The viable-but-nonculturable state induced by abiotic stress in the biocontrol agent Pseudomonas fluorescens CHA0 does not promote strain persistence in soil

Behavior of Salmonella Enteritidis and Shigella flexneri during induction and recovery of the viable but nonculturable state

Bacteria may enter into a viable but nonculturable (VBNC) state as a response to stresses, such as those found in food processing. Cells in the VBNC state lose the ability to grow in a conventional culture medium but man recover culturability. The viability, culturability and intracellular reactive oxygen species (ROS) of Salmonella Enteritidis and Shigella flexneri were evaluated under stress conditions to induce a VBNC state. Cells were maintained under nutritional, osmotic and cold stresses (long-term induction) in Butterfield's phosphate solution plus 1.2 M of NaCl at 4°C and under nutritional and oxidative stresses (short-term induction) in 10 mM of H2O2. Culture media, recovery agents, sterilization methods of media and incubation temperature, were combined and applied to recover the culturability of the VBNC cells. Salmonella entered in the VBNC state after 135 days under long-term induction, while Shigella maintained culturability after 240 days. Under short-term induction, Salmonella and Shigella lose culturability after 135 and 240 min, respectively. Flow cytometric analysis revealed viable cells and intracellular ROS in both species in VBNC. It was not possible to recover the culturability of VBNC cells using the 42 combinations of different factors.

Faucet aerator design influences aerosol size distribution and microbial contamination level

Faucet aerators have been linked to multiple opportunistic pathogen outbreaks in hospital, especially Pseudomonas aeruginosa, their complex structure promoting biofilm development. The importance of bacteria aerosolization by faucet aerators and their incidence on the risk of infection remain to be established. In this study, ten different types of aerators varying in complexity, flow rates and type of flow were evaluated in a controlled experimental setup to determine the production of aerosols and the level of contamination. The aerosol particle number density and size distribution were assessed using a particle spectrometer. The bacterial load was quantified with a 14-stage cascade impactor, where aerosol particles were captured and separated by size, then analysed by culture and flow cytometry. The water was seeded with Pseudomonas fluorescens as a bacterial indicator. Aerosol particle size and mean mass distribution varied depending on the aerator model. Devices without aeration or with laminar flow produced the lowest number and mass of aerosol particles when measured with spectrometry. Models with aeration displayed wide differences in their potential production of aerosol particles. A new aerator with a low flow, no air inlet in its structure, and a spray stream produced 12 to 395 times fewer aerosol particles containing bacteria. However, the impact of low flow on biofilm development and incorporation of pathogens should be further investigated. Repeated use of aerators resulted in fouling which increased the quantity of bacteria released through aerosol particles. An in-depth mechanical cleaning including complete dismantling of the aerator was required to recover initial performances. Aerators should be selected to minimize aerosol production, considering the ease of maintenance and the main water usage at each sink. Low flow aerators produced a lower number of contaminated aerosol particles when new but may be more susceptible to fouling and quickly lose their initial advantage.

Characteristics of ambient bioaerosols during haze episodes in China: A review

Frequent low visibility, haze pollution caused by heavy fine particulate matter (PM_2.5) loading, has been entailing significant environmental issues and health risks in China since 2013. A substantial fraction of bioaerosols was observed in PM (1.5-15%) during haze periods with intensive pollution. However, systematic and consistent results of the variations of bioaerosol characteristics during haze pollution are lacking. The role of bioaerosols in air quality and interaction with environment conditions are not yet well characterized. The present article provides an overview of the state of bioaerosol research during haze episodes based on numerous recent studies over the past decade, focusing on concentration, size distribution, community structure, and influence factors. Examples of insightful results highlighted the characteristics of bioaerosols at different air pollution levels and their pollution effects. We summarize the influences of meteorological and environmental factors on the distribution of bioaerosols. Further studies on bioaerosols, applying standardized sampling and identification criteria and investigating the influence of mechanisms of environmental or pollution factors on bioaerosols as well as the sources of bioaerosols are proposed.

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.

Cell culturability of Pseudomonas protegens CHA0 depends on soil pH

Pseudomonas inoculants may lose colony-forming ability in soil, but soil properties involved are poorly documented. Here, we tested the hypothesis that soil acidity could reduce persistence and cell culturability of Pseudomonas protegens CHA0. At 1 week in vitro, strain CHA0 was found as culturable cells at pH 7, whereas most cells at pH 4 and all cells at pH 3 were noncultured. In 21 natural soils of contrasted pH, cell culturability loss of P. protegens CHA0 took place in all six very acidic soils (pH < 5.0) and in three of five acidic soils (5.0 < pH < 6.5), whereas it was negligible in the neutral and alkaline soils at 2 weeks and 2 months. No correlation was found between total cell counts of P. protegens CHA0 and soil composition data, whereas colony counts of the strain correlated with soil pH. Maintenance of cell culturability in soils coincided with a reduction in inoculant cell size. Some of the noncultured CHA0 cells were nutrient responsive in Kogure's viability test, both in vitro and in soil. Thus, this shows for the first time that the sole intrinsic soil composition factor triggering cell culturability loss in P. protegens CHA0 is soil acidity.

Viable but nonculturable bacteria: food safety and public health perspective

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

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.

Changes of ultrastructure and stress tolerance of Vibrio parahaemolyticus upon entering viable but nonculturable state

This study examined the change of ultrastructure and stress tolerance of the marine foodborne pathogen, Vibrio parahaemolyticus 1137, when incubated under viable but nonculturable (VBNC) state induction conditions for different time intervals. The rod-shaped V. parahaemolyticus cells in the exponential phase became coccoid cells in the VBNC state, with aberrantly shaped cells formed in the initial stage. In the aberrantly shaped cells, the cell wall was loosened, flexible and allowed the cell to bulge, and the formation of new and thin cell wall or the expansion of cell wall was also discerned primarily at the polar position, enclosing an empty cellular space. The thickness of the cell wall increased with the VBNC induction time, and was increased in cultures that were removed from the induction conditions and whose temperature was upshifted to 25°C for 1 or 2days. The incubation of V. parahaemolyticus under the VBNC induction conditions significantly enhanced its tolerance to heat, H(2)O(2) and low salinity, but sensitized it to bile salts. Tolerance to heat, bile salts and low salinity was significantly higher in the temperature upshifted cultures than in the corresponding unheated cultures, and the heated cultures were also more susceptible to H(2)O(2). The V. parahaemolyticus cultures that were incubated in the VBNC state induction conditions and the corresponding temperature-upshifted cultures exhibited unique changes in ultrastructure and tolerance to various stresses, unlike the nutrient-starved cells.

Leaching of Cryptosporidium parvum oocysts, Escherichia coli, and a Salmonella enterica serovar Typhimurium bacteriophage through intact soil cores following surface application and injection of slurry

Increasing amounts of livestock manure are being applied to agricultural soil, but it is unknown to what extent this may be associated with contamination of aquatic recipients and groundwater if microorganisms are transported through the soil under natural weather conditions. The objective of this study was therefore to evaluate how injection and surface application of pig slurry on intact sandy clay loam soil cores influenced the leaching of Salmonella enterica serovar Typhimurium bacteriophage 28B, Escherichia coli, and Cryptosporidium parvum oocysts. All three microbial tracers were detected in the leachate on day 1, and the highest relative concentration was detected on the fourth day (0.1 pore volume). Although the concentration of the phage 28B declined over time, the phage was still found in leachate at day 148. C. parvum oocysts and chloride had an additional rise in the relative concentration at a 0.5 pore volume, corresponding to the exchange of the total pore volume. The leaching of E. coli was delayed compared with that of the added microbial tracers, indicating a stronger attachment to slurry particles, but E. coli could be detected up to 3 months. Significantly enhanced leaching of phage 28B and oocysts by the injection method was seen, whereas leaching of the indigenous E. coli was not affected by the application method. Preferential flow was the primary transport vehicle, and the diameter of the fractures in the intact soil cores facilitated transport of all sizes of microbial tracers under natural weather conditions.

Leaching of human pathogens in repacked soil lysimeters and contamination of potato tubers under subsurface drip irrigation in Denmark

The risk for contamination of potatoes and groundwater through subsurface drip irrigation with low quality water was explored in 30 large-scale lysimeters containing repacked coarse sand and sandy loam soils. The human pathogens, Salmonella Senftenberg, Campylobacter jejuni and Escherichia coli O157:H7, and the virus indicator Salmonella Typhimurium bacteriophage 28B, were added weekly through irrigation tubes for one month with low irrigation rates (8 mm per week). In the following six months lysimeters were irrigated with groundwater free of pathogens. Two weeks after irrigation was started, phage 28B was detected in low concentrations (2 pfu ml(-1)) in leachate from both sandy loam soil and coarse sand lysimeters. After 27 days, phage 28B continued to be present in similar concentrations in leachate from lysimeters containing coarse sand, while no phage were found in lysimeters with sandy loam soil. The added bacterial pathogens were not found in any leachate samples during the entire study period of 212 days. Under the study conditions with repacked soil, limited macropores and low water velocity, bacterial pathogens seemed to be retained in the soil matrix and died-off before leaching to groundwater. However, viruses may leach to groundwater and represent a health risk as for some viruses only few virus particles are needed to cause human disease. The bacterial pathogens and the phage 28B were found on the potato samples harvested just after the application of microbial tracers was terminated. The findings of bacterial pathogens and phage 28 on all potato samples suggest that the main risk associated with subsurface drip irrigation with low quality water is faecal contamination of root crops, in particular those consumed raw.

Soil amendment with Pseudomonas fluorescens CHA0: lasting effects on soil biological properties in soils low in microbial biomass and activity

Pseudomonas fluorescens strains are used in agriculture as plant growth-promoting rhizobacteria (PGPR). Nontarget effects of released organisms should be analyzed prior to their large-scale use, and methods should be available to sensitively detect possible changes in the environments the organism is released to. According to ecological theory, microbial communities with a greater diversity should be less susceptible to disturbance by invading organisms. Based on this principle, we laid out a pot experiment with field-derived soils different in their microbial biomass and activity due to long-term management on similar parent geological material (loess). We investigated the survival of P. fluorescens CHA0 that carried a resistance toward rifampicin and the duration of potential changes of the soil microflora caused by the inoculation with the bacterium at the sowing date of spring wheat. Soil microbial biomass (C(mic), N(mic)) basal soil respiration (BR), qCO(2), dehydrogenase activity (DHA), bacterial plate counts, mycorrhiza root colonization, and community level substrate utilization were analyzed after 18 and 60 days. At the initial stage, soils were clearly different with respect to most of the parameters measured, and a time-dependent effect between the first and the second set point were attributable to wheat growth and the influence of roots. The effect of the inoculum was small and merely transient, though significant long-term changes were found in soils with a relatively low level of microbial biomass. Community level substrate utilization as an indicator of changes in microbial community structure was mainly changed by the growth of wheat, while other experimental factors were negligible. The sensitivity of the applied methods to distinguish the experimental soils was in decreasing order N(mic), DHA, C(mic), and qCO(2). Besides the selective enumeration of P. fluorescens CHA0 rif(+), which was only found in amended soils, methods to distinguish the inoculum effect were DHA, C(mic), and the ratio of C(mic) to N(mic). The sampling time was most sensitively indicated by N(mic), DHA, C(mic), and qCO(2). Our data support the hypothesis-based on ecosystem theory-that a rich microflora is buffering changes due to invading species. In other words, a soil-derived bacterium was more effective in a relatively poor soil than in soils that are rich in microorganisms.

Stress and How Bacteria Cope with Death and Survival

Bacterial populations that are exposed to rapidly changing and sometimes hostile environments constantly switch between growth, survival, and death. Understanding bacterial survival and death are therefore cornerstones in a full comprehension of microbial life. During the last few years, new insights have emerged regarding the mechanisms of bacterial inactivation under stressful conditions. Particularly under mildly lethal stress, the ultimate cause of inactivation often seems mediated by the cell itself and is subject to additional regulation that integrates information about the global state of the cell and its environmental and social surrounding. This article explores the thin line between bacterial growth and inactivation and focuses on some emerging bacterial survival strategies, both from an individual cell and from a population perspective.

Wave-like distribution patterns of gfp-marked Pseudomonas fluorescens along roots of wheat plants grown in two soils

Culturable rhizosphere bacterial communities had been shown to exhibit wave-like distribution patterns along wheat roots. In the current work we show, for the first time, significant wave-like oscillations of an individual bacterial strain, the biocontrol agent Pseudomonas fluorescens 32 marked with gfp, along 3-week-old wheat roots in a conventionally managed and an organically managed soil. Significant wave-like fluctuations were observed for colony forming units (CFUs) on selective media and direct fluorescent counts under the microscope. Densities of fluorescent cells and of CFUs fluctuated in a similar manner along wheat roots in the conventional soil. The frequencies of the first, second, and third harmonics were similar for direct cell counts and CFUs. Survival of P. fluorescens 32-gfp introduced into organically managed soil was lower than that of the same strain added to conventionally managed soil. Thus, when root tips reached a depth of 10-35 cm below soil level, the majority of the introduced cells may have died, so that no cells or CFU"s were detected in this region at the time of sampling. As a result, significant waves in CFUs or direct counts along roots were not found in organically managed soil, except when a sufficiently long series with detectable CFUs were obtained. In this last case the wave-like fluctuation in CFUs was damped toward the root tip. In conclusion, when cells of a single bacterial strain randomly mixed in soil survived until a root tip passed, growth and death cycles after passage of the root tip resulted in oscillating patterns of population densities of this strain along 3-week-old wheat roots.

Detection of plant-modulated alterations in antifungal gene expression in Pseudomonas fluorescens CHA0 on roots by flow cytometry

The biocontrol activity of the root-colonizing Pseudomonas fluorescens strain CHA0 is largely determined by the production of antifungal metabolites, especially 2,4-diacetylphloroglucinol. The expression of these metabolites depends on abiotic and biotic environmental factors, in particular, elements present in the rhizosphere. In this study, we have developed a new method for the in situ analysis of antifungal gene expression using flow cytometry combined with green fluorescent protein (GFP)-based reporter fusions to the phlA and prnA genes essential for the production of the antifungal compounds 2,4-diacetylphloroglucinol and pyrrolnitrin, respectively, in strain CHA0. Expression of phlA-gfp and prnA-gfp in CHA0 cells harvested from the rhizosphere of a set of plant species as well as from the roots of healthy, leaf pathogen-attacked, and physically stressed plants were analyzed using a FACSCalibur. After subtraction of background fluorescence emitted by plant-derived particles and CHA0 cells not carrying the gfp reporters, the average gene expression per bacterial cell could be calculated. Levels of phlA and prnA expression varied significantly in the rhizospheres of different plant species. Physical stress and leaf pathogen infection lowered phlA expression levels in the rhizosphere of cucumber. Our results demonstrate that the newly developed approach is suitable to monitor differences in levels of antifungal gene expression in response to various plant-derived factors. An advantage of the method is that it allows quantification of bacterial gene expression in rhizosphere populations at a single-cell level. To our best knowledge, this is the first study using flow cytometry for the in situ analysis of biocontrol gene expression in a plant-beneficial bacterium in the rhizosphere.

Pseudomonas biocontrol agents of soilborne pathogens: looking back over 30 years

ABSTRACT Pseudomonas spp. are ubiquitous bacteria in agricultural soils and have many traits that make them well suited as biocontrol agents of soilborne pathogens. Tremendous progress has been made in characterizing the process of root colonization by pseudomonads, the biotic and abiotic factors affecting colonization, bacterial traits and genes contributing to rhizosphere competence, and the mechanisms of pathogen suppression. This review looks back over the last 30 years of Pseudomonas biocontrol research and highlights key studies, strains, and findings that have had significant impact on shaping our current understanding of biological control by bacteria and the direction of future research.

Assessment of the environmental fate of the biological control agent of fire blight, Pseudomonas fluorescens EPS62e, on apple by culture and real-time PCR methods

The colonization of apple blossoms and leaves by Pseudomonas fluorescens EPS62e was monitored in greenhouse and field trials using cultivable cell counting and real-time PCR. The real-time PCR provided a specific quantitative method for the detection of strain EPS62e. The detection level was around 10(2) cells g (fresh weight)(-1) and the standard curve was linear within a 5-log range. EPS62e actively colonized flowers reaching values from 10(7) to 10(8) cells per blossom. In apple flowers, no significant differences were observed between population levels obtained by real-time PCR and plating, suggesting that viable but nonculturable (VBNC) cells and residual nondegraded DNA were not present. In contrast, on apple leaves, where cultivable populations of EPS62e decreased with time, significant differences were observed between real-time PCR and plating. These differences indicate the presence of VBNC cells or nondegraded DNA after cell death. Therefore, the EPS62e population was under optimal conditions during the colonization of flowers but it was stressed and poorly survived on leaves. It was concluded that for monitoring this biological control agent, the combined use of cultivable cell count and real-time PCR is necessary.

Bacteria associated with spores of the arbuscular mycorrhizal fungi Glomus geosporum and Glomus constrictum

Spores of the arbuscular mycorrhizal fungi (AMF) Glomus geosporum and Glomus constrictum were harvested from single-spore-derived pot cultures with either Plantago lanceolata or Hieracium pilosella as host plants. PCR-denaturing gradient gel electrophoresis analysis revealed that the bacterial communities associated with the spores depended more on AMF than host plant identity. The composition of the bacterial populations linked to the spores could be predominantly influenced by a specific spore wall composition or AMF exudate rather than by specific root exudates. The majority of the bacterial sequences that were common to both G. geosporum and G. constrictum spores were affiliated with taxonomic groups known to degrade biopolymers (Cellvibrio, Chondromyces, Flexibacter, Lysobacter, and Pseudomonas). Scanning electron microscopy of G. geosporum spores revealed that these bacteria are possibly feeding on the outer hyaline spore layer. The process of maturation and eventual germination of AMF spores might then benefit from the activity of the surface microorganisms degrading the outer hyaline wall layer.

Colonization of tomato root seedling by Pseudomonas fluorescens 92 rkG5: spatio-temporal dynamics, localization, organization, viability, and culturability

The localization, viability, and culturability of Pseudomonas fluorescens 92 rkG5 were analyzed on three morphological root zones (root tip + elongation, root hair, and collar) of 3-, 5-, and 7-day-old tomato plants. Qualitative information about the localization and viability was collected by confocal laser scanning microscopy. Quantitative data concerning the distribution, viability, and culturability were obtained through combined dilution plating and flow cytometry. Colonization by P. fluorescens affected root development in a complex way, causing a general increase in the length of the collar and early stimulation of the primary root growth (3rd day), followed by a reduction in length (7th day). The three root zones showed different distribution, organization, and viability of the bacterial cells, but the distribution pattern within each zone did not change with time. Root tips were always devoid of bacteria, whereas with increasing distance from the apex, microcolonies or strings of cells became more and more prominent. Viability was high in the elongation zone, but it declined in the older parts of the roots. The so-called viable but not culturable cells were observed on the root, and their proportion in the distal (root tip + elongation) zone dramatically increased with time. These results suggest the existence of a specific temporal and spatial pattern of root colonization, related to cell viability and culturability, expressed by the plant-beneficial strain P. fluorescens 92 rkG5.

Induction of viable but nonculturable state in Vibrio parahaemolyticus and its susceptibility to environmental stresses

AIMS

This work analysed factors that influence the induction of viable but nonculturable (VBNC) state in the common enteric pathogen, Vibrio parahaemolyticus. The susceptibility of the VBNC cells to environmental stresses was investigated.

METHODS AND RESULTS

Bacterium was cultured in tryptic soy broth-3% NaCl medium, shifted to a nutrient-free Morita mineral salt-0.5% NaCl medium (pH 7.8) and further incubated at 4 degrees C in a static state to induce the VBNC state in 28-35 days. The culturability and viability of the cells were monitored by the plate count method and the Bac Light viable count method, respectively. Cells grown at the optimum growth temperature and in the exponential phase better induced the VBNC state than those grown at low temperature and in the stationary phase. Low salinity of the medium crucially and markedly shortened the induction period. The VBNC cells were highly resistant to thermal (42, 47 degrees C), low salinity (0% NaCl), or acid (pH 4.0) inactivation.

CONCLUSIONS

Optimal conditions for inducing VBNC V. parahaemolyticus were reported. The increase in resistance of VBNC V. parahaemolyticus to thermal, low salinity and acidic inactivation verified that this state is entered as part of a survival strategy in an adverse environment.

SIGNIFICANCE AND IMPACT OF THE STUDY

The methods for inducing VBNC V. parahaemolyticus in a markedly short time will facilitate further physiological and pathological study. The enhanced stress resistance of the VBNC cells should attract attention to the increased risk presented by this pathogen in food.

Stationary phase mutagenesis: mechanisms that accelerate adaptation of microbial populations under environmental stress

Microorganisms are exposed to constantly changing environmental conditions. In a growth-restricting environment (e.g. during starvation), mutants arise that are able to take over the population by a process known as stationary phase mutation. Genetic adaptation of a microbial population under environmental stress involves mechanisms that lead to an elevated mutation rate. Under stressful conditions, DNA synthesis may become more erroneous because of the induction of error-prone DNA polymerases, resulting in a situation in which DNA repair systems are unable to cope with increasing amounts of DNA lesions. Transposition may also increase genetic variation. One may ask whether the rate of mutation under stressful conditions is elevated as a result of malfunctioning of systems responsible for accuracy or are there specific mechanisms that regulate the rate of mutations under stress. Evidence for the presence of mutagenic pathways that have probably been evolved to control the mutation rate in a cell will be discussed.

Impact of biocontrol strain Pseudomonas fluorescens CHA0 on rhizosphere bacteria isolated from barley (Hordeum vulgare L.) with special reference to Cytophaga-like bacteria

AIMS

To assess the impact of the biocontrol strain Pseudomonas fluorescens CHA0 on a collection of barley rhizosphere bacteria using an agar plate inhibition assay and a plant microcosm, focusing on a CHA0-sensitive member of the Cytophaga-like bacteria (CLB).

METHODS AND RESULTS

The effect of strain CHA0 on a collection of barley rhizosphere bacteria, in particular CLB and fluorescent pseudomonads sampled during a growth season, was assessed by a growth inhibition assay. On average, 85% of the bacteria were sensitive in the May sample, while the effect was reduced to around 68% in the July and August samples. In the May sample, around 95% of the CLB and around 45% of the fluorescent pseudomonads were sensitive to strain CHA0. The proportion of CHA0-sensitive CLB and fluorescent pseudomonad isolates decreased during the plant growth season, i.e. in the July and August samples. A particularly sensitive CLB isolate, CLB23, was selected, exposed to strain CHA0 (wild type) and its genetically modified derivatives in the rhizosphere of barley grown in gnotobiotic soil microcosms. Two dry-stress periods were imposed during the experiment. Derivatives of strain CHA0 included antibiotic or exopolysaccharide (EPS) overproducing strains and a dry-stress-sensitive mutant. Despite their inhibitory activity against CLB23 in vitro, neither wild-type strain CHA0, nor any of its derivatives, had a major effect on culturable and total cell numbers of CLB23 during the 23-day microcosm experiment. Populations of all inoculants declined during the two dry-stress periods, with soil water contents below 5% and plants reaching the wilting point, but they recovered after re-wetting the soil. Survival of the dry-stress-sensitive mutant of CHA0 was most affected by the dry periods; however, this did not result in an increased population density of CLB23.

CONCLUSIONS

CLB comprise a large fraction of barley rhizosphere bacteria that are sensitive to the biocontrol pseudomonad CHA0 in vitro. However, in plant microcosm experiments with varying soil humidity conditions, CHA0 or its derivatives had no major impact on the survival of the highly sensitive CLB strain, CLB23, during two dry-stress periods and a re-wetting period; all co-existed well in the rhizosphere of barley plants.

SIGNIFICANCE AND IMPACT OF THE STUDY

Results indicate a lack of interaction between the biocontrol pseudomonad CHA0 and a sensitive CLB when the complexity increases from agar plate assays to plant microcosm experiments. This suggests the occurrence of low levels of antibiotic production and/or that the two bacterial genera occupy different niches in the rhizosphere.

Persistence and cell culturability of biocontrol strain Pseudomonas fluorescens CHA0 under plough pan conditions in soil and influence of the anaerobic regulator gene anr

Certain fluorescent pseudomonads can protect plants from soil-borne pathogens, and it is important to understand how these biocontrol agents survive in soil. The persistence of the biocontrol strain Pseudomonas fluorescens CHA0-Rif under plough pan conditions was assessed in non-sterile soil microcosms by counting total cells (immunofluorescence microscopy), intact cells (BacLight membrane permeability test), viable cells (Kogure's substrate-responsiveness test) and culturable cells (colony counts on selective plates) of the inoculant. Viable but non-culturable cells of CHA0-Rif (106 cells g-1 soil) were found in flooded microcosms amended with fermentable organic matter, in which the soil redox potential was low (plough pan conditions), in agreement with previous observations of plough pan samples from a field inoculated with CHA0-Rif. However, viable but non-culturable cells were not found in unamended flooded, amended unflooded or unamended unflooded (i.e. control) microcosms, suggesting that such cells resulted from exposure of CHA0-Rif to a combination of low redox potential and oxygen limitation in soil. CHA0-Rif is strictly aerobic. Its anaerobic regulator ANR is activated by low oxygen concentrations and it controls production of the biocontrol metabolite hydrogen cyanide under microaerophilic conditions. Under plough pan conditions, an anr-deficient mutant of CHA0-Rif and its complemented derivative displayed the same persistence pattern as CHA0-Rif, indicating that anr was not implicated in the formation of viable but non-culturable cells of this strain at the plough pan.

Effect of stress on the ability of a phlA-based quantitative competitive PCR assay to monitor biocontrol strain Pseudomonas fluorescens CHA0

A quantitative competitive PCR (QC-PCR) assay targeting the phlA gene of Pseudomonas fluorescens CHA0 was developed and tested in vitro. Statistically significant, positive correlations were found between QC-PCR and both CFU and total cell number when studying cells in log or stationary phase. The correlations disappeared when considering stressed cells.

Adoption of the transiently non-culturable state — a bacterial survival strategy?

Microbial culturability can be ephemeral. Cells are not merely either dead or alive but can adopt physiological states in which they appear to be (transiently) non-culturable under conditions in which they are known normally to be able to grow and divide. The reacquisition of culturability from such states is referred to as resuscitation. We here develop the idea that this "transient non-culturability" is a consequence of a special survival strategy, and summarise the morphological, physiological and genetic evidence underpinning such behaviour and its adaptive significance.

Inactivation of the regulatory gene algU or gacA can affect the ability of biocontrol Pseudomonas fluorescens CHA0 to persist as culturable cells in nonsterile soil

Rifampin-resistant Pseudomonas fluorescens CHA0-Rif and mutants in which the regulatory gene algU (encoding sigma factor sigma(E)) or gacA (encoding a global regulator of secondary metabolism) was inactivated were compared for persistence in three nonsterile soils. Functional algU and (particularly) gacA were needed for CHA0-Rif to maintain cell culturability in soil.

Analysis of seawaters for the recovery of culturable Vibrio parahaemolyticus and some other vibrios

We investigated the recovery of dormant and injured cells along with the normally culturable cells of Vibrio species with special emphasis on V. parahaemolyticus using both selective and non-selective media at moderate (20 C) and standard (37 C) culture temperatures from a bay water environment. Culture temperatures (20 or 37 C) did not affect the recovery of V. parahaemolyticus but did for other vibrios. We observed similar seasonality of V parahaemolyticus as in most other environmental studies. V. parahaemolyticus and other Vibrio species were recovered in higher numbers by a replica plating method compared to most probable number (MPN) and direct TCBS (thiosulfate citrate bile-salt sucrose) agar counts. Even with the replica plating method, however, vibrios number goes down to a minimum level and V. parahaemolyticus was undetectable during the cool temperature period of the year, although total bacterial cells and CFU on nutrient agar (with 2% NaCl) did not vary so much during the study period.

A matter of bacterial life and death

Over 50 years ago, standard microbiological methods were established for determining whether bacterial cells were dead or alive. Recently there has been a flurry of reports suggesting that bacteria may exist in an eclipsed state, escaping detection by standard methods. Whether there really is such a state is of more than academic interest, considering the implications for public health. The ensuing debate has been unusually energetic for the normally cultured community of microbiologists.