Survival of cells and DNA of Aeromonas salmonicida released into aquatic microcosms

Microbial Diversity in Groundwater and Its Response to Seawater Intrusion in Beihai City, Southern China

Seawater intrusion is a major concern commonly found in coastal aquifers worldwide. Because of the intense aquifer exploitation and land-based marine aquaculture in the coastal area of Beihai City, Guangxi Zhuang Autonomous Region, China, numerous underground aquifers in this area have been affected by seawater intrusion. However, the microbial communities in freshwater aquifers and their response to seawater intrusion are still unclear. In this study, groundwater from three aquifers was collected from three monitoring sites at different distances from the coastline in the coastal area of Beihai City, and the hydrochemical characteristics of these groundwater samples and the structure of the associated microbial communities were analyzed. The Cl- concentration of the samples indicated that seawater intrusion had occurred in the research area up to 1.5 km away from the coastline, but the monitoring site 2 km away from the coastline had yet to be affected. Statistical analysis showed that the bacterial communities in different groundwater aquifers were significantly correlated with the Cl- concentration, thereby suggesting that the extent of seawater intrusion might be one of the primary factors shaping bacterial composition in groundwater of this area, but the composition and distribution of archaea did not show a significant response to seawater intrusion and presented no apparent correlation with the Cl- concentration. α-, γ-Proteobacteria and Bacteroidota were the dominant bacterial lineages, accounting for about 58-95% of the bacterial communities. Meanwhile, the predominant archaeal taxa were mainly composed of Crenarchaeota, Nanoarchaeota, and Thermoplasmatota, as accounting for 83-100%. Moreover, there was significant spatial heterogeneity of microbial communities in the aquifers affected by varying degrees of seawater intrusion. The microbial communities inhabiting the unconfined aquifer were influenced by the geochemical fluctuation caused by seawater infiltration from land-based marine aquaculture ponds and the diffusion of eutrophic surface water. In contrast, changes in microbial community structure in the confined aquifers were closely related to the environmental gradient caused by different degrees of seawater intrusion. In addition, we also found that the tidal cycle did not significantly affect the structure of microbial communities inhabiting confined aquifers that had been long affected by seawater intrusion.

Abundance, diversity, and host assignment of total, intracellular, and extracellular antibiotic resistance genes in riverbed sediments

Human health risk assessment for environmental antibiotic resistant microbes requires not only quantifying the abundance of antibiotic resistance genes (ARGs) in environmental matrices, but also understanding their hosts and genetic context. Further, differentiating ARGs in intracellular and extracellular DNA (iDNA and eDNA) fractions may help refine our understanding of ARG transferability. The objectives of this study were to understand the (O1) abundance and diversity of extracellular, intracellular, and total ARGs along a land use gradient and (O2) impact of bioinformatics pipeline on the assignment of putative hosts for the ARGs observed in the different DNA fractions. Sediment samples were collected along a land use gradient in the Raritan River, New Jersey, USA. DNA was extracted to separate eDNA and iDNA and qPCR was performed for select ARGs and the 16S rRNA gene. Shotgun metagenomic sequencing was performed on DNA extracts for the different DNA fractions. ARG hosts were assigned via two different bioinformatic pipelines: network analysis of raw reads versus assembly. Results of the two pipelines were compared to evaluate their performance in terms of number and diversity of linkages and accuracy of in silico matrix spike host assignments. No differences were observed in the 16S rRNA gene normalized sul1 concentrations between the DNA fractions. The overall microbial community structure was more similar for iDNA and total DNA compared to eDNA and generally clustered by sampling site. ARGs associated with mobile genetic elements increased in iDNA for the downstream sites. Regarding host assignment, the raw reads pipeline via network analysis identified 247 ARG hosts as compared to 53 hosts identified by assembly pipeline. Other comparisons between the pipelines were made including ARG assignment to taxa containing waterborne pathogens and practical considerations regarding processing time.

A duplex real-time NASBA assay targeting serotype-specific gene for rapid detection of viable S. enterica serovar Paratyphi C in retail foods of animal origin

Salmonella enterica serovars Paratyphi C is highly adapted to humans and can cause a typhoid-like disease with high mortality rates. In this study, three serovar-specific genes were determined for S. Paratyphi C, SPC_0871，SPC_0872, and SPC_0908, by comparative genomics method. Based on SPC_0908 and xcd gene for testing Salmonella spp., we have developed a duplex real-time nucleic acid sequence-based amplification (real-time NASBA) with molecular beacon approach for simultaneous detection of viable cells of Salmonella spp. and serotype Paratyphi C. The test selectively and consistently detected 53 Salmonella spp. (representing 31 serotypes) and 18 non-Salmonella strains. Additionally, the method showed high resistance to interference by natural background flora in pork and chicken samples. The sensitivity of the established approach was determined to be 4.89 CFU/25 g in artificially contaminated pork and chicken samples after pre-enrichment. We propose this NASBA-based protocol as a potential detection method for Salmonella spp. and serotype Paratyphi C in food of animal origin.

Extracellular DNA in environmental samples: Occurrence, extraction, quantification, and impact on microbial biodiversity assessment

Environmental DNA, i.e., DNA directly extracted from environmental samples, has been applied to understand microbial communities in the environments and to monitor contemporary biodiversity in the conservation context. Environmental DNA often contains both intracellular DNA (iDNA) and extracellular DNA (eDNA). eDNA can persist in the environment and complicate environmental DNA sequencing-based analyses of microbial communities and biodiversity. Although several studies acknowledged the impact of eDNA on DNA-based profiling of environmental communities, eDNA is still being neglected or ignored in most studies dealing with environmental samples. In this article, we summarize key findings on eDNA in environmental samples and discuss the methods used to extract and quantify eDNA as well as the importance of eDNA on the interpretation of experimental results. We then suggest several factors to consider when designing experiments and analyzing data to negate or determine the contribution of eDNA to environmental DNA-based community analyses. This field of research will be driven forward by: (i) carefully designing environmental DNA extraction pipelines by taking into consideration technical details in methods for eDNA extraction/removal and membrane-based filtration and concentration; (ii) quantifying eDNA in extracted environmental DNA using multiple methods including qPCR and fluorescent DNA binding dyes; (iii) carefully interpretating effect of eDNA on DNA-based community analyses at different taxonomic levels; and (iv) when possible, removing eDNA from environmental samples for DNA-based community analyses.

Assessment of the Effect of Thermotherapy on 'Candidatus Liberibacter asiaticus' Viability in Woody Tissue of Citrus via Graft-Based Assays and RNA Assays

In 2019, citrus production in Florida declined by more than 70%, mostly because of Huanglongbing (HLB), which is caused by the bacterium 'Candidatus Liberibacter asiaticus' (CLas). Thermotherapy for HLB-affected trees was proposed as a short-term management solution to maintain field productivity. It was hypothesized that thermotherapy could eliminate HLB from affected branches; therefore, the study objectives were to show which time-temperature combinations eliminated CLas from woody tissues. Hardening, rounded Valencia twigs collected from HLB-affected field trees were treated in a steam chamber at different time-temperature combinations (50°C for 60 s; 55°C for 0, 30, 60, 90, and 120 s; 60°C for 30 s; and an untreated control). Three independent repetitions of 13 branches per treatment were grafted onto healthy rootstocks and tested to detect CLas after 6, 9, and 12 months. For the RNA-based CLas viability assay, three branches per treatment were treated and bark samples were peeled for RNA extraction and subsequent gene expression analyses. During the grafting study, at 12 months after grafting, a very low frequency of trees grafted with twigs treated at 55°C for 90 s and 55°C for 120 s had detectable CLas DNA. In the few individuals with CLas, titers were significantly lower (P ≤ 0.0001) and could have been remnants of degrading DNA. Additionally, there was a significant decrease (P ≤ 0.0001) in CLas 16S rRNA expression at 55°C for 90 s, 55°C for 120 s, and 60°C for 30 s (3.4-fold change, 3.4-fold change, and 2.3-fold change, respectively) in samples 5 days after treatment. Heat injury, not total CLas kill, could explain the limited changes in transcriptional activity; however, failed recovery and eventual death of CLas resulted in no CLas detection in most of the grafted trees treated with the highest temperatures or longest durations.

Differential decay and prediction of persistence of Enterococcus spp. and Escherichia coli culturable cells and molecular markers in freshwater and seawater environments

To quantify the impact of fecal pollution on the microbiological bathing water quality, predictive modeling is being increasingly used in which the decay rate of the fecal indicators plays an important role. The decay of sewage-sourced enterococci and Escherichia coli culturable cells and their associated molecular markers (16SrRNA) quantified by Quantitative Reverse transcription PCR were measured in controlled microcosms as well in in situ conditions using different water types, from marine waters to fresh waters with intermediate salinity. All bacterial decays were fitted to a first order decay model. In the laboratory study, the light radiation was the most influent factor affecting E. coli and enterococci survival by culture methods although environmental conditions weakly impacted the decay of molecular markers. The results also indicated differential persistence of genetic markers and culturable organisms of fecal indicator bacteria in different water systems. For each bacteria indicator and analytical method, four equations were obtained to predict the time required to have a 90% reduction (T90) according irradiance, salinity and temperature parameters. The weighted model RMSE (Root Mean Square Error) calculated for all field experiments showed that quantification obtained with the equations defined by laboratory-based study compared reasonably well with in-situ observed quantification (0.4 and 0.2 log by standard culture methods for E. coli and Enterococcus spp. and 0.6 and 0.3 log by RT-qPCR for E. coli and Enterococcus spp. respectively). The modeling tool can be used to predict the presence of fecal pollution in marine and fresh waters in combination with either culture based- or rapid molecular methods.

Development of a real-time nucleic acid sequence-based amplification assay for the rapid detection of Salmonella spp. from food

Salmonella spp. is one of the most common foodborne infectious pathogen. This study aimed to develop a real-time nucleic acid sequence-based amplification (NASBA) assay for detecting Salmonella in foods. Primers and a molecular beacon targeting the Salmonella-specific xcd gene were designed for mRNA transcription, and 48 Salmonella and 18 non-Salmonella strains were examined. The assay showed a high specificity and low detection limit for Salmonella (7 × 10^-1 CFU/mL) after 12 h of pre-enrichment. Importantly, it could detect viable cells. Additionally, the efficacy of the NASBA assay was examined in the presence of pork background microbiota; it could detect Salmonella cells at 9.5 × 10³ CFU/mL. Lastly, it was successfully used to detect Salmonella in pork, beef, and milk, and its detection limit was as low as 10 CFU/25 g (mL). The real-time NASBA assay developed in this study may be useful for rapid, specific, and sensitive detection of Salmonella in food of animal origin.

A novel immunotherapy of Brucellosis in cows monitored non invasively through a specific biomarker

Brucellosis is an important zoonotic disease causing huge economic losses worldwide. Currently no effective immunotherapy for Brucellosis or any biomarker to monitor the efficacy of therapy is available. Treatment is ineffective and animals remain carrier lifelong. S19 and RB51 are live attenuated vaccine strains of Brucella abortus. However, S19 induces only antibody, ineffective for intracellular pathogen. RB51 induces cell mediated immunity (CMI) but it is Rifampicin resistant. Both organisms are secreted in milk and can infect humans and cause abortions in animals. Phage lysed bacteria (lysates) retain maximum immunogenicity as opposed to killing by heat or chemicals. We report here the successful immunotherapy of bovine Brucellosis by phage lysates of RB51 (RL) and S19 (SL). The SL induced strong antibody response and RL stimulated CMI. In vitro restimulation of leukocytes from RL immunized cattle induced interferon gamma production. A single subcutaneous dose of 2 ml of cocktail lysate (both RL and SL), eliminated live virulent Brucella from Brucellosis affected cattle with plasma level of Brucella specific 223 bp amplicon undetectable by RT-PCR and blood negative for live Brucella by culture in 3 months post-immunization. This is the first report on minimally invasive monitoring of the efficacy of antibacterial therapy employing plasma RNA specific for live bacteria as a biomarker as well as on the use of RB51 phage lysate for successful immunotherapy of Brucellosis in cattle.

Survival behaviour and virulence of the fish pathogen Vibrio ordalii in seawater microcosms

Vibrio ordalii, the causative agent of atypical vibriosis, is a Gram-negative, motile, rod-shaped bacterium that severely affects the salmonid aquaculture industry. V. ordalii has been biochemically, antigenically and genetically characterized. However, studies on the survival behaviour of this bacterium in aquatic environments are scarce, and there is no information regarding its disease transmission and infectious abilities outside of the fish host or regarding water as a possible reservoir. The present study investigated the survival behaviour of V. ordalii Vo-LM-06 and Vo-LM-18 in sterile and non-sterile seawater microcosms. After a year in sterile seawater without nutrients, 1% of both V. ordalii strains survived (~10(3) colony-forming units ml(-1)), and long-term maintenance did not affect bacterial biochemical or genetic properties. Additionally, V. ordalii maintained for 60 d in sterile seawater remained infective in rainbow trout Oncorhynchus mykiss. However, after 2 d of natural seawater exposure, this bacterium became non-culturable, indicating that autochthonous microbiota may play an important role in survival. Recuperation assays that added fresh medium to non-sterile microcosms did not favour V. ordalii recovery on solid media. Our results contribute towards a better understanding of V. ordalii survival behaviour in seawater ecosystems.

Persistence and renaturation efficiency of thermally treated waste recombinant DNA in defined aquatic microcosms

To validate the possibility of horizontal gene transfer (HGT) from thermally denatured recombinant DNA discharged into the eco-system, a constructed plasmid was used to investigate the persistence and renaturation efficiency of thermally denatured recombinant DNA in defined aquatic microcosms. The results revealed that there was undecayed recombinant plasmid pMDLKJ material being discharged into the aquatic microcosms even after thermal treatment at either 100°C (using boiling water) or at 120°C (using an autoclave). The plasmid had a relatively long persistence time. At least 10(2) copies μL(-1) of a specific 245 bp fragment of the plasmid could be detected after 12 h and a specific 628 bp fragment could be detected up to 2 h. The thermally denatured recombinant DNA could efficiently renature and recover its functional double stranded structure in aquatic microcosms and the highest concentration of double-stranded DNA (dsDNA) occurred around 1 h after the thermally denatured DNA was added to the system. These results imply that when thermally treated recombinant DNAs are discharged into aquatic environments, they have enough time to renature and possibly transfer to other organisms. In addition, the recombinant DNA added to aquatic microcosms could be absorbed by the seston particles in water, such as mineral, organic and colloids particles with a maximum absorption value of about 5.18 ng L(-1). This absorbed DNA could persist longer in aquatic environments than free recombinant DNA, thus further favoring HGT.

Fate and Survival of Campylobacter coli in Swine Manure at Various Temperatures

Campylobacter coli is the most common Campylobacter species found in pig (95%), but the ability of this bacterium to survive in swine manure as well as the potential for causing human illness are poorly understood. We present here laboratory-scale experiments to investigate the effect of temperature on the survival of C. coli in spiked swine manure samples at temperatures from 4 to 52°C. The survival of C. coli during storage for 30 days was studied by three different methods: bacterial culture (plate counting), DNA qPCR, and mRNA RT-qPCR. The results indicate that C. coli could survive in swine manure up to 24 days at 4°C. At higher temperatures, this bacterium survived only 7 days (15°C) or 6 days (22°C) of storage. The survival of C. coli was extremely short (few hours) in samples incubated at 42 and 52°C. The results from the RT-qPCR method were consistent with the data from the bacterial culture method, indicating that it detected only viable C. coli cells, thus eliminating false-positive resulting from DNA from dead C. coli cells.

Differential decay of human faecal Bacteroides in marine and freshwater

Genetic markers from Bacteroides and other faecal bacteria are being tested for inclusion in regulations to quantify aquatic faecal contamination and estimate public health risk. For the method to be used quantitatively across environments, persistence and decay of markers must be understood. We measured concentrations of contaminant molecular markers targeting Enterococcus and Bacteroides spp. in marine and freshwater microcosms spiked with human sewage and exposed to either sunlight or dark treatments. We used Bayesian statistics with a delayed Chick-Watson model to estimate kinetic parameters for target decay. DNA- and RNA-based targets decayed at approximately the same rate. Molecular markers persisted (could be detected) longer in marine water. Sunlight increased the decay rates of cultured indicators more than those of molecular markers; sunlight also limited persistence of molecular markers. Within each treatment, Bacteroides markers had similar decay profiles, but some Bacteroides markers significantly differed in decay rates. The role of extracellular DNA in persistence appeared unimportant in the microcosms. Because conditions were controlled, microcosms allowed the effects of specific environmental variables on marker persistence and decay to be measured. While marker decay profiles in more complex environments would be expected to vary from those observed here, the differences we measured suggest that water matrix is an important factor affecting quantitative source tracking and microbial risk assessment applications.

Nucleic acids as viability markers for bacteria detection using molecular tools

A large set of nucleic acid detection methods with good sensitivity and specificity are now available for the detection of pathogens in clinical, food and environmental samples. Given increasing demand, many efforts have been made to combine these methods to assess viability. Genomic DNA PCR amplification has been shown to be inappropriate for distinguishing viable from dead bacteria owing to DNA stability. Many authors have tried to bypass this difficulty by switching to RNA amplification methods such as reverse transcription-PCR and nucleic acid sequence-based amplification. More recently, researchers have developed methods combining specific sample pretreatment with nucleic acid detection methods, notably ethidium or propidium monoazide pretreatment coupled with PCR DNA detection or direct viable count methods and subsequent fluorescent in situ hybridization of 16S rRNA. This review evaluates the performance of these different methods for viability assessment.

The use of multiplex PCR to detect and differentiate food- and beverage-associated microorganisms: a review

Regarding food safety, rapid detection of microbial species is crucial to develop effective preventive and/or adjustment measures. Classical methods for determining the presence of certain species are time-consuming and labor-intensive, hence, molecular methods, which offer speed, sensitivity and specificity, have been developed to address this problem. Multiplex PCR (MPCR) is widely applied in the various fields of microbiology for the rapid differentiation of microbial species without compromising accuracy. This paper describes the method and reports on the state-of-the-art application of this technique to the identification of microorganisms vehiculated with foods and beverages. The identification of both pathogens and probiotics and the species important for food fermentation or deterioration will be discussed. Applications of MPCR in combination with other techniques are also reviewed. Potentials, pitfalls, limitations and future prospects are summarised.

Quantification of toxic Microcystis spp. during the 2003 and 2004 blooms in western Lake Erie using quantitative real-time PCR

In August of 2003 and August of 2004, blooms of potentially toxic cyanobacteria Microcystis spp. persisted in western Lake Erie. Samples collected from the bloom were analyzed for the cyanobacterial toxin microcystin and the presence of Microcystis spp. cells. Estimates of microcystin toxicity exceeding 1 microg L(-1) (microcystin-LR activity equivalents), the safety limit set by the World Health Organization, were found from the samples in both 2003 and 2004. The presence of Microcystis spp. in water samples was confirmed through standard polymerase chain reaction (PCR) using a combination of four primer sets. Quantification of Microcystis was accomplished by a real-time PCR assay utilizing specific primer-Taq-man probe sets targeted on a conserved, Microcystis-specific 16S rDNA fragment and a microcystin toxin synthetase gene mcyD. This approach allowed us to specifically study the distribution and abundance of toxic Microcystis in the lake in contrast to previous studies that have assessed Microcystis populations with less refined methods. On the basis of quantification by quantitative real-time PCR analysis, the total abundance of Microcystis cells in the bloom area varied from 4 x 10(8) to 2 x 10(3) cells L(-1). The results of this study provide novel insight regarding the distribution and abundance of Microcystis spp. in the western basin of Lake Erie, a region plagued in recent years by large-scale (>20 km2) blooms. Our results suggest that the Maumee River and Bay may serve as a source for Microcystis to western and central Lake Erie.

Persistence of extracellular baculoviral DNA in aquatic microcosms: extraction, purification, and amplification by the polymerase chain reaction (PCR)

Genetically-modified baculoviruses have potential uses as bio-pesticides in forestry. However, the baculoviral occlusion bodies (OBs) may release genetically-modified DNA into the forest environment. In this research, outdoor aquatic microcosms, spiked with 673 microg of genomic DNA (4.4 x 10(12) target copies) from the genetically modified baculovirus Choristoneura fumiferana MNPVegt-/lacZ+, were exposed to natural summer conditions. A 530 bp DNA fragment from the genome of CfMNPVegt-/lacZ+ was detected in field microcosm water samples for about 24 h. The introduced DNA may have persisted for a longer time, but was below the detection limit of the PCR analysis (13.5 pg DNA or 8.9 x 10(4) target copies ml(-1) water). The detection limit of PCR was determined by spiking water samples with a dilution series of CfMNPVegt-/lacZ+ genomic DNA, extracting and purifying the DNA, and then PCR analysis. This study provides some of the first information on the persistence and detection limits of this viral DNA under aquatic ecological conditions, and the methods that can be used to conduct such a molecular-based field study.

Relationship of messenger RNA reverse transcriptase-polymerase chain reaction signal to Campylobacter spp. viability

SUMMARY. Discriminating viable from dead cells is of importance in the development of bacterial detection methods. A positive reverse transcriptase-polymerase chain reaction (RT-PCR) amplification signal was tested as a potential predictor of chick colonization. Some researchers have suggested that the presence of messenger RNA (mRNA) may not correlate with cell viability. Chicken colonization by cells that have positive mRNA signal but that are noncultivable would provide a correlation in cell viability and persistence of mRNA. The role of a viable but noncultivable (VBNC) form of Campylobacter spp. for colonization of poultry could be verified by such an mRNA signal. The levels of four strains of Campylobacter spp., previously isolated from poultry feces, declined progressively over time, and loss of cultivability occurred after 6 to 7 wk incubation in phosphate-buffered saline (PBS) at 4 C. Cold-stored, noncultivable and heat-inactivated (60 C for 10 min) Campylobacter spp. produced inconsistent amplified products from RT-PCR assay, depending on the target transcripts and strains used, although all fresh cultures showed mRNA signals. For the most part, signals of mRNA species from VBNC and heat-killed Campylobacter spp. AH-1, AH-2, and CH-3 persisted. RT-PCR amplification of transcripts originating from the tkt and cmp genes and a 256-base pair amplicon (from a previously described putative haem-copper oxidase) provided consistent signals, whereas transcripts from the flaA gene did not. Presumed VBNC and heat-inactivated Campylobacter spp., which produced positive mRNA signal but was not cultivable by conventional culture-based methods, did not establish colonization in the intestine of chicks 7 days after challenge. These results lead us to question the correlation between mRNA durability with cell viability as well as the significance of the VBNC cells in environmental transmission of Campylobacter spp.

Molecular methods for the assessment of bacterial viability

A significant number of pathogenic microorganisms can be found in environmental reservoirs (air, water, soil). It is important to assess the viability status of these organisms to determine whether they pose a threat to public health. Classical methods for determining viability are time consuming. Hence, molecular methods have been developed to address this problem. Molecular methods offer speed, sensitivity and specificity. Both DNA and RNA have been analysed using molecular amplification methods such as polymerase chain reaction (PCR), reverse transcriptase PCR (RT-PCR) and nucleic acid sequence-based amplification (NASBA). However, due to the variable persistence of nucleic acids in cells post-death, the correlation between presence of DNA and RNA and viability is not clear-cut. Similarly, the choice of target and sensitivity of the method can significantly affect the validity of the viability assay. This review assesses the molecular methods currently available and evaluates their ability to assess cell viability with emphasis on environmental pathogens.

A comparison of nucleic acid amplification techniques for the assessment of bacterial viability

AIMS

The ability to determine the presence and viability status of bacteria by molecular methods could offer significant advantages to the food, environmental and health sectors, in terms of improved speed and sensitivity of detection.

METHODS AND RESULTS

In this study, we have assessed three amplification techniques, PCR, RT-PCR and NASBA, for their ability to detect nucleic acid persistence in an E. coli strain following heat-killing. NASBA offered the greatest sensitivity of the three methods tested. The presence of residual DNA and mRNA could be detected by PCR and NASBA, respectively, for up to 30 h postdeath, by which time cell death had been confirmed by culture methods. Thus a single quantitative measurement based on nucleic acid amplification did not permit unequivocal determination of cell viability.

CONCLUSIONS, SIGNIFICANCE AND IMPACT OF THE STUDY

The correlation between cell viability and persistence of nucleic acids must be well characterized for a particular analytical situation before molecular techniques can be substituted for traditional culture methods.

Growth and survival of clinical vs. environmental species of Aeromonas in tap water

The ability of four species of Aeromonas (two of clinical and two of environmental origin) to survive and/or grow in tap water microcosms supplemented with sodium thiosulphate was tested. After bottling, the autochthonous microflora reached 6 x 10(5) cfu ml(-1) after a 5-day incubation period in tap water unfiltered and which was non-autoclaved. In filtered tap water, "ultramicrocells" were detected and final populations of ca. 10(6) cfu ml(-1) after 7 days were obtained. Aeromonas was inoculated at an initial cell concentration of ca. 10(4) cfu ml(-1). All strains were able to grow in tap water samples, which were filtered and autoclaved, and a final concentration of 10(5)-10(6) cfu ml(-1) was observed. Any inherent capability of Aeromonas to grow in tap water was eliminated by the presence of autochthonous microflora and "ultramicrocells" bacteria. Survival rates were strain- and microcosm-dependent. In unfiltered-non-autoclaved water, viable counts declined to below the detection limit (i.e. 1 log cfu ml(-1)) in 1.5 to 20 days. The declines in viable counts were even more pronounced in the filtered microcosm. Although inoculation ratios (100/1 in unfiltered-non-autoclaved and 1,000/1 in filtered microcosms) were favourable for aeromonads, at least for I to 3 days, the organisms disappeared in these microcosms. Thus, competition for nutrients was an unlikely cause of the limitation of aeromonads. The bacteriolytic effect of enzymes released by membrane vesicles from the autochthonous microflora and of "tail phage-like particles" bacteriocins were suggested as an in situ control of aeromonad populations. The present study showed that environmental strains of Aeromonas had no ecological advantage over clinical isolates. Thus, waterborne infections and contaminations of foods by pathogenic Aeromonas species could not be discounted.

Survival of biocontrol Pseudomonas fluorescens CHA0 in lysimeter effluent water depends on time of the year and soil type

AIMS

To assess the effects of soil type and time of the year on survival of the biocontrol inoculant Pseudomonas fluorescens CHA0 under aerobic conditions in lysimeter effluent water.

METHODS AND RESULTS

Effluent water was collected at different times from large outdoor lysimeters (2.5 m deep), which contained a well-drained or a poorly-drained cambisol, and inoculated with CHA0. The inoculant was monitored for 175 d by colony counts, total immunofluorescence cell counts and Kogure's viable cell counts. Cell numbers obtained with the three methods were similar. The inoculant declined exponentially in time and its population level varied considerably depending on the time of the year at which effluent water had been collected and soil type in the lysimeter. Positive correlations were found between the number of resident culturable aerobic bacteria and subsequent survival of the inoculant.

CONCLUSION

The fluctuations of inoculant survival patterns correlated with differences in biological properties of lysimeter water that were related to soil type and time of the year.

SIGNIFICANCE AND IMPACT OF THE STUDY

Results suggest that predictability of the survival of bacterial soil inoculants transported to groundwater level by heavy rainfall may be improved by taking into account key biological properties of the water.