Retention of enteropathogenicity by viable but nonculturable Escherichia coli exposed to seawater and sunlight

Specific and rapid enumeration of viable but nonculturable and viable-culturable gram-negative bacteria by using flow cytometry

An issue of critical concern in microbiology is the ability to detect viable but nonculturable (VBNC) and viable-culturable (VC) cells by methods other than existing approaches. Culture methods are selective and underestimate the real population, and other options (direct viable count and the double-staining method using epifluorescence microscopy and inhibitory substance-influenced molecular methods) are also biased and time-consuming. A rapid approach that reduces selectivity, decreases bias from sample storage and incubation, and reduces assay time is needed. Flow cytometry is a sensitive analytical technique that can rapidly monitor physiological states of bacteria. This report outlines a method to optimize staining protocols and the flow cytometer (FCM) instrument settings for the enumeration of VBNC and VC bacterial cells within 70 min. Experiments were performed using the FCM to quantify VBNC and VC Escherichia coli O157:H7, Pseudomonas aeruginosa, Pseudomonas syringae, and Salmonella enterica serovar Typhimurium cells after staining with different fluorescent probes: SYTO 9, SYTO 13, SYTO 17, SYTO 40, and propidium iodide (PI). The FCM data were compared with those for specific standard nutrient agar to enumerate the number of cells in different states. By comparing results from cultures at late log phase, 1 to 64% of cells were nonculturable, 40 to 98% were culturable, and 0.7 to 4.5% had damaged cell membranes and were therefore theoretically dead. Data obtained using four different gram-negative bacteria exposed to heat and stained with PI also illustrate the usefulness of the approach for the rapid and unbiased detection of dead versus live organisms.

Coliform accumulation in Amphibalanus amphitrite (Darwin, 1854) (Cirripedia) and its use as an organic pollution bioindicator in the estuarine area of Recife, Pernambuco, Brazil

Samples of water and barnacles Amphibalanus amphitrite were collected from Recife, Brazil, to assess if it accumulates total (TC) and thermotolerant coliforms (TTC) related with sewage pollution. The Most Probable Number (MPN) values and the standard procedures for examination of shellfish were used. Comparatively with the water samples, the highest coliform values came from the barnacles, with TC values ranging from < 3.0 × 10³ to > 2.4 × 10(6) MPN.g-1, and TTC ranging from > 2.4 × 10³ to 2.9 × 10(5) MPN.g-1. Barnacles accumulate the TC Ewingella americana, and the TTC Escherichia coli, Enterobacter gergoviae, Enterobacter aerogenes, and Enterobacter sakazakii. The results provided an indication of the level of organic contamination at the sampling locations and that this species could be a good organic pollution bioindicator.

Rapid detection of Escherichia coli in water using a hand-held fluorescence detector

The quantification of pathogenic bacteria in an environmental or clinical sample commonly involves laboratory-based techniques and results are not obtained for 24-72 h after sampling. Enzymatic analysis of microbial activity in water and other environmental samples using fluorescent synthetic substrates are well-established and highly sensitive methods in addition to providing a measure of specificity towards indicative bacteria. The enzyme beta-d-glucuronidase (GUD) is a specific marker for Escherichia coli and 4-methylumbelliferone-beta-D-glucuronide (MUG) a sensitive substrate for determining the presence of E. coli in a sample. However, currently used procedures are laboratory-based and require bench-top fluorimeters for the measurement of fluorescence resulting from the enzyme-substrate reaction. Recent developments in electronic engineering have led to the miniaturisation of fluorescence detectors. We describe the use of a novel hand-held fluorimeter to directly analyse samples obtained from the River Thames for the presence of E. coli. The results obtained by the hand-held detector were compared with those obtained with an established fluorescent substrate assay and by quantifying microbial growth on a chromogenic medium. Both reference methods utilised filtration of water samples. The miniaturised fluorescence detector was used and incubation times reduced to 30 min making the detection system portable and rapid. The developed hand-held system reliably detected E. coli as low as 7 cfu/mL river water sample. Our study demonstrates that new hand-held fluorescence measurement technology can be applied to the rapid and convenient detection of bacteria in environmental samples. This enables rapid monitoring to be carried out on-site. The technique described is generic and it may, therefore, be used in conjunction with different fluorescent substrates which allows the assessment of various target microorganisms in biological samples.

The impact of pond depth and environmental conditions on sunlight inactivation of Escherichia coli and enterococci in wastewater in a warm climate

Microcosm experiments were carried out under dark and real sunlight conditions in Ouagadougou (Burkina Faso) to investigate the survival of faecal indicators (Escherichia coli and enterococci) in secondary wastewater. Light damage was estimated by loss of bacterial culturability. The results clearly show that sunlight has a deleterious effect on the survival of both indicators. The mean dark inactivation coefficients for E. coli and enterococci were 0.045 and 0.047 h(-1), respectively, whereas inactivation coefficients in the shallowest microcosm (0.1 m) in illuminated conditions were 0.796 and 0.559 h(-1), respectively. No significant effect of pond depth (0.1-0.9 m) on the inactivation of both indicators was observed in the dark. However, the effect of depth was significant in the microcosms exposed to sunlight, probably because of attenuation. In illuminated conditions, enterococci were broadly inactivated more rapidly than E. coli (T90 = 26.81 h for E. coli and 15.67 h for enterococci in the 0.4 m microcosm). However, E. coli presented greater variability in the survival capabilities, suggesting difficulties in interpreting data using only E. coli as an indicator. Therefore, the use of both indicators together should be advisable for the assessment of effluent quality from waste stabilization ponds in the Sahelian region.

Detection of viable but non cultivable Escherichia coli after UV irradiation using a lytic Qbeta phage

In order to qualify the germicidal efficacy of ultraviolet (UV) disinfection system, we generally determine the reduction of viable bacteria after UV-C irradiation. However, the simple count of viable and cultivable bacteria in usual media cannot reflect whether or not the UV dose applied to disinfect water is sufficient to inactivate bacteria. Indeed, there is a bacterial mix in the UV-treated water: dead bacteria, viable and cultivable bacteria and viable but noncultivable bacteria (VBNC). The third type of bacteria can constitute a potential risk for public health. In fact, VBNC bacteria can be active and cause diseases. Consequently, the combination of a conventional method used to measure colony-forming ability after UV disinfection and the determination of adsorption constants of a lytic Qbeta phage in relation to irradiated host cells by an increased UV dose (Escherichia coli ATCC 13965) allows the detection of active bacteria, which lose their cultivability in usual growth media, but keep the phage susceptibility.

Recent findings on the viable but nonculturable state in pathogenic bacteria

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

Abundance of culturable versus viable Escherichia coli in freshwater

Approved methods traditionally used for Escherichia coli enumeration in waters are culture-based. However, these methods can underestimate the E. coli abundance in aquatic systems because they do not take into account cells that remain viable but have lost the ability to grow in or on culture media. We investigated, in freshwater samples, the abundance of (i) culturable E. coli, enumerated by the most probable number microplate method and (ii) viable E. coli, estimated using a procedure called DVC-FISH, which couples fluorescent in situ hybridization (FISH) and a viability testing technique (direct viable count (DVC)). The ratio of culturable to viable E. coli was close to 1 in highly contaminated waters (samples with a high concentration of culturable E. coli), but decreased drastically for weakly contaminated samples. This indicates a large fraction of viable but nonculturable (VBNC) E. coli in the latter samples. Microcosm experiments showed that some environmental factors, such as nutrient scarcity and solar irradiation, could lead to the presence of a high proportion of VBNC E. coli.

Effect of stress on production of heat labile enterotoxin by Escherichia coli

Enterotoxigenic Escherichia coli (ETEC) is an important pathogen responsible for secretory diarrhoea. The production of heat labile enterotoxin (LT), by ETEC, is largely responsible for the pathogenesis of diarrhoea. In the present study we investigated the effect of stress factors such as temperature, pH, osmotic stress and nutritional limitation on the production of LT by ETEC using in-house GMI-ELISA. Four strains of E. coli consisting, one standard strain MTCC 723 and three clinical isolates were used in the study. Maximum amount of LT (OD 3.285) was produced at 37 0 C followed by 40 0 C (OD 3.305). Growth of E. coli in medium with pH 8.6 resulted in maximum amount of LT production (OD 3.489). LT was not detectable when bacteria were grown in medium with pH < or =7.2 and > or = 9.2. Sodium chloride concentration of 0.2 M stimulated maximum amount of LT production. Maximum amount of LT was produced when the bacteria were grown in medium containing 2.5 g/l of glucose. All the stress factors had a significant effect on the LT production by E. coli , though quantitative differences in the various strains were observed.

Persistence of nucleic acid markers of health-relevant organisms in seawater microcosms: implications for their use in assessing risk in recreational waters

In the last decade, the use of culture-independent methods for detecting indicator organisms and pathogens in recreational waters has increased and has led to heightened interest in their use for routine water quality monitoring. However, a thorough understanding of the persistence of genetic markers in environmental waters is lacking. In the present study, we evaluate the persistence of enterococci, enterovirus, and human-specific Bacteroidales in seawater microcosms. Two microcosms consisted of seawater seeded with human sewage. Two additional seawater microcosms were seeded with naked Enterococcus faecium DNA and poliovirus RNA. One of each replicate microcosm was exposed to natural sunlight; the other was kept in complete darkness. In the sewage microcosms, concentrations of enterococci and enterovirus were measured using standard culture-dependent methods as well as QPCR and RT-QPCR respectively. Concentrations of human-specific Bacteroidales were determined with QPCR. In the naked-genome microcosms, enterococci and enterovirus markers were enumerated using QPCR and RT-QPCR, respectively. In the sewage microcosm exposed to sunlight, concentrations of culturable enterococci fell below the detection limit within 5 days, but the QPCR signal persisted until the end of the experiment (day 28). Culturable enterococci did not persist as long as infectious enteroviruses. The ability to culture enteroviruses and enterococci was lost before detection of the genetic markers was lost, but the human-specific Bacteroidales QPCR signal persisted for a similar duration as infectious enteroviruses in the sewage microcosm exposed to sunlight. In the naked-genome microcosms, DNA and RNA from enterococci and enterovirus, respectively, persisted for over 10d and did not vary between the light and dark treatments. These results indicate differential persistence of genetic markers and culturable organisms of public health relevance in an environmental matrix and have important management implications.

Stress, sublethal injury, resuscitation, and virulence of bacterial foodborne pathogens

Environmental stress and food preservation methods (e.g., heating, chilling, acidity, and alkalinity) are known to induce adaptive responses within the bacterial cell. Microorganisms that survive a given stress often gain resistance to that stress or other stresses via cross-protection. The physiological state of a bacterium is an important consideration when studying its response to food preservation techniques. This article reviews the various definitions of injury and stress, sublethal injury of bacteria, stresses that cause this injury, stress adaptation, cellular repair and response mechanisms, the role of reactive oxygen species in bacterial injury and resuscitation, and the potential for cross-protection and enhanced virulence as a result of various stress conditions.

Survival and persistence of human and ruminant-specific faecal Bacteroidales in freshwater microcosms

Amplification of host-specific markers from Bacteroidales faecal anaerobes can rapidly identify the source of faecal pollution. It is necessary to understand persistence and survival of these markers and marker cells, both to interpret quantitative source-tracking data, and to use such data to predict pathogen occurrence. We measured marker persistence and cell survival of two human (HF134, HF183) and two ruminant (CF128, CF193) faecal Bacteroidales markers, compared with Escherichia coli and enterococci. Freshwater microcosms were inoculated with fresh cattle or human faeces and incubated at 13 degrees C in natural light or darkness. Marker persistence was measured by polymerase chain reaction (PCR) and quantitative PCR. Survival of marker cells was measured by real-time quantitative PCR. There was no difference in persistence between the two human-specific Bacteroidales DNA markers in the light and dark microcosms. Cell survival profiles of the two human markers were also similar; both were significantly affected by light. Ruminant markers persisted and survived longer than human markers (14 versus 6 days respectively). CF193 decreased more rapidly than CF128, and light significantly affected CF128 but not CF193. These results support use of host-specific faecal Bacteroidales markers as indicators of recent faecal pollution, but suggest that caution is needed in interpreting quantitative results to indicate proportional contribution of different sources, as individual markers differ in their survival, persistence and response to environmental variables. The survival and persistence profiles for Bacteroidales markers are consistent with survival profiles for several faecal pathogens.

Comparative assessment of human and farm animal faecal microbiota using real-time quantitative PCR

Pollution of the environment by human and animal faecal pollution affects the safety of shellfish, drinking water and recreational beaches. To pinpoint the origin of contaminations, it is essential to define the differences between human microbiota and that of farm animals. A strategy based on real-time quantitative PCR (qPCR) assays was therefore developed and applied to compare the composition of intestinal microbiota of these two groups. Primers were designed to quantify the 16S rRNA gene from dominant and subdominant bacterial groups. TaqMan probes were defined for the qPCR technique used for dominant microbiota. Human faecal microbiota was compared with that of farm animals using faecal samples collected from rabbits, goats, horses, pigs, sheep and cows. Three dominant bacterial groups (Bacteroides/Prevotella, Clostridium coccoides and Bifidobacterium) of the human microbiota showed differential population levels in animal species. The Clostridium leptum group showed the lowest differences among human and farm animal species. Human subdominant bacterial groups were highly variable in animal species. Partial least squares regression indicated that the human microbiota could be distinguished from all farm animals studied. This culture-independent comparative assessment of the faecal microbiota between humans and farm animals will prove useful in identifying biomarkers of human and animal faecal contaminations that can be applied to microbial source tracking methods.

The impact of sunlight on inactivation of indicator microorganisms both in river water and benthic biofilms

A detailed knowledge on decay or inactivation kinetics of faecal indicator microorganism in rivers is essential for control of bathing water quality. Both reliable inactivation rate coefficients for such kinetics and the knowledge on pathogen accumulation in benthic biofilms are needed for the assessment of river self purification capability and for being able to make decisions for an optimized water management. Therefore, the inactivation kinetics for main indicator microorganisms like faecal coliforms and intestinal Enterococci in water and on stones of the river Isar (Germany) were measured at artificial sunlight radiation. A flume was built, containing Isar water and sediment. By adding a pulse of pre-treated sewage water, the subsequently change of indicator bacteria levels in the bulk and biofilm phase is measured. Bacterial inactivation was mainly dependent on sunlight intensity. Mean inactivation rate coefficients in the experimental set up with river water obtained for a radiation intensity comparable to average midday sunlight in June in 50 degrees north latitude (I((290-390nm))=40.0W/m(2)) were found to be 21.4d(-1) for faecal coliforms and 20.0d(-1) for intestinal Enterococci. For a radiation intensity conform with the annual mean radiation in Germany (I((290-390nm))=8.0W/m(2)) inactivation rate coefficients were 12.7d(-1) for faecal coliforms and 9.3d(-1) for intestinal Enterococci. A measurement without direct artificial sunlight (I((290-390nm))=0.08W/m(2)) yields inactivation rate coefficients of 3.4d(-1) for faecal coliforms and 1.7d(-1) for intestinal Enterococci, which were similar to those found in wastewater treatment ponds. Due to re-growth and better environmental conditions the concentrations of faecal coliforms and intestinal Enterococci within the biofilm were 10(2) to 10(4) fold higher compared to the bulk water depending on the exposure time.

State transitions of Vibrio parahaemolyticus VBNC cells evaluated by flow cytometry

BACKGROUND

Vibrio parahaemolyticus, in response to environmental conditions, may be present in a viable but nonculturable state (VBNC), which can still be responsible for cases of infectious diseases in humans.

METHODS

The characterization of the cellular states of V. parahaemolyticus during entry into, persistence in, and resuscitation from the VBNC state, was assessed through plate culture method and epifluorescence microscope evaluation of actively respiring cells. Flow cytometry (FCM) in combination with SYBR Green I (SG) and propidium iodide allowed us to distinguish between viable, dead, and damaged-cells. Immunofluorescence labeling detected by FCM was used to study changes in antibody affinity.

RESULTS

Two groups of bacteria, one with High Nucleic Acid (HNA) and one having Low Nucleic Acid (LNA) content, were differentiated using SG and FCM and each was correlated with cell viability. With the aging of the microcosm, the LNA bacteria population increased while the HNA population gradually disappeared. Cytofluorimetric immunofluorescence analyses showed that the bacterial cell levels dropped from 95% at day 0 to 40% at day 26 and by day 29, antibody affinity was virtually lost. FCM analyses of light scatter signals expressed by cell population highlighted morphological changes indicating a reduction in cell size, as also shown by scanning electron microscopy images and variations in cell structure.

CONCLUSIONS

The methodology used has provided useful data in relation to the state transitions of V. parahaemolyticus regarding cell viability, antigenic surface components, and the quantification of morphological variations during its entry into the VBNC state.

Development of a rapid quantitative PCR assay for direct detection and quantification of culturable and non-culturable Escherichia coli from agriculture watersheds

A real-time quantitative polymerase chain reaction (Q-PCR) assay was developed for detecting and quantifying Escherichia coli in water samples from agricultural watersheds. The assay included optimization of DNA extraction and purification from water samples, and Q-PCR amplification conditions using newly designed species-specific oligonucleotide primers derived from conserved flanking regions of the 16S rRNA gene, the internal transcribed spacer region (ITS) and the 23S rRNA gene. The assay was optimized using a pure culture of E. coli with known quantities spiked into autoclaved agricultural water samples. The optimized assay was capable of a minimum quantification limit of 10 cells/ml of E. coli in the spiked agricultural water samples. A total of 121 surface water samples from three agricultural watersheds across Canada were analyzed, and results were compared with conventional culture-based enumerations of E. coli. The Q-PCR assay revealed significantly higher numbers of E. coli in water samples than the culture-based assay in each agricultural watershed. The new Q-PCR assay can facilitate the quantification of E. coli in a single water sample in < 3 h, including melt curve analysis, across a range of agricultural water quality conditions.

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

The aim of this study was to assess the efficiency of the embryonated egg model to recover Viable But Non Culturable (VBNC) cells of Listeria monocytogenes. L. monocytogenes cells were incubated in filtered sterilised distilled water. The VBNC state was obtained after a 25 to 47 days incubation period (concentration of culturable cells less than 1 cfu/mL). Fifteen days after the VBNC state was reached, non culturability was checked in various media. One milliliter of each VBNC suspension that contained 10(4) metabolically active cells (i.e. Direct Viable Count + cells) was inoculated into the vitellus fluid of embryonated and non-embryonated eggs. Culturable cells were detected in a large proportion of the embryonated eggs (18/32), but not in the non-embryonated eggs (1/32). The recovery rate was higher after culture of the vitellus fluid plus embryo (18/32) than after culture of the vitellus fluid alone (6/32). The results indicate that the embryo likely plays a prominent part in the recovery process. The virulence of recovered cells was assessed by the ability to form plaques in HT-29 cell monolayers and by the ability to colonise mouse spleens. Although the cells were classified as avirulent when in the VBNC state, the virulence was recovered after resuscitation.

Characterization and resuscitation of viable but nonculturable Vibrio alginolyticus VIB283

The aim of this study was to investigate the viable but nonculturable (VBNC) state of the bacterium. Vibrio alginolyticus VIB283 was cultured in sterilized seawater microcosm at 4 degrees C. Culturability of the cells in the microcosm was monitored by spread plate count (PC) on 2216E agar, PCs declined to undetectable levels (<0.1 CFU/ml) within 90 days. Total cell counts remained constant throughout the period as determined by acridine orange direct count (AODC). The direct viable counts, on the other hand, declined from 10(10) to 10(9) CFU/ml active cells and remained fairly constant at this level by direct viable count (DVC), which indicated that a large population of cells entered into the VBNC state. The VBNC cells could be resuscitated by temperature upshift with and without the presence of nutrition. The resuscitated time were 16 h and 8 days respectively. The resuscitation was not achieved in chick embryos. The morphology of the VBNC, normal and resuscitated cells was studied with scanning electron microscope and flow cytometry. The cells changed from rod or arc to coccoid and decreased in size when entered into the VBNC state. The resuscitated and the normal cells had almost no morphological differences.

Survival of Mycobacterium avium, Legionella pneumophila, Escherichia coli, and caliciviruses in drinking water-associated biofilms grown under high-shear turbulent flow

Most of the bacteria in drinking water distribution systems are associated with biofilms. In biofilms, their nutrient supply is better than in water, and biofilms can provide shelter against disinfection. We used a Propella biofilm reactor for studying the survival of Mycobacterium avium, Legionella pneumophila, Escherichia coli, and canine calicivirus (CaCV) (as a surrogate for human norovirus) in drinking water biofilms grown under high-shear turbulent-flow conditions. The numbers of M. avium and L. pneumophila were analyzed with both culture methods and with peptide nucleic acid fluorescence in situ hybridization (FISH) methods. Even though the numbers of pathogens in biofilms decreased during the experiments, M. avium and L. pneumophila survived in biofilms for more than 2 to 4 weeks in culturable forms. CaCV was detectable with a reverse transcription-PCR method in biofilms for more than 3 weeks. E. coli was detectable by culture for only 4 days in biofilms and 8 days in water, suggesting that it is a poor indicator of the presence of certain waterborne pathogens. With L. pneumophila and M. avium, culture methods underestimated the numbers of bacteria present compared to the FISH results. This study clearly proved that pathogenic bacteria entering water distribution systems can survive in biofilms for at least several weeks, even under conditions of high-shear turbulent flow, and may be a risk to water consumers. Also, considering the low number of virus particles needed to result in an infection, their extended survival in biofilms must be taken into account as a risk for the consumer.

Reactivation and growth of non-culturable indicator bacteria in anaerobically digested biosolids after centrifuge dewatering

Recent literature has reported that high concentrations of indicator bacteria such as fecal coliforms (FCs) were measured in anaerobically digested sludges immediately after dewatering even though low concentrations were measured prior to dewatering. This research hypothesized that the indicator bacteria can enter a non-culturable state during digestion, and are reactivated during centrifuge dewatering. Reactivation is defined as restoration of culturability. To examine this hypothesis, a quantitative polymerase chain reaction (qPCR) method was developed to enumerate Escherichia coli, a member of the FC group, during different phases of digestion and dewatering. For thermophilic digestion, the density of E. coli measured by qPCR could be five orders of magnitude greater than the density measured by standard culturing methods (SCMs), which is indicative of non-culturable bacteria. For mesophilic digestion, qPCR enumerated up to about one order of magnitude more E. coli than the SCMs. After centrifuge dewatering, the non-culturable organisms could be reactivated such that they are enumerated by SCMs, and the conditions in the cake allowed rapid growth of FCs and E. coli during cake storage.

Retention of virulence in a viable but nonculturable Edwardsiella tarda isolate

Edwardsiella tarda is pathogen of fish and other animals. The aim of this study was to investigate the viable but nonculturable (VBNC) state and virulence retention of this bacterium. Edwardsiella tarda CW7 was cultured in sterilized aged seawater at 4 degrees C. Total cell counts remained constant throughout the 28-day period by acridine orange direct counting, while plate counts declined to undetectable levels (<0.1 CFU/ml) within 28 days by plate counting. The direct viable counts, on the other hand, declined to ca. 10(9) CFU/ml active cells and remained fairly constant at this level by direct viable counting. These results indicated that a large population of cells existed in a viable but nonculturable state. VBNC E. tarda CW7 could resuscitate in experimental chick embryos and in the presence of nutrition with a temperature upshift. The resuscitative times were 6 days and 8 days, respectively. The morphological changes of VBNC, normal, and resuscitative E. tarda CW7 cells were studied with a scanning electron microscope. The results showed that when the cells entered into the VBNC state, they gradually changed in shape from short rods to coccoid and decreased in size, but the resuscitative cells did not show any obvious differences from the normal cells. The VBNC and the resuscitative E. tarda CW7 cells were intraperitoneally inoculated into turbot separately, and the fish inoculated with the resuscitative cells died within 7 days, which suggested that VBNC E. tarda CW7 might retain pathogenicity.

Persistence and growth of fecal Bacteroidales assessed by bromodeoxyuridine immunocapture

Extraintestinal growth of fecal bacteria can impair accurate assessment of watershed health. Anaerobic fecal bacteria belonging to the order Bacteroidales are attractive candidates for fecal source tracking because they have host-specific distributions and do not grow well in the presence of high oxygen concentrations. Growth of general and human-specific fecal Bacteroidales marker organisms in environmental samples (sewage) and persistence of the corresponding genetic markers were investigated using bromodeoxyuridine (BrdU) DNA labeling and immunocapture, followed by PCR detection. Background amplification of unlabeled controls occasionally occurred when a high number of PCR cycles was used. By using fluorescent detection of PCR products obtained after 15 cycles, which was determined to be quantitative, we enriched for BrdU-labeled DNA and did not detect unlabeled DNA. By using pure cultures of Bacteroides vulgatus, the ability of Bacteroidales bacteria to take up and incorporate BrdU into nascent DNA was confirmed. Fecal Bacteroidales organisms took up and incorporated BrdU into DNA during growth. In sewage incubated aerobically at the in situ temperature, Bacteroidales genetic marker sequences persisted for at least 24 h and Bacteroidales fecal bacteria grew for up to 24 h as well. Detection by PCR using a low, quantitative cycle number decreased the sensitivity of the assay such that we were unable to detect fecal Bacteroidales human-specific marker sequences in unlabeled or BrdU-labeled fractions, even when fluorescent detection was used. Using 30 PCR cycles with unlabeled fractions, human-specific Bacteroidales sequences were detected, and they persisted for up to 24 h in sewage. These data support the utility of BrdU labeling and immunocapture followed by length heterogeneity PCR or fluorescent detection using low numbers of PCR cycles. However, this method may not be sensitive enough to identify cells that are present at low densities in aquatic environments.

Virulence of viable but nonculturable S. Typhimurium LT2 after peracetic acid treatment

S. Typhimurium LT2 cells suspended in sterilized sewage effluent water (SEW) and in distilled water microcosms were exposed to 0, 7, 15 and 20 mg/l peracetic acid, and tested for viability and virulence. After treatment for one hour, colony forming units decreased by at least 5 log units at peracetic acid concentration of 7 mg/l. In SEW, at peracetic acid concentration of 15 mg/l, the cells were nonculturable (VNC), but retained virulence as demonstrated by invasion assays of HeLa cells. Higher concentrations (greater than or equal to 20 mg/l) resulted in bacterial death, i.e. substrate non-responsive cells. Despite morphological alterations of the bacteria after peracetic acid treatment, visualized by transmission electronic microscopy, conservation of both adhesive and invasive capacities was confirmed by scanning electron microscopy after exposure to 0-15 mg/l peracetic acid. Public health professionals need to recognize that peracetic acid-treated Salmonella is capable of modifying its physiological characteristics, including entering and recovering from the viable but nonculturable state, and may remain virulent after a stay in SEW followed by peracetic acid treatment.

Survival of Vibrio fluvialis in seawater under starvation conditions

The viability of Vibrio fluvialis in seawater microcosms, with and without sediment was investigated. The strain survived as culturable bacteria for at least 1 year and the expression of its virulence factors was maintained. In microcosms containing sediment Vibrio fluvialis was more stable. Viable but nonculturable (VBNC) cells of Vibrio fluvialis were able to resuscitate to the culturable state up to 6 years of incubation in marine sediment. These cells recuperate their initial biochemical characteristics after 3 months of incubation in marine broth. Amplified 16S ribosomal DNA (rDNA) restriction analysis (ARDRA) was used to confirm that it is the same strain of Vibrio fluvialis which resists in all microcosms during a long period of time.

Resuscitation of the viable but non-culturable state of Salmonella enterica serovar Oranienburg by recombinant resuscitation-promoting factor derived from Salmonella Typhimurium strain LT2

A gene encoding the resuscitation-promoting factor (Rpf) from Salmonella Typhimurium LT2 was cloned and characterized. The amino acid sequence encoded by S. Typhimurium LT2 rpf gene shares 24.2% homology with Micrococcus luteus Rpf, which is secreted by growing cells, and required to resuscitate from viable but non-culturable (VNC) state. The S. Typhimurium LT2 rpf gene is 696 bp long, and shared a conserved segment with Salmonella enterica serovar Oranienburg (99.4%). Recombinant Rpf (rRpf) proteins of S. Typhimurium LT2 after expression in E. coli BL21 harboring the pET15-b plasmid was approximately 25 kDa. Since S. Oranienburg cells are relatively quick to enter the VNC state just after incubating in the presence of 7% NaCl at 37 degrees C for 3 days, we evaluated the biological effect of rRpf by using S. Oranienburg VNC cells. The rRpf not only promoted proliferation but also induced resuscitation of VNC cells to the culturable state in a dose-dependent manner. Therefore, rRpf may be useful for detection of bacterial contaminants present in the VNC form in food samples and the environment.

Fluorescent Antibody‐Viability Staining and β‐Glucuronidase Assay as Rapid Methods for MonitoringEscherichia coliViability in Coastal Marine Waters

A faecal pollution monitoring of coastal Messina waters was performed by comparing three (microscopic, enzyme, and culture) methods. Evidence of Escherichia coli cells (29.99 to 96.79% of the total enteropathogenic serotypes) retaining their viability into the marine environment was shown. beta-Glucuronidase activity rates suggested that living cells were also metabolically active. Heavily polluted sites were detected, where improperly treated urban wastes were discharged. Significant relationships between microscopic and enzymatic data proved both methods to be suitable alternatives to the culture method for E. coli detection, improving environmental quality assessment.

Occurrence and growth characteristics ofEscherichia coliand enterococci within the accumulated fluid of the northern pitcher plant (Sarracenia purpureaL.)

Sarracenia purpurea L., a carnivorous bog plant (also known as the pitcher plant), represents an excellent model of a well-defined, self-contained ecosystem; the individual pitchers of the plant serve as a microhabitat for a variety of micro- and macro-organisms. Previously, fecal indicator bacteria (Escherichia coli and enterococci) were shown as incidental contaminants in pitcher fluid; however, whether their occurrence in pitcher fluid is incidental or common has not been established. The purpose of this study was to investigate the occurrence, distribution, and growth potential of E. coli and enterococci in pitcher plant fluid from a protected bog in northwest Indiana. Escherichia coli and enterococci were recovered in pitcher fluids (n = 43 plants), with mean densities (log CFU mL–1) of 1.28 ± 0.23 and 1.97 ± 0.27, respectively. In vitro experiments showed that E. coli growth in fluid not containing insects or indigenous organisms was directly proportional to the fluid concentration (growth was 10-fold in 24 h in 100% fluid); however, in the presence of other indigenous organisms, E. coli and enterococci were only sustained for 5 days at 26 °C. Pulsed-field gel electrophoresis (PFGE) analysis showed that the plant Enterococcus faecalis isolates were genetically distinct from the human isolates; identical PFGE patterns were observed among plant isolates that fell into one of six clonal groups. These findings suggest that (i) E. coli and enterococci occurrence in pitcher plants is rather common in the bog studied, although their originating source is unclear, and (ii) the pitcher fluid contains adequate nutrients, especially carbon and energy sources, to promote the growth of indicator bacteria; however, under natural conditions, the biotic factors (e.g., competition for nutrients) may restrict their growth.Key words: indicator bacteria, environmental occurrence, microcosm, pitcher plant fluid, temperate bog.

Monitoring of active but non-culturable bacterial cells by flow cytometry

Flow cytometric signatures (i.e., light scatter, red and green fluorescence) were obtained for the active but non-culturable (ABNC) cells of E. coli and a coliform isolate H03N1, in seawater microcosms using BacLight, a live-dead assay kit from Molecular Probes (Eugene/Portland, OR). Previous studies have reported that there are two major adaptations, which cells undergo during the formation of ABNC states: cell wall toughening and DNA condensation. Therefore, we hypothesized that the matured ABNC forms should be more resistant to extreme temperature treatments (i.e., by freezing in liquid nitrogen and thawing at room temperature) than the normal and transition populations. It was shown that the membrane-compromised cells (comprising of normal wild-type and dead cells which are less resistant to rapid freeze thaw) could be differentiated from the matured ABNC using BacLight staining and fluorescence detection by flow cytometry. The population of ABNC cells, which could not be cultured using m-FC media (for the enumeration of fecal coliforms), was resuscitated in phosphate buffer saline followed by growth in Luria broth. Flow cytometry was thus able to detect and differentiate the ABNC cells against a mixed population comprising of culturable cells, transition populations, and dead cells. The results also showed that the formation of ABNC is as early as 2 days in seawater microcosms. By directly comparing the coliform levels enumerated by the BacLight based flow cytometry assays and m-FC technique, it was shown that the presence of coliforms can be undetected by the membrane filtration method.

Detection of Salmonella in environmental water and sediment by a nested-multiplex polymerase chain reaction assay

From 1995 to 2002, 53 serovars of Salmonella were isolated in the Seine estuary (France). The 3 serovars most frequently found were S. enterica serovar Typhimurium, S. enterica serovar Infantis and S. enterica serovar Virchow. A nested multiplex PCR (nm-PCR) assay was developed to detect the presence of Salmonella in estuarine water and sediment samples. The target gene used was the phase 1 flagellin fliC chromosomal gene, present in all Salmonella serovars. A set of 4 primers was first used to amplify an 890-bp sequence of the fliC gene, and then a second set of 3 primers was used for the nested PCR. The nmPCR method has been successfully tested for 28 serovars, 13 of which are of epidemiological significance. The detection limit of the assay, without any pre-enrichment step, was estimated at 1 CFU in deionized water, and at 4-5 CFU in the reaction mixture when tested on estuarine water seeded with a Salmonella strain. When the nmPCR was used together with the classical culture method in environmental samples, it gave additional positive results for 11.3% of the sediment samples and 20% of the water samples despite a high background of other bacteria. Overall, the results demonstrated that this molecular approach informed us about the contamination by Salmonella of estuarine water and sediment samples. Positive amplifications suggested the presence of Salmonella DNA and could thus provide information about a recent (culturable) or past (non-culturable, released DNA) contamination of environmental samples by this pathogenic bacteria.

Biochemical and virulence characterization of viable but nonculturable cells of Vibrio parahaemolyticus

Vibrio parahaemolyticus is a common foodborne pathogen frequently causing outbreaks in summer. Maintenance of virulence by the viable but nonculturable (VBNC) state of this pathogen would allow its threat to human health to persist. This study reports on the change in virulence and concomitant changes in activity of two enzymes and fatty acid profiles when V. parahaemolyticus ST550 entered the VBNC state in the modified Morita mineral salt-0.5% NaCl medium incubated at 4 degrees C. The major change in fatty acid composition occurred in the first week, with a rapid increase in C15:0 fatty acid and saturated/unsaturated ratio while a rapid decrease in C16:1 was observed. The activity level of the inducible protective enzyme superoxide dismutase became undetectable in the VBNC state, whereas that of constitutive glucose-6-phosphate dehydrogenase did not change in either the exponential phase or the VBNC state. Cytotoxicity against HEp-2 cells and a suckling mouse assay showed that virulence was lowered in the VBNC state compared with exponential-phase cells. Longer incubation times were required by the VBNC cells to achieve the same level of virulence as seen in exponential-phase cells. Culturable cells were recovered on selective agar medium from the VBNC cultures injected into suckling mice, probably as the result of in vivo resuscitation. Results of this study add to our understanding of the biochemical and physiological changes that have not been reported when V. parahaemolyticus enters into the VBNC state.

Solar and temporal effects on Escherichia coli concentration at a Lake Michigan swimming beach

Studies on solar inactivation of Escherichia coli in freshwater and in situ have been limited. At 63rd St. Beach, Chicago, Ill., factors influencing the daily periodicity of culturable E. coli, particularly insolation, were examined. Water samples for E. coli analysis were collected twice daily between April and September 2000 three times a week along five transects in two depths of water. Hydrometeorological conditions were continuously logged: UV radiation, total insolation, wind speed and direction, wave height, and relative lake level. On 10 days, transects were sampled hourly from 0700 to 1500 h. The effect of sunlight on E. coli inactivation was evaluated with dark and transparent in situ mesocosms and ambient lake water. For the study, the number of E. coli samples collected (n) was 2,676. During sunny days, E. coli counts decreased exponentially with day length and exposure to insolation, but on cloudy days, E. coli inactivation was diminished; the E. coli decay rate was strongly influenced by initial concentration. In situ experiments confirmed that insolation primarily inactivated E. coli; UV radiation only marginally affected E. coli concentration. The relationship between insolation and E. coli density is complicated by relative lake level, wave height, and turbidity, all of which are often products of wind vector. Continuous importation and nighttime replenishment of E. coli were evident. These findings (i) suggest that solar inactivation is an important mechanism for natural reduction of indicator bacteria in large freshwater bodies and (ii) have implications for management strategies of nontidal waters and the use of E. coli as an indicator organism.

Maintenance of pathogenicity during entry into and resuscitation from viable but nonculturable state in Aeromonas hydrophila exposed to natural seawater at low temperature

AIMS

To investigate the fate of Aeromonas hydrophila pathogenicity when cells switch, in nutrient-poor filtered sterilized seawater, between the culturable and nonculturable state.

METHODS AND RESULTS

Aeromonas hydrophila ATCC 7966, rendered non culturable within 50-55 days of exposure to marine stress conditions, was tested for its ability to maintain haemolysin and to adhere to McCoy cells. Results showed that pathogenicity was lost concomitantly with culturability, whereas cell viability remained undamaged, as determined by the Kogure cell elongation test. However, this loss is only temporary because, following temperature shift from 5 to 23 degrees C, multiple biological activities of recovered Aer. hydrophila cells, which include their ability to lyse human erythrocytes and to attach and destroy McCoy cells were regained. During the temperature-induced resuscitation, constant total cell counts were observed. Moreover, no significant improvement in recovery yield was obtained on brain-heart infusion (BHI) agar plates amended with catalase. We suggest that in addition to the growth of the few undetected culturable cells, there is repair and growth of some mildly injured viable but nonculturable cells.

CONCLUSIONS

The possibility that nonculturable cells of normally culturable Aer. hydrophila in natural marine environment may constitute a source of infectious diseases posing a public health problem was demonstrated.

SIGNIFICANCE AND IMPACT OF THE STUDY

These experiments may mimic what happens when Aer. hydrophila cells are released in natural seawater with careful attention to the conditions in which surrounding waters gradually become warmer in late summer/early autumn.

Enumeration of viable E. coli in rivers and wastewaters by fluorescent in situ hybridization

A combination of direct viable count (DVC) and fluorescent in situ hybridization (FISH) procedures was used to enumerate viable Escherichia coli in river waters and wastewaters. A probe specific for the 16S rRNA of E. coli labeled with the CY3 dye was used; enumeration of hybridized cells was performed by epifluorescence microscopy. Data showed that the method was able to accurately enumerate a minimum of 3000 viable E. coli among a large number of non-fecal bacteria. When applied to river water and wastewater samples, the DVC-FISH method gave systematically higher E. coli counts than a reference culture-based method (miniaturized MPN method). The ratio between both counts (DVC-FISH/MPN) increased with decreasing abundance of culturable E. coli indicating that the proportion of viable but non-culturable (VBNC) E. coli (detectable by the DVC-FISH procedure and not by a culture-based method) was higher in low contaminated environments. We hypothesized that the more stressing conditions, i.e. nutritional stress and sunlight effect, met in low contaminated environments were responsible for the larger fraction of VBNC E. coli. A survival experiment, in which sterile mineral water was inoculated with a pure E. coli strain and incubated, confirmed that stressing conditions induced the apparition of non-culturable E. coli detectable by the DVC-FISH procedure. The analysis of the E. coli concentration along a Seine river longitudinal profile downstream a large input of fecal bacteria by a WWTP outfall showed an increasing fraction of VBNC E. coli with increasing residence time of the E. coli in the river after release. These data suggest that the DVC-FISH method is useful tool to analyze the dynamics of fecal bacteria in river water.

Viable but nonculturable bacteria are present in mouse and human urine specimens

The presence of viable but nonculturable bacteria in human clean-catch and mouse bladder-isolated urine specimens was investigated. Viable but nonculturable bacteria are alive but do not give rise to visible growth under nonselective growth conditions. Urine specimens obtained from human female volunteers with or without an active urinary tract infection were found to contain, on average, significantly more viable than culturable forms of bacteria. Additional support for the presence of viable but nonculturable cells in urine specimens considered sterile was obtained from examination of urine specimens obtained directly from the bladder of healthy mice. Because the viability assay used to study the viable but nonculturable condition is by necessity growth independent, and hence indirect, the accuracy of this assay that scores cells with intact cell membranes as being viable was studied. Greater than 95% of Escherichia coli cells exposed to lethal doses of UV irradiation were found to lose their membrane integrity within a day, a time frame similar to that used to examine urine specimens. These data suggest that viable but nonculturable cells can occur within regions of the urinary tract previously considered sterile.

Detection of Salmonella Senftenberg associated with high saline environments in mussel processing facilities

A contamination by Salmonella Senftenberg in frozen mussels was detected in 1998 during a routine analytical surveillance. From June 1998 to December 2001, a total of 3,410 samples of steamed frozen mussels and items related to their manufacture were analyzed for the presence of Salmonella. Salmonella Senftenberg was isolated in 573 (16.8%) samples, and no other serovar was detected. The contamination episodes extended for several months. Salmonella Senftenberg colonies from the first contamination events showed a rugose morphology on agar with a shiny crystalline layer and limited colony formation on microbiological media. These contaminations were mainly associated with brine (300 g of NaCl per liter), while the live molluscs that were being processed were free of Salmonella. When the brine contaminations were nearly controlled, new episodes were detected that were associated with live mussels. In the new episodes, colonies showed the typical characteristics of Salmonella and normal growth on agar. Salmonella Senftenberg presented a high resistance to unfavorable environments and showed a preference for clean environments. While Salmonella Senftenberg could be isolated from mussels after steam treatment, it could not survive after immersion in water at 80 degrees C for 1 min. This fact was used to develop a process to remove contamination from products, minimizing the health risk associated with frozen mussel consumption. The general incidence of Salmonella Senftenberg in facilities and mussels was reduced from 31.2% in 1998 to 2.5% in 2001. During this study, no cases of illness from consumption of frozen mussels were reported, indicating a possible lack of virulence of Salmonella Senftenberg in these contamination events.

Retention of virulence in viable but non-culturable halophilic Vibrio spp

The viable but non-culturable (VBNC) forms of two environmental strains of Vibrio alginolyticus 1 and Vibrio parahaemolyticus 66 and one strain of V. parahaemolyticus ATCC 43996 showing virulence characteristics (hemolysin production, adhesive and/or cytotoxic ability, in vivo enteropathogenicity) were obtained by culturing bacteria in a microcosm consisting of artificial sea water (ASW) and incubating at 5 degrees C with shaking. Every 2 days, culturability of the cells in the microcosm was monitored by spread plates on BHI agar and total count and the percentage of viable cells were determined by double staining with DAPI and CTC. When cell growth was not detectable (<0.1 CFU/ml), the population was considered non-culturable and, then, the VBNC forms were resuscitated in a murine model. For each strain, eight male Balb/C mice were intragastrically inoculated with 0.1 ml of concentrated ASW bacterial culture. Two mice from each group were sacrificed at 2, 4, 8, and 12 days after challenge for autopsy and re-isolation of the microorganisms from the intestinal tissue cultures. Isolation was obtained in 25% of the animals challenged with the VBNC V. alginolyticus strain, in 37.5% of those challenged with the VBNC V. parahaemolyticus strain of environmental origin and in 50% of the animals infected with VBNC V. parahaemolyticus ATCC 43996. The strains thus isolated were again subjected to biological assays to determine the retention of pathogenicity. The virulence characteristics that seemed to disappear after resuscitation in the mouse were subsequently reactivated by means of two consecutive passages of the strains in the rat ileal loop model. The results obtained indicate that VBNC forms of the strains examined can be resuscitated and retain their virulence properties.

Combined fluorescent antibody assay and viability staining for the assessment of the physiological states of Escherichia coli in seawaters

AIMS

A comparison of methods that combine the use of immune sera with specific fluorescent probes for testing viability at single cell level was performed in order to estimate different living attributes of Escherichia coli in natural seawater samples.

METHODS AND RESULTS

Cell culturability was assayed by plate method, respiratory activity and membrane integrity were determined by an indirect fluorescent antibody assay, combined with 5-cyano-2, 3 ditolyl tetrazolium chloride and propidium iodide, respectively. Results showed the coexistence of different physiological states within the E. coli population, of which a large fraction (46%) of cells was actively respiring.

CONCLUSIONS

The methodological approach used offer interesting perspectives in water pollution monitoring, particularly when the differentiation between dead and living E. coli cells is required for a more precise assessment of the bacteriological quality of seawaters.

SIGNIFICANCE AND IMPACT OF THE STUDY

The study suggests the importance of knowledge of the viability status of faecal bacteria in aquatic environments as a fundamental issue for the preservation of public health; the availability of rapid analytical procedures for this purpose may find significant applications in the evaluation of the sanitary risk consequent to water use.

A stable bioluminescent construct of Escherichia coli O157:H7 for hazard assessments of long-term survival in the environment

A chromosomally lux-marked (Tn5 luxCDABE) strain of nontoxigenic Escherichia coli O157:H7 was constructed by transposon mutagenesis and shown to have retained the O157, H7, and intimin phenotypes. The survival characteristics of this strain in the experiments performed (soil at -5, -100, and -1,500 kPa matric potential and artificial groundwater) were indistinguishable from the wild-type strain. Evaluation of potential luminescence was found to be a rapid, cheap, and quantitative measure of viable E. coli O157:H7 Tn5 luxCDABE populations in environmental samples. In the survival studies, bioluminescence of the starved populations of E. coli O157:H7 Tn5 luxCDABE could be reactivated to the original levels of light emission, suggesting that these populations remain viable and potentially infective to humans. The attributes of the construct offer a cheap and low-risk substitute to the use of verocytotoxin-producing E. coli O157:H7 in long-term survival studies.

Immunofluorescence detection of Escherichia coli in seawater: a comparison of various commercial antisera

Through a microscopical method, relying on the interaction between fluorescent antibodies and target antigen, it is possible to detect and enumerate Escherichia coli in seawaters. Various commercial monoclonal and polyclonal antisera have been tested in an indirect immunofluorescence (IIF) assay developed for microbiological monitoring of coastal waters. Prior to use, they have been titrated and screened for cross-reactions with a collection of clinical and environmental isolates. A comparison among counts obtained on field samples showed higher performance for microscopical than for plate methods, due to the ability of all antisera to label target cells specifically, regardless of their viability. Because of their different specificities, polyclonal antisera yielded better quantitative results than monoclonal antisera. The study further suggested the usefulness of the immunofluorescence assay as a rapid alternative analytical tool for the specific detection of bacterial pathogens in aquatic environments.

Inactivation of coliform bacteria in Black Sea waters due to solar radiation

The effects of solar radiation and temperature on bacterial die-off rates in Black Sea coastal waters using total coliform as the indicator organism were studied. Coliform die-off experiments were carried out in seawater samples collected along the coastline. The experiments were conducted in beakers filled with seawater that were kept at constant temperatures and exposed to solar radiation. The membrane filter technique was used for the coliform analysis. Temperature ranging between 9 and 26 degrees C and solar radiation between 20 and 60 cal/cm(2) h were tested. Experiments in the dark were also conducted to isolate the effect of solar radiation from the other factors and, furthermore, to determine the effect of temperature on bacterial die-off. The solar radiation was found to be the most significant factor affecting the mortality of coliform bacteria.

Flow cytometry and conventional enumeration of microorganisms in ships' ballast water and marine samples

Conventional methods for bacteriological testing of water quality take long periods of time to complete. This makes them inappropriate for a shipping industry that is attempting to comply with the International Maritime Organization's anticipated regulations for ballast water discharge. Flow cytometry for the analysis of marine and ship's ballast water is a comparatively fast and accurate method. Compared to a 5% standard error for flow cytometry analysis the standard methods of culturing and epifluorescence analysis have errors of 2-58% and 10-30%, respectively. Also, unlike culturing methods, flow cytometry is capable of detecting both non-viable and viable but non-culturable microorganisms which can still pose health risks. The great variability in both cell concentrations and microbial content for the samples tested is an indication of the difficulties facing microbial monitoring programmes. The concentration of microorganisms in the ballast tank was generally lower than in local seawater. The proportion of aerobic, microaerophilic, and facultative anaerobic microorganisms present appeared to be influenced by conditions in the ballast tank. The gradual creation of anaerobic conditions in a ballast tank could lead to the accumulation of facultative anaerobic microorganisms, which might represent a potential source of pathogenic species.

Effect of temperature, salinity and nutrient content on the survival responses of Vibrio splendidus biotype I

The aim of this study was to evaluate the survival responses of two strains of Vibrio splendidus, both in natural and in defined media. For this purpose, freshwater and defined media containing different salinities (3.3-0.9 %) and nutrient concentrations (17-0.005 mg x l(-1)) were assayed. The incubation temperatures were established at 4, 10 and 22 degrees C. The acridine orange staining technique was used for total cell enumeration and the number of viable cells was determined using two direct assays, nalidixic acid and tetrazolium salt reduction and plate spreading. Resuscitation assays of viable but non-culturable (VBNC) cells were conducted. According to the counting procedures employed, at least four different subpopulations were found: (i). active (positive response in both nalidixic acid and tetrazolium assays) culturable cells; (ii). active non-culturable cells; (iii). tetrazolium-salt-responsive non-culturable cells and (iv). non-active (responsive to none of the direct viable assays) non-culturable cells. Long-term survival was found at salinities and nutrient concentrations of seawater environments (3.3 % and 5 mg x l(-1) or 1 g l(-1)), whereas the strains entered a VBNC state in freshwater and in brackish (0.9 or 1.6 % salinities) or high nutrient content (17 g x l(-1)) defined medium. The recovery of VBNC cells was not achieved.

Rapid and sensitive enumeration of viable diluted cells of members of the family enterobacteriaceae in freshwater and drinking water

Water quality assessment involves the specific, sensitive, and rapid detection of bacterial indicators and pathogens in water samples, including viable but nonculturable (VBNC) cells. This work evaluates the specificity and sensitivity of a new method which combines a fluorescent in situ hybridization (FISH) approach with a physiological assay (direct viable count [DVC]) for the direct enumeration, at the single-cell level, of highly diluted viable cells of members of the family Enterobacteriaceae in freshwater and drinking water after membrane filtration. The approach (DVC-FISH) uses a new direct detection device, the laser scanning cytometer (Scan RDI). Combining the DVC-FISH method on a membrane with Scan RDI detection makes it possible to detect as few as one targeted cell in approximately 10(8) nontargeted cells spread over the membrane. The ability of this new approach to detect and enumerate VBNC enterobacterial cells in freshwater and drinking water distribution systems was investigated and is discussed.