Investigation of the first events leading to loss of culturability during Escherichia coli starvation: future nonculturable bacteria form a subpopulation

Phenotypic and transcriptional characterization of F. tularensis LVS during transition into a viable but non-culturable state

Francisella tularensis is a gram-negative, intracellular pathogen which can cause serious, potentially fatal, illness in humans. Species of F. tularensis are found across the Northern Hemisphere and can infect a broad range of host species, including humans. Factors affecting the persistence of F. tularensis in the environment and its epidemiology are not well understood, however, the ability of F. tularensis to enter a viable but non-culturable state (VBNC) may be important. A broad range of bacteria, including many pathogens, have been observed to enter the VBNC state in response to stressful environmental conditions, such as nutrient limitation, osmotic or oxidative stress or low temperature. To investigate the transition into the VBNC state for F. tularensis, we analyzed the attenuated live vaccine strain, F. tularensis LVS grown under standard laboratory conditions. We found that F. tularensis LVS rapidly and spontaneously enters a VBNC state in broth culture at 37°C and that this transition coincides with morphological differentiation of the cells. The VBNC bacteria retained an ability to interact with both murine macrophages and human erythrocytes in in vitro assays and were insensitive to treatment with gentamicin. Finally, we present the first transcriptomic analysis of VBNC F. tularensis, which revealed clear differences in gene expression, and we identify sets of differentially regulated genes which are specific to the VBNC state. Identification of these VBNC specific genes will pave the way for future research aimed at dissecting the molecular mechanisms driving entry into the VBNC state.

Transcriptomic responses of the fast-growing bacterium Vibrio natriegens during cold-induced loss of culturability

Vibrio natriegens has massive biotechnological potential owing to its fast growth rate. However, this bacterium rapidly loses its culturability during low-temperature preservation (LTP), the reason for which is still unknown. To reveal the metabolic responses of V. natriegens during LTP, we analyzed and compared the transcriptome before and after 8 days of preservation at 4 or 25 °C (room-temperature preservation (RTP)) in liquid culture medium. Most genes exhibited significant transcriptional responses to LTP. Using gene set enrichment analysis, we compared the transcriptional responses of different V. natriegens Gene Ontology (GO) sets during LTP or RTP. The enrichment of the GO set "SOS response" during LTP, but not RTP, indicated the occurrence of DNA damage during LTP. The GO set "respiratory electron transport chain" was suppressed during LTP and RTP. Although the GO set "response to oxidative stress" was not significantly altered, we observed an increase in reactive oxygen species (ROS) during LTP, suggesting a relationship between ROS and cold-induced loss of culturability (CILC) in V. natriegens. The faster loss of culturability and accumulation of ROS in 20 mL compared to 100 mL of liquid culture medium further suggested a relationship between CILC and oxygen availability. Furthermore, we showed that the deletion of Na⁺-translocating NADH-ubiquinone oxidoreductase, but not type-II NADH dehydrogenase, accelerated CILC and increased intracellular ROS levels in V. natriegens. These findings will help to understand the cause of CILC which may lead to improving the stability of V. natriegens at low temperatures.

Current Perspectives on Viable but Non-Culturable Foodborne Pathogenic Bacteria: A Review

Foodborne diseases caused by foodborne pathogens pose risks to food safety. Effective detection and efficient inactivation of pathogenic bacteria has always been a research hotspot in the field of food safety. Complicating these goals, bacteria can be induced to adopt a viable but non-culturable (VBNC) state under adverse external environmental stresses. When in the VBNC state, pathogens cannot form visible colonies during traditional culture but remain metabolically active and toxic. The resulting false negative results in growth-related assays can jeopardize food safety. This review summarizes the latest research on VBNC foodborne pathogens, including induction conditions, detection methods, mechanism of VBNC formation, and possible control strategies. It is hoped that this review can provide ideas and methods for future research on VBNC foodborne pathogenic bacteria.

Characteristics of the copper-induced viable-but-non-culturable state in bacteria

The antimicrobial applications of copper (Cu) are exploited in several industries, such as agriculture and healthcare settings. While Cu is capable of efficiently killing microorganisms, sub-lethal doses can induce a viable-but-non-culturable (VBNC) state in bacteria of many distinct clades. VBNC cells cannot be detected by standard culture-based detection methods, and can become a threat to plants and animals as they often retain virulent traits upon resuscitation. Here we discuss the putative mechanisms of the Cu-induced VBNC state. Common observations in Cu-induced VBNC cells include a cellular response to reactive oxygen species, the exhaustion of energy reserves, and a reconfiguration of the proteome. While showing partial overlap with other VBNC state-inducing stressors, these changes seem to be part of an adaptive response to Cu toxicity. Furthermore, we argue that Cu resistance mechanisms such as P-type ATPases and multicopper oxidases may ward off entry into the VBNC state to some extent. The spread of these mechanisms across multi-species populations could increase population-level resistance to Cu antimicrobials. As Cu resistance mechanisms are often co-selected with antibiotic resistance mechanisms, this threat is exacerbated.

Size Matters: Biological and Food Safety Relevance of Leaf Damage for Colonization of Escherichia coli O157:H7 gfp

This study examined the biological and food safety relevance of leaf lesions for potential invasion of food pathogens into the plant tissue (internalization). This was done by determining the role of artificial leaf damage in terms of damaged leaf area on proliferation of E. coli O157:H7 gfp+. In a two-factorial experiment, unwashed fresh baby leaf spinach (Spinacia oleracea L.) was subjected to four damage levels (undamaged, low, moderate, high damage; factor 1) and three incubation intervals (0, 1, 2 days post-inoculation; factor 2). Individual leaves were immersed for 15 s in a suspension loaded with E. coli O157:H7 gfp+ (10⁶ CFU × mL^–1). The leaves were analyzed individually using image analysis tools to quantify leaf area and number and size of lesions, and using confocal laser scanning and scanning electron microscopy to visualize leaf lesions and presence of the introduced E. coli strain on and within the leaf tissue. Prevalence of E. coli O157:H7 gfp+ was assessed using a culture-dependent technique. The results showed that size of individual lesions and damaged leaf area affected depth of invasion into plant tissue, dispersal to adjacent areas, and number of culturable E. coli O157:H7 gfp+ directly after inoculation. Differences in numbers of the inoculant retrieved from leaf macerate evened out from 2 days post-inoculation, indicating rapid proliferation during the first day post-inoculation. Leaf weight was a crucial factor, as lighter spinach leaves (most likely younger leaves) were more prone to harbor E. coli O157:H7 gfp+, irrespective of damage level. At the high inoculum density used, the risk of consumers’ infection was almost 100%, irrespective of incubation duration or damage level. Even macroscopically intact leaves showed a high risk for infection. These results suggest that the risk to consumers is correlated with how early in the food chain the leaves are contaminated, and the degree of leaf damage. These findings should be taken into account in different steps of leafy green processing. Further attention should be paid to the fate of viable, but non-culturable, shiga-toxigenic E. coli on and in ready-to-eat leafy vegetables.

Induction of a viable but non-culturable state in Salmonella Typhimurium by thermosonication and factors affecting resuscitation

The objective of this work was to analyze the effects of thermosonication (TS) on induction of a viable but non-culturable (VBNC) state in Salmonella Typhimurium and to examine incubation factors affecting subsequent resuscitation of cells. A TS treatment of 380 W at 53°C for 30 min induced the VBNC state in S. Typhimurium cells in beef peptone yeast (BPY) broth, apple/carrot juice, physiological saline and phosphate buffer solution. The logarithmic and decline phases of growth were more sensitive to the TS treatment compared to stationary phase cells. Meanwhile, VBNC S. Typhimurium could be resuscitated back to culturable cells by using suitable incubation temperatures and media. Addition of Tween 20 hindered resuscitation compared to the use of BPY medium alone. The optimal growth temperature (i.e. 37°C) was the most suitable temperature to resuscitate cells from the VBNC state. The VBNC incidence index decreased with the addition of sodium pyruvate during TS treatment, as it accelerated resuscitation. The results demonstrated that free radicals produced during TS processing and the growth phase of cells affected induction of the VBNC state in S. Typhimurium. Hence, the kinds and amounts of free radicals generated during TS treatment should be analyzed in the future.

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.

cAMP-CRP acts as a key regulator for the viable but non-culturable state in Escherichia coli

A variety of bacteria, including Escherichia coli, are known to enter the viable but non-culturable (VBNC) state under various stress conditions. During this state, cells lose colony-forming activities on conventional agar plates while retaining signs of viability. Diverse environmental stresses including starvation induce the VBNC state. However, little is known about the genetic mechanism inducing this state. Here, we aimed to reveal the genetic determinants of the VBNC state of E. coli. We hypothesized that the VBNC state is a process wherein specific gene products important for colony formation are depleted during the extended period of stress conditions. If so, higher expression of these genes would maintain colony-forming activities, thereby restraining cells from entering the VBNC state. From an E. coli plasmid-encoded ORF library, we identified genes that were responsible for maintaining high colony-forming activities after exposure to starvation condition. Among these, cpdA encoding cAMP phosphodiesterase exhibited higher performance in the maintenance of colony-forming activities. As cpdA overexpression decreases intracellular cAMP, cAMP or its complex with cAMP-receptor protein (CRP) may negatively regulate colony-forming activities under stress conditions. We confirmed this using deletion mutants lacking adenylate cyclase or CRP. These mutants fully maintained colony-forming activities even after a long period of starvation, while wild-type cells lost most of this activity. Thus, we concluded that the lack of cAMP-CRP effectively retains high colony-forming activities, indicating that cAMP-CRP acts as a positive regulator necessary for the induction of the VBNC state in E. coli.

A General Method for Measuring Persister Levels in Escherichia coli Cultures

Genetically homogeneous bacterial cultures contain persisters, cells that are not killed by bactericidal antibiotics. These cells are suggested to be involved in the establishment of chronic infections. Persister levels depend on growth conditions. Here, we discuss the parameters that have to be considered when measuring persister levels and provide a sample protocol to do it.

The viable but non-culturable state in pathogenic Escherichia coli: A general review

Background

The persistence and pathogenicity of pathogenic bacteria are dependent on the ability of the species to survive in adverse conditions. During the infectious process, the organism may need to pass through certain hostile anatomical sites, such as the stomach. Under various environmental stresses, many bacteria enter into the viable but non-culturable (VBNC) state, where they are ‘alive’ or metabolically active, but will not grow on conventional media. Escherichia coli bacteria encounter several diverse stress factors during their growth, survival and infection and thus may enter into the VBNC state.

Objectives

This review discusses various general aspects of the VBNC state, the mechanisms and possible public health impact of indicator and pathogenic E. coli entering into the VBNC state.

Method

A literature review was conducted to ascertain the possible impact of E. coli entering into the VBNC state.

Results

Escherichia coli enter into the VBNC state by means of several induction mechanisms. Various authors have found that E. coli can be resuscitated post-VBNC. Certain strains of pathogenic E. coli are still able to produce toxins in the VBNC state, whilst others are avirulent during the VBNC state but are able to regain virulence after resuscitation.

Conclusion

Pathogenic and indicator E. coli entering into the VBNC state could have an adverse effect on public health if conventional detection methods are used, where the number of viable cells could be underestimated and the VBNC cells still produce toxins or could, at any time, be resuscitated and become virulent again.

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.

Mechanism to control the cell lysis and the cell survival strategy in stationary phase under heat stress

An array of stress signals triggering the bacterial cellular stress response is well known in Escherichia coli and other bacteria. Heat stress is usually sensed through the misfolded outer membrane porin (OMP) precursors in the periplasm, resulting in the activation of σ(E) (encoded by rpoE), which binds to RNA polymerase to start the transcription of genes required for responding against the heat stress signal. At the elevated temperatures, σ(E) also serves as the transcription factor for σ(H) (the main heat shock sigma factor, encoded by rpoH), which is involved in the expression of several genes whose products deal with the cytoplasmic unfolded proteins. Besides, oxidative stress in form of the reactive oxygen species (ROS) that accumulate due to heat stress, has been found to give rise to viable but non-culturable (VBNC) cells at the early stationary phase, which is in turn lysed by the σ(E)-dependent process. Such lysis of the defective cells may generate nutrients for the remaining population to survive with the capacity of formation of colony forming units (CFUs). σ(H) is also known to regulate the transcription of the major heat shock proteins (HSPs) required for heat shock response (HSR) resulting in cellular survival. Present review concentrated on the cellular survival against heat stress employing the harmonized impact of σ(E) and σ(H) regulons and the HSPs as well as their inter connectivity towards the maintenance of cellular survival.

Genetic and phenotypic evidence of the Salmonella enterica serotype Enteritidis human-animal interface in Chile

Salmonella enterica serotype Enteritidis is a worldwide zoonotic agent that has been recognized as a very important food-borne bacterial pathogen, mainly associated with consumption of poultry products. The aim of this work was to determine genotypic and phenotypic evidence of S. Enteritidis transmission among seabirds, poultry and humans in Chile. Genotyping was performed using PCR-based virulotyping, pulse-field gel electrophoresis (PFGE) and multi-locus sequence typing (MLST). Pathogenicity-associated phenotypes were determined with survival to free radicals, acidic pH, starvation, antimicrobial resistance, and survival within human dendritic cells. As result of PCR and PFGE assays, some isolates from the three hosts showed identical genotypic patterns, and through MLST it was determined that all of them belong to sequence type 11. Phenotypic assays show diversity of bacterial responses among isolates. When results were analyzed according to bacterial host, statistical differences were identified in starvation and dendritic cells survival assays. In addition, isolates from seabirds showed the highest rates of resistance to gentamycin, tetracycline, and ampicillin. Overall, the very close genetic and phenotypic traits shown by isolates from humans, poultry, and seabirds suggest the inter-species transmission of S. Enteritidis bacteria between hosts, likely through anthropogenic environmental contamination that determines infection of seabirds with bacteria that are potentially pathogenic for other susceptible organism, including humans.

Viable but not culturable forms of Legionella pneumophila generated after heat shock treatment are infectious for macrophage-like and alveolar epithelial cells after resuscitation on Acanthamoeba polyphaga

Legionella pneumophila, the causative agent of legionellosis is transmitted to human through aerosols from environmental sources and invades lung's macrophages. It also can invade and replicate within various protozoan species in environmental reservoirs. Following exposures to various stresses, L. pneumophila enters a non-replicative viable but non-culturable (VBNC) state. Here, we evaluated whether VBNC forms of three L. pneumophila serogroup 1 strains (Philadelphia GFP 008, clinical 044 and environmental RNN) infect differentiated macrophage-like cell lines (U937 and HL-60), A549 alveolar cells and Acanthamoeba polyphaga. VBNC forms obtained following shocks at temperatures ranging from 50 to 70 °C for 5 to 60 min were quantified using a flow cytometric assay (FCA). Their loss of culturability was checked on BCYE agar medium. VBNC forms were systematically detected upon a 70 °C heat shock for 30 min. When testing their potential to resuscitate upon amoebal infection, VBNC forms obtained after 30 min at 70 °C were re-cultivated except for the clinical strain. No resuscitation or cell lysis was evidenced when using U937, HL-60, or A549 cells despite the use of various contact times and culture media. None of the strains tested could infect A. polyphaga, macrophage-like or alveolar epithelial cells after a 60-min treatment at 70 °C. However, heat-treated VBNC forms were able to infect macrophage-like or alveolar epithelial cells following their resuscitation on A. polyphaga. These results suggest that heat-generated VBNC forms of L. pneumophila (i) are not infectious for macrophage-like or alveolar epithelial cells in vitro although resuscitation is still possible using amoeba, and (ii) may become infectious for human cell lines following a previous interaction with A. polyphaga.

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.

Characterization and resuscitation of 'non-culturable' cells of Legionella pneumophila

BACKGROUND

Legionella pneumophila is a waterborne pathogen responsible for Legionnaires' disease, an infection which can lead to potentially fatal pneumonia. After disinfection, L. pneumophila has been detected, like many other bacteria, in a "viable but non culturable" state (VBNC). The physiological significance of the VBNC state is unclear and controversial: it could be an adaptive response favoring long-term survival; or the consequence of cellular deterioration which, despite maintenance of certain features of viable cells, leads to death; or an injured state leading to an artificial loss of culturability during the plating procedure. VBNC cells have been found to be resuscitated by contact with amoebae.

RESULTS

We used quantitative microscopic analysis, to investigate this "resuscitation" phenomenon in L. pneumophila in a model involving amending solid plating media with ROS scavengers (pyruvate or glutamate), and co-culture with amoebae. Our results suggest that the restoration observed in the presence of pyruvate and glutamate may be mostly due to the capacity of these molecules to help the injured cells to recover after a stress. We report evidence that this extracellular signal leads to a transition from a not-culturable form to a culturable form of L. pneumophila, providing a technique for recovering virulent and previously uncultivated forms of L. pneumophila.

CONCLUSION

These new media could be used to reduce the risk of underestimation of counts of virulent of L. pneumophila cells in environmental samples.

Whole-genome transcriptional analysis of Escherichia coli during heat inactivation processes related to industrial cooking

Escherichia coli K-12 was grown to the stationary phase, for maximum physiological resistance, in brain heart infusion (BHI) broth at 37°C. Cells were then heated at 58°C or 60°C to reach a process lethality value \[\mathbf{\left(}{{\mathit{F}}^{\mathit{o}}}_{\mathbf{70}}^{\mathbf{10}}\mathbf{\right)} \] of 2 or 3 or to a core temperature of 71°C (control industrial cooking temperature). Growth recovery and cell membrane integrity were evaluated immediately after heating, and a global transcription analysis was performed using gene expression microarrays. Only cells heated at 58°C with F(o) = 2 were still able to grow on liquid or solid BHI broth after heat treatment. However, their transcriptome did not differ from that of bacteria heated at 58°C with F(o) = 3 (P value for the false discovery rate [P-FDR] > 0.01), where no growth recovery was observed posttreatment. Genome-wide transcriptomic data obtained at 71°C were distinct from those of the other treatments without growth recovery. Quantification of heat shock gene expression by real-time PCR revealed that dnaK and groEL mRNA levels decreased significantly above 60°C to reach levels similar to those of control cells at 37°C (P < 0.0001). Furthermore, despite similar levels of cell inactivation measured by growth on BHI media after heating, 132 and 8 genes were differentially expressed at 71°C compared to 58°C and 60°C at F(o) = 3, respectively (P-FDR < 0.01). Among them, genes such as aroA, citE, glyS, oppB, and asd, whose expression was upregulated at 71°C, may be worth investigating as good biomarkers for accurately determining the efficiency of heat treatments, especially when cells are too injured to be enumerated using growth media.

Surveillance of single-cell behavior in different subpopulations of Ralstonia pickettii AR1 during growth and polyhydroxybutyrate production phases by flow cytometry

Most bacterial strains accumulate intracellular polyhydroxybutyrate (PHB) granules as an energy reservoir, in response to fluctuations in their microenvironment. Flow cytometry was applied for the analysis of single cells of Ralstonia pickettii AR1 in response to changes in the culture conditions. Two parameters, the PHB production-related FL2 and side scatter (SSC) parameters, were used to monitor, distinguish and characterize different subpopulations in the growth and PHB production phases. A high SSC level was observed in the mid-log exponential growth phase. When the nitrogen source reached a limiting level, the SSC started to decrease, in contrast to the intracellular PHB granules-related FL2 parameter. The results show that ammonium limitation is a critical and important factor for the accumulation of reserve compounds. Four subpopulations were observed and distinguished upon entrance of the cells into the exponential growth phase. When the cells entered the late exponential growth or early stationary phase, two subpopulations had disappeared and only two, different subpopulations were dominant. One of the subpopulations with changed SSC and PHB production activity switched to another subpopulation that was only active in PHB production in the stationary phase. The whole cells of R. pickettii AR1 tended to form a homogeneous population at the end of the stationary phase. In fact, the changes in the subpopulations of a single strain are related to different physiological states of the cells. The observation of different subpopulations suggests that each subpopulation shows a specific response to changes in the surrounding microenvironment, nutrients and limiting factors.

Genetic diversity among offspring from archived Salmonella enterica ssp. enterica serovar typhimurium (Demerec Collection): in search of survival strategies

Extensive phenotypic and genomic diversity was detected among offspring of Salmonella enterica ssp. enterica serovar Typhimurium LT2 (nonmutator) and LT7 (mutator, mutL) strains after decades of storage in sealed nutrient agar stabs. In addition to numerous losses in carbon and nitrogen metabolism, the acquired new metabolites indicated that alternate pathways were established. Particularly striking was the array of phage types when this phenotype was expected to be a stable feature. Evidence is presented regarding the role of mutator gene mutL(-) in the establishment of diversity as well as the ability of cells to return to mutL(+) genetic stabilization. Mutations included deletions, duplications, frameshifts, inversions and transpositions. In competition tests, survivors were more fit than were wild type. Because survival strategies continue to intrigue microbiologists, observations are compared with those of others who have addressed related questions. A brief genealogy of the archived strains is also recorded.

A microscope automated fluidic system to study bacterial processes in real time

Most time lapse microscopy experiments studying bacterial processes ie growth, progression through the cell cycle and motility have been performed on thin nutrient agar pads. An important limitation of this approach is that dynamic perturbations of the experimental conditions cannot be easily performed. In eukaryotic cell biology, fluidic approaches have been largely used to study the impact of rapid environmental perturbations on live cells and in real time. However, all these approaches are not easily applicable to bacterial cells because the substrata are in all cases specific and also because microfluidics nanotechnology requires a complex lithography for the study of micrometer sized bacterial cells. In fact, in many cases agar is the experimental solid substratum on which bacteria can move or even grow. For these reasons, we designed a novel hybrid micro fluidic device that combines a thin agar pad and a custom flow chamber. By studying several examples, we show that this system allows real time analysis of a broad array of biological processes such as growth, development and motility. Thus, the flow chamber system will be an essential tool to study any process that take place on an agar surface at the single cell level.

Dissection of sigma(E)-dependent cell lysis in Escherichia coli: roles of RpoE regulators RseA, RseB and periplasmic folding catalyst PpiD

To understand the mechanism of sigma(E)-dependent cell lysis, we examined the consequences of deletion derivatives of rpoE regulators rseA, rseB and rseC on sigma(E) transcription, on levels of free versus membrane-bound sigma(E) and on OMP-biogenesis limiting factor(s) that could impact cell lysis. RT-PCR showed that individual nonpolar DeltarseA and DeltarseB increased the rpoE expression to varying extents, with pronounced induction in DeltarseA. Significantly the ratio of soluble (free) versus membrane-bound form of RpoE increased in DeltarseA, however without increase of its total amount, unraveling furthermore complexity in RpoE regulation. Significant characteristics of cell lysis, accompanied by a severe reduction in the levels of periplasmic OMP-folding factor (PpiD), were observed in DeltarseA. The cell-lysis phenotype of DeltarseA was suppressed by either rseA or ppiD plasmids, but neither by rseB nor by rseC clones. However, the cell lysis of the wild-type strain was almost completely repressed not only by the rseA clone but also by the rseB clone, suggesting RseB might be limiting in vivo. Thus, increase in the ratio of free sigma(E) in rseA mutants with a concomitant reduction in PpiD levels can account for sigma(E)-dependent lysis in concert with a potential role of small RNAs on the lysis process.

Respective roles of culturable and viable-but-nonculturable cells in the heterogeneity of Salmonella enterica serovar typhimurium invasiveness

The existence of Salmonella enterica serovar Typhimurium viable-but-nonculturable (VBNC) cells is a public health concern since they could constitute unrecognized sources of infection if they retain their pathogenicity. To date, many studies have addressed the ability of S. Typhimurium VBNC cells to remain infectious, but their conclusions are conflicting. An assumption could explain these conflicting results. It has been proposed that infectivity could be retained only temporarily after entry into the VBNC state and that most VBNC cells generated under intense stress could exceed the stage where they are still infectious. Using a Radioselectan density gradient centrifugation technique makes it possible to increase the VBNC-cell/culturable-cell ratio without increasing the exposure to stress and, consequently, to work with a larger proportion of newly VBNC cells. Here, we observed that (i) in the stationary phase, the S. Typhimurium population comprised three distinct subpopulations at 10, 24, or 48 h of culture; (ii) the VBNC cells were detected at 24 and 48 h; (iii) measurement of invasion gene (hilA, invF, and orgA) expression demonstrated that cells are highly heterogeneous within a culturable population; and (iv) invasion assays of HeLa cells showed that culturable cells from the different subpopulations do not display the same invasiveness. The results also suggest that newly formed VBNC cells are either weakly able or not able to successfully initiate epithelial cell invasion. Finally, we propose that at entry into the stationary phase, invasiveness may be one way for populations of S. Typhimurium to escape stochastic alteration leading to cell death.

Localization and expression of MreB in Vibrio parahaemolyticus under different stresses

MreB, the homolog of eukaryotic actin, may play a vital role when prokaryotes cope with stress by altering their spatial organization, including their morphology, subcellular architecture, and localization of macromolecules. This study investigates the behavior of MreB in Vibrio parahaemolyticus under various stresses. The behavior of MreB was probed using a yellow fluorescent protein-MreB conjugate in merodiploid strain SC9. Under normal growth conditions, MreB formed helical filaments in exponential-phase cells. The shape of starved or stationary-phase cells changed from rods to small spheroids. The cells differentiated into the viable but nonculturable (VBNC) state with small spherical cells via a "swelling-waning" process. In all cases, drastic remodeling of the MreB cytoskeleton was observed. MreB helices typically were loosened and fragmented into short filaments, arcs, and spots in bacteria under these stresses. The disintegrated MreB exhibited a strong tendency to attach to the cytoplasmic membrane. The expression of mreB generally declined in bacteria in the stationary phase and under starvation but was upregulated during the initial periods of cold shock and VBNC state differentiation and decreased afterwards. Our findings demonstrated the behavior of MreB in the morphological changes of V. parahaemolyticus under intrinsic or extrinsic stresses and may have important implications for studying the cellular stress response and aging.

Protein aggregates: an aging factor involved in cell death

In a previous study, we demonstrated the presence of protein aggregates in an exponentially grown Escherichia coli culture. In light of these observations, protein aggregates could be considered damage to cells that is able to pass from one generation to the next. Based on the assumption that the amount of aggregate protein could represent an aging factor, we monitored this amount in a bacterial culture during senescence. In doing so, we observed (i) a significant increase in the amount of aggregate protein over time, (ii) a proportional relationship between the amount of aggregate protein and the level of dead cells, (iii) a larger amount in dead cells than in culturable cells, (iv) a heterogeneous distribution of different amounts within a homogenous population of culturable cells entering stasis, and (v) that the initial amount of aggregate protein within a culturable population conditioned the death rate of the culture. Together, the results presented in this study suggest that protein aggregates indeed represent one aging factor leading to bacterial cell death.

Cell division in Escherichia coli cultures monitored at single cell resolution

Background

A fundamental characteristic of cells is the ability to divide. To date, most parameters of bacterial cultures, including cell division, have been measured as cell population averages, assuming that all bacteria divide at a uniform rate.

Results

We monitored the division of individual cells in Escherichia coli cultures during different growth phases. Our experiments are based on the dilution of green fluorescent protein (GFP) upon cell division, monitored by flow cytometry. The results show that the vast majority of E. coli cells in exponentially growing cultures divided uniformly. In cultures that had been in stationary phase up to four days, no cell division was observed. However, upon dilution of stationary phase culture into fresh medium, two subpopulations of cells emerged: one that started dividing and another that did not. These populations were detectable by GFP dilution and displayed different side scatter parameters in flow cytometry. Further analysis showed that bacteria in the non-growing subpopulation were not dead, neither was the difference in growth capacity reducible to differences in stationary phase-specific gene expression since we observed uniform expression of several stress-related promoters. The presence of non-growing persisters, temporarily dormant bacteria that are tolerant to antibiotics, has previously been described within growing bacterial populations. Using the GFP dilution method combined with cell sorting, we showed that ampicillin lyses growing bacteria while non-growing bacteria retain viability and that some of them restart growth after the ampicillin is removed. Thus, our method enables persisters to be monitored even in liquid cultures of wild type strains in which persister formation has low frequency.

Conclusion

In principle, the approaches developed here could be used to detect differences in cell division in response to different environmental conditions and in cultures of unicellular organisms other than E. coli.

Mutations in two global regulators lower individual mortality in Escherichia coli

There has been considerable investigation into the survival of bacterial cells under stress conditions, but little is known about the causes of mortality in the absence of exogenous stress. That there is a basal frequency of cell death in such populations may reflect that it is either impossible to avoid all lethal events, or alternatively, that it is too costly. Here, through a genetic screen in the model organism Escherichia coli, we identify two mutants with lower frequencies of mortality: rssB and fliA. Intriguingly, these two genes both affect the levels of different sigma factors within the cell. The rssB mutant displays enhanced resistance to multiple external stresses, possibly indicating that the cell gains its increased vitality through elevated resistance to spontaneous, endogenous stresses. The loss of fliA does not result in elevated stress resistance; rather, its survival is apparently due to a decreased physical stress linked to the insertion of the flagellum through the membrane and energy saved through the loss of the motor proteins. The identification of these two mutants implies that reducing mortality is not impossible; rather, due to its cost, it is subject to trade-offs with other traits that contribute to the competitive success of the organism.

Is Urografin density gradient centrifugation suitable to separate nonculturable cells from Escherichia coli populations?

The ability of Urografin or Percoll density gradient centrifugations to separate nonculturable subpopulations from heterogeneous Escherichia coli populations was analysed. Bacterial counts (total, active and culturable cells) and flow cytometric analyses were carried out in all recovered bands. After Urografin centrifugation, and despite the different origin of E. coli populations, a common pattern was obtained. High-density bands were formed mainly by nonculturable cells. However, the increase in cell density would not be common to all nonculturable cells, since part of this subpopulations banded in low-density zones, mixed with culturable cells. Bands obtained after Percoll centrifugation were heterogeneous and culturable and nonculturable cells were recovered along the gradient. Thus, fractionation in Urografin cannot be only attributed to changes in buoyant densities during the transition from culturable to nonculturable state. Urografin density gradients allow us to obtain enriched fractions in nonculturable subpopulations from a heterogeneous population, but working conditions should be carefully chosen to avoid Urografin toxicity.

Survival of Escherichia coli O157:H7 in wastewater from dairy lagoons

AIM

To determine the survival of Escherichia coli O157:H7 in dairy wastewater from on-site holding lagoons equipped with or without circulating aerators.

METHODS AND RESULTS

Survival was monitored in dairy lagoon microcosms equipped with or without scale-size circulators. Both laboratory strains of E. coli O157:H7 and an isolate of E. coli H7 from wastewater had poor survival rates and none proliferated in water from waste lagoons with or without circulators. Furthermore, the decline of E. coli O157:H7 was not enhanced in those microcosms equipped with circulators. Strain variation in survival was observed in both circulated and settling waters. The decline rate of E. coli O157:H7 Odwalla strain increased proportionately with the inoculum load. Escherichia coli failed to establish itself in wastewater even after four sequential inoculations simulating continuous faecal input into the lagoon. The native aerobic bacteria survived longer with a decimal reduction time of 21.3 days vs either introduced or native E. coli, which declined rapidly with decimal reduction time of 0.5-9.4 days.

CONCLUSIONS

Escherichia coli O157:H7 failed to establish and proliferate in dairy wastewater microcosms equipped with or without circulating aerators.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study furthers our knowledge of pathogen survival in wastewater, and suggests that proper management of wastewater before its use in irrigation is essential to reduce pathogen transfer to crops.

Culturability of stream bacteria assessed at the assemblage and population levels

Lotic bacterial communities can be examined at multiple levels: from the assemblage level to populations of individual species. In stream environments, as in many other systems, the percentage of bacteria that are culturable is quite low. In this study, the culturability of the overall bacterial assemblage, as well as the culturability of three common species (Acinetobacter calcoaceticus, Burkholderia cepacia, and Pseudomonas putida), was determined in samples collected from four streams on three dates. Colony hybridization (colonies were grown on modified nutrient agar) and fluorescent in situ hybridization were used to calculate the percentage of cells of a given species that were culturable. Approximately half of the overall assemblage was estimated to be viable but nonculturable cells (VBNC). The culturability of two of the species was low (0.29% for A. calcoaceticus and 0.46% for P. putida), whereas the value for B. cepacia (2.48%) exceeded the overall assemblage level culturability (0.90%). Overall, both bacterial assemblages and populations were dominated by VBNC. These results show quantitatively that not all members of a species that has culturable representatives are culturable when retrieved from natural populations, likely because of interspecific phenotypic and genotypic variability. Thus, the large pool of nonculturable cells includes representatives of species that are, under some circumstances, culturable.