus in sterile oyster meats. The 30°C incubation temperature was also the optimum temperature for recovery of V. parahaemolyticus in pressure-treated live oysters. The results of this study indicate that the growth conditions for V. parahaemolyticus before and after high hydrostatic pressure treatment should be taken into consideration when assessing the efficacy of pressure inactivation.

Entry of Yersinia pestis into the viable but nonculturable state in a low-temperature tap water microcosm

Yersinia pestis, the causative agent of plague, has caused several pandemics throughout history and remains endemic in the rodent populations of the western United States. More recently, Y. pestis is one of several bacterial pathogens considered to be a potential agent of bioterrorism. Thus, elucidating potential mechanisms of survival and persistence in the environment would be important in the event of an intentional release of the organism. One such mechanism is entry into the viable but non-culturable (VBNC) state, as has been demonstrated for several other bacterial pathogens. In this study, we showed that Y. pestis became nonculturable by normal laboratory methods after 21 days in a low-temperature tap water microcosm. We further show evidence that, after the loss of culturability, the cells remained viable by using a variety of criteria, including cellular membrane integrity, uptake and incorporation of radiolabeled amino acids, and protection of genomic DNA from DNase I digestion. Additionally, we identified morphological and ultrastructural characteristics of Y. pestis VBNC cells, such as cell rounding and large periplasmic spaces, by electron microscopy, which are consistent with entry into the VBNC state in other bacteria. Finally, we demonstrated resuscitation of a small number of the non-culturable cells. This study provides compelling evidence that Y. pestis persists in a low-temperature tap water microcosm in a viable state yet is unable to be cultured under normal laboratory conditions, which may prove useful in risk assessment and remediation efforts, particularly in the event of an intentional release of this organism.

The cell membrane as a major site of damage during aerosolization of Escherichia coli

This study aimed to provide data on the survival and site of damage of Escherichia coli cells following aerosolization using two different techniques, nebulization and flow focusing. Four metabolic stains were assessed for their ability to detect respiratory activities and membrane homeostasis in aerosolized E. coli cells. The degree of sublethal injury increased significantly over the 10-min period of aerosolization in E. coli cells aerosolized by using the Collison nebulizer, reaching up to 99.9% of the population. In contrast, a significantly lower proportion of the population was sublethally damaged during aerosolization using the flow-focusing aerosol generator (FFAG). Concomitantly, loss of membrane homeostasis increased at a higher rate in nebulized cells (68 to 71%) than in those aerosolized by using the FFAG (32 to 34%). The activities of respiratory enzymes decreased at increased rates in nebulized cells (27 to 37%) compared to the rates of decrease in cells aerosolized by using the FFAG (59 to 61%). The results indicate that the physiology of an aerosolized bacterium is linked to the method of aerosol generation and may affect the interpretation of a range of aerobiological phenomenon.

Inactivation of the gene katA or sodA affects the transient entry into the viable but non-culturable response of Staphylococcus aureus in natural seawater at low temperature

We have investigated the fate of Staphylococcus aureus by starving the cells and maintaining them in natural seawater at 22 and 4 °C. At 22 °C, cells developed a long-term survival state where about 0.037% of the initial population remained culturable over more than 7 months, whereas at 4 °C, bacteria lost culturability and transiently entered into the viable but non-culturable state (VBNC). However, after 22 days of entry into the VBNC state, the number of viable cells detected via the direct viable count method decreased significantly. We show here that mutational inactivation of catalase (KatA) or superoxide dismutase (SodA) rendered strains hypersensitive to seawater stress at 4 °C and consequently, part of the seawater lethality on S. aureus at low temperature is mediated by reactive oxygen species (ROS) during microcosm-survival process. Shifting the temperature from 4 to 22 °C of totally non-culturable wild-type cells induced a partial recovery of the population. However, deficiencies in catalase or superoxide dismutase prevent resuscitation ability.

In vivo gene expression of cold shock and other stress-related genes in Vibrio vulnificus during shellstock temperature control conditions in oysters

AIMS

To determine Vibrio vulnificus response to shellstock refrigeration conditions while the bacterium was embedded in oysters (in vivo).

METHODS AND RESULTS

Depurated oysters were artificially inoculated with V. vulnificus. Several cold-shock conditions were examined according to the National Sanitation Shellfish Program guidelines. Culturability of cells along the refrigeration period was measured using specific colony dot-blot. Gene expression of putative cold-shock genes (csp1, csp3, csp4 and csp5) as well as three stress-related genes (rpoS, oxyR and katG) was determined by reverse transcriptase polymerase chain reaction. Vibrio vulnificus exhibited a decline in cell viability after cold shock but a cold adaptation response was observed when oysters were kept at suboptimal (15 degrees C) temperatures. 16SrRNA, and rpoS genes were constitutively expressed while expression of csp genes varied among strains and time points.

CONCLUSIONS

Vibrio vulnificus culturability was reduced after oysters were subjected to shellstock refrigeration conditions. When V. vulnificus was allowed to acclimate to cold temperatures, its survival after cold shock was higher. None of the cold shock genes analysed behaved as csp type I genes.

SIGNIFICANCE AND IMPACT OF THE STUDY

A model for artificially inoculated specific strains of V. vulnificus into oysters has been established. For the first time, V. vulnficus gene expression was assessed with the pathogen embedded in oysters.