Explanation for the decline of bacteria introduced into lake water

Influence of combined abiotic/biotic factors on decay of P. aeruginosa and E. coli in Rhine River water

Understanding the dynamic change in abundance of both fecal and opportunistic waterborne pathogens in urban surface water under different abiotic and biotic factors helps the prediction of microbiological water quality and protection of public health during recreational activities, such as swimming. However, a comprehensive understanding of the interaction among various factors on pathogen behavior in surface water is missing. In this study, the effect of salinity, light, and temperature and the presence of indigenous microbiota, on the decay/persistence of Escherichia coli and Pseudomonas aeruginosa in Rhine River water were tested during 7 days of incubation with varying salinity (0.4, 5.4, 9.4, and 15.4 ppt), with light under a light/dark regime (light/dark) and without light (dark), temperature (3, 12, and 20 °C), and presence/absence of indigenous microbiota. The results demonstrated that light, indigenous microbiota, and temperature significantly impacted the decay of E. coli. Moreover, a significant (p<0.01) four-factor interactive impact of these four environmental conditions on E. coli decay was observed. However, for P. aeruginosa, temperature and indigenous microbiota were two determinate factors on the decay or growth. A significant three-factor interactive impact between indigenous microbiota, temperature, and salinity (p<0.01); indigenous microbiota, light, and temperature (p<0.01); and light, temperature, and salinity (p<0.05) on the decay of P. aeruginosa was found. Due to these interactive effects, caution should be taken when predicting decay/persistence of E. coli and P. aeruginosa in surface water based on a single environmental condition. In addition, the different response of E. coli and P. aeruginosa to the environmental conditions highlights that E. coli monitoring alone underestimates health risks of surface water by non-fecal opportunistic pathogens, such as P. aeruginosa. KEY POINTS: Abiotic and biotic factors interactively affect decay of E. coli and P. aeruginosa E.coli and P.aeruginosa behave significantly different under the given conditions Only E. coli as an indicator underestimates the microbiological water quality.

Agrochemicals indirectly increase survival of E. coli O157:H7 and indicator bacteria by reducing ecosystem services

Storm water and agricultural runoff frequently contain agrochemicals, fecal indicator bacteria (FIB), and zoonotic pathogens. Entry of such contaminants into aquatic ecosystems may affect ecology and human health. This study tested the hypothesis that the herbicide atrazine and the fungicide chlorothalonil indirectly affect the survival of FIB (Escherichia coli and Enterococcus faecalis) and a pathogen (E. coli O157:H7) by altering densities of protozoan predators or by altering competition from autochthonous bacteria. Streptomycin-resistant E. coli, En. faecalis, and E. coli O157:H7 were added to microcosms composed of Florida river water containing natural protozoan and bacterial populations. FIB, pathogen, and protozoan densities were monitored over six days. Known metabolic inhibitors, cycloheximide and streptomycin, were used to inhibit autochthonous protozoa or bacteria, respectively. The inhibitors made it possible to isolate the effects of predation or competition on survival of allochthonous bacteria, and each treatment increased the survival of FIB and pathogens. Chlorothalonil's effect was similar to that of cycloheximide, significantly reducing protozoan densities and elevating densities of FIB and pathogens relative to the control. Atrazine treatment did not affect protozoan densities, but, through an effect on competition, resulted in significantly greater densities of En. faecalis and E. coli O157:H7. Hence, by reducing predaceous protozoa and bacterial competitors that facilitate purifying water bodies of FIBs and human pathogens, chlorothalonil and atrazine indirectly diminished an ecosystem service of fresh water.

Protozoan predation in soil slurries compromises determination of contaminant mineralization potential

Soil suspensions (slurries) are commonly used to estimate the potential of soil microbial communities to mineralize organic contaminants. The preparation of soil slurries disrupts soil structure, however, potentially affecting both the bacterial populations and their protozoan predators. We studied the importance of this "slurry effect" on mineralization of the herbicide 2-methyl-4-chlorophenoxyacetic acid (MCPA, (14)C-labelled), focussing on the effects of protozoan predation. Mineralization of MCPA was studied in "intact" soil and soil slurries differing in soil:water ratio, both in the presence and absence of the protozoan activity inhibitor cycloheximide. Protozoan predation inhibited mineralization in dense slurry of subsoil (soil:water ratio 1:3), but only in the most dilute slurry of topsoil (soil:water ratio 1:100). Our results demonstrate that protozoan predation in soil slurries may compromise quantification of contaminant mineralization potential, especially when the initial density of degrader bacteria is low and their growth is controlled by predation during the incubation period.

Influence of native microbiota on survival of Ralstonia solanacearum phylotype II in river water microcosms

Ralstonia solanacearum phylotype II biovar 2 causes bacterial wilt in solanaceous hosts, producing severe economic losses worldwide. Waterways can be major dissemination routes of this pathogen, which is able to survive for long periods in sterilized water. However, little is known about its survival in natural water when other microorganisms, such as bacteriophages, other bacteria, and protozoa, are present. This study looks into the fate of a Spanish strain of R. solanacearum inoculated in water microcosms from a Spanish river, containing different microbiota fractions, at 24 degrees C and 14 degrees C, for a month. At both temperatures, R. solanacearum densities remained constant at the initial levels in control microcosms of sterile river water while, by contrast, declines in the populations of the introduced strain were observed in the nonsterile microcosms. These decreases were less marked at 14 degrees C. Lytic bacteriophages present in this river water were involved in the declines of the pathogen populations, but indigenous protozoa and bacteria also contributed to the reduced persistence in water. R. solanacearum variants displaying resistance to phage infection were observed, but only in microcosms without protozoa and native bacteria. In water microcosms, the temperature of 14 degrees C was more favorable for the survival of this pathogen than 24 degrees C, since biotic interactions were slower at the lower temperature. Similar trends were observed in microcosms inoculated with a Dutch strain. This is the first study demonstrating the influence of different fractions of water microorganisms on the survival of R. solanacearum phylotype II released into river water microcosms.

Effect of Solar Radiation on Survival of Indicator Bacteria in Bathing Waters

Sunlight exposure is considered to be the most important cause of "natural disinfection" in surface water environments. The UV-B portion of the solar spectrum is the most bactericidal, causing direct (photo-biological) DNA damage. In the present experimental study, the effect of solar radiation on the elimination of bacteria in water, especially in surface water, was studied. The influence of depth and UV-B transmittance of water was determined. Comparing Escherichia coli, Enterococcus faecalis, Pseudomonas aeruginosa and Staphylococcus aureus, Enterococcus faecalis proved to be the most resistant organism. Pseudomonas aeruginosa was shown to be the most sensitive indicator bacterium among the tested microorganisms. Results show a significant correlation between radiation intensity and reduction rates. Best elimination of microorganisms occurs on the water surface; with increasing water depth, there is less UV radiation to inactivate bacteria. High turbidity substantially reduces UV-B transmittance in water causing decreased elimination efficiency. The results of the present study show that sunlight, given an appropriate intensity and good water transparency is suitable to inactivate fecal indicator bacteria within a few hours in surface waters and therefore also in bathing waters.

Protozoan grazing increases mineralization of naphthalene in marine sediment

Bacterial decomposition of organic matter is frequently enhanced when protozoa are present. Various mechanisms have been proposed to account for this phenomenon, including effects associated with grazing by protozoa (such as increased recycling of limiting nutrients, removal of senescent cells, or reduction of competition among bacteria) and indirect effects of grazers (such as excretion of bacterial growth factors). Few studies have examined the role of protozoa in bacterial degradation of xenobiotic compounds in sediment containing a natural community of microbes. The effect of protozoa on mineralization of naphthalene was investigated in this study. Laboratory experiments were conducted using field-contaminated estuarine sediment, with the indigenous microbial populations. Mineralization of naphthalene was up to four times greater in treatments with actively grazing protozoa than in treatments containing the grazing inhibitor cytochalasin B. Control experiments confirmed that the grazing inhibitor was not toxic to ciliates but did prevent them from grazing. The grazing inhibitor did not affect growth rates of a mixed culture of sediment bacteria or a pure polycyclic-aromatic-hydrocarbon-degrading strain. Once grazing had been inhibited, supplementing treatments with inorganic N and P, glucose, or additional protozoa failed to stimulate naphthalene mineralization. Naphthalene-degrading bacteria were four to nine times less abundant when protozoan grazing was suppressed. We suggest that protozoa enhance naphthalene mineralization by selectively grazing on those sediment bacteria that ordinarily would outcompete naphthalene-degrading bacteria.

Salmonella typhimurium survival and viability is unaltered by suspended particles in freshwater

Rolling microcosm experiments were conducted to determine whether suspended particles affect the survival and viability of a model pathogen, Salmonella choleraesuis, serotype typhimurium (American Type Culture Collection no. 23567), in a freshwater microbial community. Water from the Duluth, MN harbor of Lake Superior (including native microorganisms) was inoculated with clay, silt, or flocculent organic particles in a range of concentrations and a streptomycin-resistant strain of S. typhimurium. Microcosms (incubated at 20 degrees C) were rolled horizontally (3 rpm) and sampled periodically for total bacteria and total, viable, and culturable S. typhimurium. Total S. typhimurium abundance decreased rapidly in all experiments (8.5-73.1% d-1). Total bacteria did not decrease as rapidly as the S. typhimurium population in any experiment, suggesting that a microcosm effect was not responsible for the decline in S. typhimurium populations. Loss rates of attached and free cells were similar, indicating that attachment to particles did not enhance the persistence of Salmonella cells beyond our minimum detectable differences. After eight days, only 0.1 to 11.9% of the initial S. typhimurium inocula were detected by direct counts. Suspended particles had a minimal effect on the survival and viability of S. typhimurium; the losses of total, viable, or culturable Salmonella were generally the same across particle treatments and concentrations. Silt and flocculent particles affected loss rates of total and viable S. typhimurium similarly to inorganic particles (clay). It appears unlikely that suspended particles would provide a means for S. typhimurium to persist at hazardous levels in freshwater.

The effect of toluidine blue on the survival, dormancy and outer membrane porin proteins (OmpC and OmpF) of Salmonella typhimurium LT2 in seawater

AIMS

To study the relationship between changes in the composition of the outer membrane proteins and the survival of Salmonella typhimurium LT2 in filtered autoclaved seawater containing Toluidine Blue (TB) dye as a photosensitizer.

METHODS AND RESULTS

In samples exposed to TB and excited by artificial visible light, the total viable (TVC) and respiring cell counts (RCC) showed that, although the TVC declined to an undetectable level in 6.5 h, the RCC showed that some cells were still capable of respiration. The porin protein composition changed gradually with OmpC and OmpF becoming undetectable by sodium dodecyl sulphate-polyacrylamide gel electrophoresis after 8 h of incubation. Hydrogen peroxide-pretreated cells survived longer compared with the control.

CONCLUSIONS

Oxidative pretreatment of Salm. typhimurium protects cells from some of the effects of sunlight in the presence of photosensitizers. The changes in porin proteins may play a role in this protection.

SIGNIFICANCE AND IMPACT OF THE STUDY

The study shows that the survival of bacteria under conditions of stress is the result of a linked series of reactions.

Use of potassium tellurite for testing the survival and viability of Pseudomonas pseudoalcaligenes KF707 in soil microcosms contaminated with polychlorinated biphenyls

This study shows that the oxyanion tellurite TeO3(2-) can be used as a tool to detect and quantify the release in soil microcosms of Pseudomonas pseudoalcaligenes KF707, a strain spontaneously resistant to tellurite with a minimal inhibitory concentration (MIC) of 150 microg ml(-1). KF707 cells which carry the genes for degradation of a wide range of polychlorinated biphenyl congeners (PCBs) were used for inoculation of laboratory microcosms prepared with two different PCB-contaminated soils (Ci/s and Di/s) in the presence or absence of biphenyl as carbon source. In all microcosms supplemented with biphenyl, significant survival of strain KF707 was noted over a time period of 35 days; conversely, in microcosms containing Ci/s soil without biphenyl addition a rapid decrease in KF707 inoculated cells was observed. By comparing the number of inoculated KF707 cells with the number of indigenous bacteria growing on biphenyl (IBGB) of both Ci/s and Di/s microcosms, it could be concluded that the KF707/IBGB ratio is a relevant parameter in determining the fate of the added strain. The efficacy of potassium tellurite as a selective marker to monitor strain KF707 in laboratory microcosms was confirmed by ARDRA analyses of the 16S rDNA, while the isolated indigenous bacteria growing on biphenyl were identified as members of three different species of the genus Pseudomonas. We also report that in microcosms inoculated with KF707 cells in the absence of biphenyl, only low chlorinated biphenyls were degraded.

Strain-specific differentiation of environmental Escherichia coli isolates via denaturing gradient gel electrophoresis (DGGE) analysis of the 16S-23S intergenic spacer region

Denaturing gradient gel electrophoresis (DGGE) was applied to the 16S-23S rRNA intergenic spacer region (ISR) as a means to evaluate strain level differences in Escherichia coli. The ISRs of 81 environmental E. coli isolates obtained from bovine, poultry, and human sources yielded a total of 41 unique DGGE banding patterns, with identical patterns and common bands within each source and no overlapping patterns among sources. An additional 51 isolates from two nearby streams yielded 45 unique banding patterns with no overlap between sites. However, two of the isolates from the streams had identical banding patterns to those from two of the source isolates, resulting in a total of 84 unique DGGE banding patterns out of 132 isolates identified in this study. These results revealed high diversity among environmental E. coli isolates, which made it difficult to unambiguously ascribe strains found in water samples to specific host organisms.

Persistence of PCR-detectable Bacteroides distasonis from human feces in river water

To evaluate the persistence of PCR-detectable Bacteroides distasonis in surface water, whole human feces were dispersed into water from the Ohio River and incubated in flasks in the laboratory or in diffusion chambers in situ. Duplicate samples were taken daily, and material that pelleted at 16,000 x g was assayed by PCR. Persistence of PCR-detectable DNA from this anaerobe depended upon temperature and predation, two of the factors shown by others to influence the survival of aerobic bacteria detected by culture. B. distasonis was detected by PCR for at least 2 weeks at 4 degrees C but for only 4 to 5 days at 14 degrees C, 1 to 2 days at 24 degrees C, and 1 day at 30 degrees C. In filtered water or in the presence of cycloheximide, a eukaryotic inhibitor, persistence at 24 degrees C was extended by at least a week.

Bacteria in post-glacial freshwater sediments

Prokaryote communities in post-glacial profundal freshwater sediments of Windermere, representing 10-12,000 years of deposition, were examined for culturability, viability and community structure. The potential for active geochemical cycles was inferred from the presence of specific groups of bacteria. Direct count procedures revealed 10(12) cells (g dry wt sediment)-1 in the surface sediments, which declined to approximately 10(9) cells (g dry wt sediment)-1 at 6 m depth of core (Representing approximately 10,000 years of deposition). The majority of the cells in the upper sediments were metabolically active when challenged with viability probes and responded to the direct viable count method. Below 250 cm, viability shown by 5-cyano-2,3-diotyl tetrazolium chloride (CTC) dye was not significantly different from the direct count; however, counts obtained with 5-carboxyfluorescein diacetate (CFDA) and the direct viable count both declined significantly from the direct count below 250 cm and 1 m, respectively. Culture was achieved from samples throughout the core, although the numbers of culturable bacteria decreased significantly with depth, from 10(7) c.f.u. (g dry wt sediment)-1 to 10(1)-10(2) c.f.u. (g dry wt sediment)-1 below 3 m depth. Among culturable isolates, Gram-positives and Gram-negatives were found at all levels of the core, and spore-forming heterotrophs dominated. Although sulphate-reducing bacteria were not detected below 20 cm, isolates demonstrating denitrifying activity were detected at all depths. PCR performed on samples taken below 3 m (deposited more than 7000 years ago) using eubacterial and archaeal primers revealed sequences similar to those found in deep sediments of the Pacific Ocean and the presence of methanogenic archaea. These observations indicate that bacteria and archaea are capable of long-term persistence and activity in deep, aged freshwater sediments.

Death of the Escherichia coli K-12 strain W3110 in soil and water

Whether Escherichia coli K-12 strain W3110 can enter the "viable but nonculturable" state was studied with sterile and nonsterile water and soil at various temperatures. In nonsterile river water, the plate counts of added E. coli cells dropped to less than 10 CFU/ml in less than 10 days. Acridine orange direct counts, direct viable counts, most-probable-number estimates, and PCR analyses indicated that the added E. coli cells were disappearing from the water in parallel with the number of CFU. Similar results were obtained with nonsterile soil, although the decline of the added E. coli was slower. In sterile water or soil, the added E. coli persisted for much longer, often without any decline in the plate counts even after 50 days. In sterile river water at 37 degrees C and sterile artificial seawater at 20 and 37 degrees C, the plate counts declined by 3 to 5 orders of magnitude, while the acridine orange direct counts remained unchanged. However, direct viable counts and various resuscitation studies all indicated that the nonculturable cells were nonviable. Thus, in either sterile or nonsterile water and soil, the decline in plate counts of E. coli K-12 strain W3110 is not due to the cells entering the viable but nonculturable state, but is simply due to their death.

Survival and activity of Pseudomonas sp. strain B13(FR1) in a marine microcosm determined by quantitative PCR and an rRNA-targeting probe and its effect on the indigenous bacterioplankton

Genetically engineered Pseudomonas sp. strain B13(FR1) was released into laboratory-scale marine ecosystem models (microcosms). Survival of the introduced population in the water column and the sediment was determined by plating on a selective medium and by quantitative competitive PCR. The activity of the released bacteria was determined by in situ hybridization of single cells with a specific rRNA-targeting oligonucleotide probe. Two microcosms were inoculated with 10(6) cells ml-1, while an uninoculated microcosm served as a control. The number of Pseudomonas sp. strain B13(FR1) cells decreased rapidly to ca. 10(2) cells ml-1 within 2 days after the release, which is indicative of grazing by protozoa. Three days after the introduction into seawater, cells were unculturable, but PCR continued to detect cells in low numbers. Immediately after the release, the ribosomal content of Pseudomonas sp. strain B13(FR1) corresponded to a generation time of 2 h. The growth rate decreased to less than 0.04 h-1 in 5 days and remained low, probably because of carbon limitation of the cells. Specific amendment of the microcosms with 10 mM 4-chlorobenzoate resulted in a rapid increase of the growth rate and an exponentially increasing number of cells detected by PCR, but not in resuscitation of the cells to a culturable state. The release of Pseudomonas sp. strain B13(FR1) into the microcosms seemed to affect only the indigenous bacterioplankton community transiently. Effects on the community were also apparent from the handling of water during filling of the microcosms and the amendment with 4-chlorobenzoate.

Validation of microbial community structure and ecological functional parameters in an aquatic microcosm designed for testing genetically engineered microorganisms

Microcosms were designed to facilitate studies of the fate, functioning, and ecological effects of microorganisms released into the aquatic environment. The microcosms were three-phase systems (sediment/water/air) with three compartments (a primary producer component, a herbivore grazer component, and intact sediment cores). The microcosms were validated by comparing gross ecological parameters and microbial community structure between the microcosms and the eutrophic Lake Bagsværd, which was simulated in the model. The photosynthetic potential and chlorophyll a concentrations were significantly lower in the microcosms than in the lake, which apparently was due to inorganic nutrient limitation. In the microcosms, total bacterial numbers and metabolic activity by [(3)H]thymidine incorporation were unaffected by the reduced algal biomass and primary production, simulating field conditions closely, with a strong dependence on temperature. Two days after filling the microcosms, the percentage of similarity of the microbial communities in the microcosm and Lake Bagsværd was 40%, measured by hybridizations of total microbial DNA. The similarity increased during the 10-day experimental period to 63-76%. In two experiments, Alcaligenes eutrophus AEO106(pRO101) was released into the microcosms. The release reduced the similarity between microcosms and lake to 2% and 27%, depending on the number of introduced cells. Concomitant to a decline in the A. eutrophus AEO106(pRO101) population, the similarity gradually recovered. It is concluded that the microcosms can simulate a freshwater lake ecosystem, but care has to be taken when extrapolating microcosm results to the source ecosystem because of the possible different selective conditions in the microcosm.

Fate of Pseudomonas putida after release into lake water mesocosms: Different survival mechanisms in response to environmental conditions

To study the fate of Pseudomonas putida DSM 3931 in an aquatic environment, cultures of the strain were released into lake water mesocosms. P. putida, bearing the TOL-plasmid, was released as a representative xenobiotic-degrading microorganism. The release was carried out in mesocosms with unamended lake water and in lake water with added culture medium to compare the survival of the strain due to the influence of different organic load. As a comparison, the survival of P. putida was followed in microcosms with sterile lake water. Survival and fate of the strain were determined by means of immunofluorescence with highly specific monoclonal antibodies and growth on selective agar medium for up to ten weeks after release. Addition of medium had a pronounced influence on survival in mesocosms. In mesocosms without added medium, the number of P. putida cells decreased within ten days by over 2 orders of magnitude. In mesocosms with medium, cell numbers increased in the first two days by an order of magnitude and were, after ten days, in the same range as at the time of introduction. Over time, cell numbers decreased but remained detectable in both types of mesocosms for up to ten weeks after release. In mesocosms with unamended lake water, the major fraction of the cells was attached to particles after two days. In mesocosms with medium, large aggregates of P. putida cells formed which included algae. The observed decrease in cell numbers in mesocosms was attributed mainly to grazing. Sedimentation was an additional factor contributing to loss of cells out of the water column, which especially affected aggregate-forming cells in mesocosms with medium in the long run (beyond two weeks). These studies demonstrate that experimental tools on a mesoscale are crucial in order to understand the complex processes microorganisms are subjected to after release into a natural environment, and that single cell detection, such as immunofluorescence, is essential to understand mechanisms of survival and elimination.

Survival of 2,4-dichlorophenoxyacetic acid degrading Alcaligenes eutrophus AE0106(pR0101) in lake water microcosms

Survival of the 2,4-dichlorophenoxyacetic acid (2,4-D) degrading Alcaligenes eutrophus strain AEO 106 harboring the catabolic plasmid pRO101 was studied in lake water from the eutrophic lake Frederiksborg Slotssø. Survival experiments were performed for periods of 7 days in laboratory microcosms containing filtered (0.2-µm pore size) or natural lake water amended with increasing concentrations of 2,4-D. A. eutrophus AE0106 was detected by combining the fluorescent antibody method with selective and nonselective plating followed by colony blotting and colony hybridization. Comparison of colony blotting and colony hybridization demonstrated that the A. eutrophus AE0106 host organism and the catabolic plasmid pRO101 had similar fates in the model system employed. In all experiments culturable counts of A. eutrophus AE0106 were lower than fluorescent antibody counts and frequently a decline in culturable counts occurred at times when the fluorescent antibody method showed an increasing population size. Amendment with 2,4-D increased survival of A. eutrophus AE0106 both in filtered and in natural lake water. Survival was always poorer in model systems with natural water than in 0.2 µm-filtered water.

Influence of ecosystematic factors on survival of Escherichia coli after large-scale release into lake water mesocosms

Mass cultures of an Escherichia coli K-12 strain were released into exposed mesocosms in a eutrophic lake. The release was performed with and without additional input of the E. coli culture medium to stimulate the scenario of leakage of a production fermenter on one hand and to compare the influence of the added organic nutrients with that of the added strain on the other hand. The survival of the introduced strain and the influence on ecological processes in the mesocosms were monitored for 10 weeks after release. For comparison, survival of the strain in microcosms with sterile lake water was also monitored. Survival of the strain was determined by means of immunofluorescence and growth on selective agar medium. In lake mesocosms, E. coli showed a rapid and constant dieback during the first week. After 4 days, cells were mostly restricted to particles, which seemed to provide niches for survival. From the second week onward, survival was improved in mesocosms with culture medium added. In microcosms with sterile lake water, plate counts of E. coli showed a strong decrease within 2 weeks, while total cell numbers remained approximately the same. The rapid elimination of E. coli from the free-water phase of the mesocosms was probably due to the combined effect of the inability to grow in lake water and grazing. The better survival of E. coli (mainly on particles) in mesocosms with added medium was attributed to the medium-induced enhancement of primary production, which was the source of a large quantity of particles. These particles, in turn, may have functioned as niches for prolonged survival as well as transport vehicles for sedimentation of the E. coli cells.

Fate in water of a recombinantEscherichia coli K-12 strain used in the commercial production of bovine somatotropin

The fate in water of Escherichia coli K-12 strain LBB269, both plasmid-free and carrying the recombinant plasmid pBGH1, was studied. E. coli K-12 strain LBB269 (pBGH1) is a nalidixic acid resistant derivative of W3110G (pBGH1), the microorganism used by Monsanto Company for the commercial production of bovine somatotropin. Water samples were obtained from the Missouri River and from the Monsanto Life Sciences Research Center aqueous waste basin. Strains LBB269 and LBB269 (pBGH1) were grown in fermentation vessel under bovine somatotropin (BST) production conditions, and inoculated into the water samples. The inoculated water samples were incubated at 26 degrees C, and the number of viable E. coli cells was determined as a function of time. In sterile water from both sources, the two strains remained at a constant level for at least 28 days; LBB269 (pBGH1) remained at a constant level in sterile water for at least 300 days. In non-sterile water from both sources, the two strains declined from an initial concentration of about 3.0 x 10(6) cells per ml to less than 10 cells per ml in 147 h. The study conditions did not adversely affect the populations of indigenous microorganisms. The selective loss of strains LBB269 and LBB269 (pBGH1) demonstrates that these E. coli strains do not survive in environmental sources of water. In addition, it was observed that the presence of pBGH1 had essentially no effect on the disappearance of strain LBB269 from either source of water.

Survival of Bacillus licheniformis in Seawater Model Ecosystems

The fate of Bacillus licheniformis DSM 13 was monitored after introduction into laboratory microcosms and mesocosms established in the Knebel Vig estuary, Denmark. The model organism was detected by a combination of immunofluorescence microscopy and nonselective plating followed by colony blotting. This allowed simultaneous quantification of intact cells and culturable cells. B. licheniformis DSM 13 adapted poorly to the conditions in filtered (0.2-μm-pore-size filter) seawater. Results from additional microcosm studies using natural seawater demonstrated that protozoan grazing also was important in regulating the population of the introduced model organism. In experiments using mesocosms, B. licheniformis DSM 13 also showed a rapid die-off. The introduction of the organism led to increased nutrient levels and to increased growth of both autotrophic and heterotrophic components of the plankton community compared with those of control enclosures. Thereby, a more intensive predation impact on the bacterioplankton community was induced. The combination of microcosm and mesocosm experiments provides a scenario in which the influence of single biotic and abiotic factors on survival of introduced organisms can be tested and in which the effect of the introduction on ecosystem structure and function can be evaluated. This test concept might prove useful in risk assessment of genetically modified microorganisms.