Survival of Escherichia coli exposed to visible light in seawater: analysis of rpoS-dependent effects

Health risk assessment of swimming beaches microbial contamination: a case study - Mahmoudabad, Iran

Waterborne organisms in marine water generally originate from untreated wastewater discharged into the sea. The presence of numerous leisure beaches in Mahmoudabad city, Iran, annually attracts thousands of tourists from all over the country to participate in recreational swimming activities. This study probabilistically characterized the health risks associated with recreational swimming engendered by waterborne pathogens, such as intestinal enterococci and Escherichia coli (E. coli) at 15 sampling points along the beaches of the study using quantitative microbial risk assessment (QMRA). The mean annual infection risk of E. coli in children and adults was 0.424 and 0.229, respectively. The respective risk in terms of enterococci was 0.999 and 0.997, which were higher than the level recommended by the WHO and EPA. The results show that the risk of infection for children was higher than adults. Related authorities have to consider measures to improve environmental quality to protect tourists' and residents' well-being.

Modeling the Endogenous Sunlight Inactivation Rates of Laboratory Strain and Wastewater E. coli and Enterococci Using Biological Weighting Functions

Models that predict sunlight inactivation rates of bacteria are valuable tools for predicting the fate of pathogens in recreational waters and designing natural wastewater treatment systems to meet disinfection goals. We developed biological weighting function (BWF)-based numerical models to estimate the endogenous sunlight inactivation rates of E. coli and enterococci. BWF-based models allow the prediction of inactivation rates under a range of environmental conditions that shift the magnitude or spectral distribution of sunlight irradiance (e.g., different times, latitudes, water absorbances, depth). Separate models were developed for laboratory strain bacteria cultured in the laboratory and indigenous organisms concentrated directly from wastewater. Wastewater bacteria were found to be 5-7 times less susceptible to full-spectrum simulated sunlight than the laboratory bacteria, highlighting the importance of conducting experiments with bacteria sourced directly from wastewater. The inactivation rate models fit experimental data well and were successful in predicting the inactivation rates of wastewater E. coli and enterococci measured in clear marine water by researchers from a different laboratory. Additional research is recommended to develop strategies to account for the effects of elevated water pH on predicted inactivation rates.

Detection and risk assessment of diarrheagenic E. coli in recreational beaches of Brazil

Marine beaches are important recreational and economic resources in Brazil, but the beaches' water quality is negatively impacted by the discharge of domestic sewage effluent. The occurrence of diarrheagenic Escherichiacoli among the E. coli isolated from three Brazilian marine beaches was investigated. Multiplex and single step PCR were used to screen 99 E. coli isolates for ten target toxin genes. Six toxin genes, stx1, eae, estp, esth, astA, and bfpA, were identified in 1% to 35% of the isolates. A quantitative microbial risk assessment (QMRA) of human exposure to diarrheagenic E. coli during marine recreation was carried out. The results indicated that the diarrheagenic E. coli risk is well below the U.S. EPA's recommended daily recreational risk benchmark. However, the overall recreational health risk due to all pathogens in the water could be much higher and exceeded the U.S. EPA's benchmark.

Prevalence and Characterization of Shiga Toxin-Producing and Enteropathogenic Escherichia coli in Shellfish-Harvesting Areas and Their Watersheds

more strains formed a strong biofilm at 18 than at 30°C. Finally, more than 85% of analyzed strains were found to be sensitive to the 16 tested antibiotics. These data suggest the low risk of human infection by STEC if shellfish from these shellfish-harvesting areas were consumed.

Diurnal variation in Enterococcus species composition in polluted ocean water and a potential role for the enterococcal carotenoid in protection against photoinactivation

Enterococcus species composition was determined each hour for 72 h at a polluted marine beach in Avalon, Santa Catalina Island, CA. Species composition during the day was significantly different from that at night, based on an analysis of similarity. Enterococcus faecium and E. faecalis were more prevalent at night than during the day, while E. hirae and other Enterococcus species were more prevalent during the day than the night. Enterococcus spp. containing a yellow pigment were more common during the day than the night, suggesting that the pigmented phenotype may offer a competitive advantage under sunlit conditions. A laboratory microcosm experiment established that the pigmented E. casseliflavus isolate and a pigmented E. faecalis isolate recovered from the field site decay slower than a nonpigmented E. faecalis isolate in a solar simulator in simulated, clear seawater. This further supports the idea that the yellow carotenoid pigment in Enterococcus provides protection under sunlit conditions. The findings are in accordance with previous work with other carotenoid-containing nonphotosynthetic and photosynthetic bacteria that suggests that the carotenoid is able to quench reactive oxygen species capable of causing photoinactivation and photostress. The results suggest that using enterococcal species composition as a microbial source tracking tool may be hindered by the differential environmental persistence of pigmented and nonpigmented enterococci.

Identification of a c-di-GMP-regulated polysaccharide locus governing stress resistance and biofilm and rugose colony formation in Vibrio vulnificus

As an etiological agent of bacterial sepsis and wound infections, Vibrio vulnificus is unique among the Vibrionaceae. Its continued environmental persistence and transmission are bolstered by its ability to colonize shellfish, form biofilms on various marine biotic surfaces, and generate a morphologically and physiologically distinct rugose (R) variant that yields profuse biofilms. Here, we identify a c-di-GMP-regulated locus (brp, for biofilm and rugose polysaccharide) and two transcription factors (BrpR and BrpT) that regulate these physiological responses. Disruption of glycosyltransferases within the locus or either regulator abated the inducing effect of c-di-GMP on biofilm formation, rugosity, and stress resistance. The same lesions, or depletion of intracellular c-di-GMP levels, abrogated these phenotypes in the R variant. The parental and brp mutant strains formed only scant monolayers on glass surfaces and oyster shells, and although the R variant formed expansive biofilms, these were of limited depth. Dramatic vertical expansion of the biofilm structure was observed in the parental strain and R variant, but not the brp mutants, when intracellular c-di-GMP levels were elevated. Hence, the brp-encoded polysaccharide is important for surface colonization and stress resistance in V. vulnificus, and its expression may control how the bacteria switch from a planktonic lifestyle to colonizing shellfish to invading human tissue.

Impact of rpoS deletion on the proteome of Escherichia coli grown planktonically and as biofilm

To investigate the role of rpoS in gene expression of Escherichia coli cells grown as biofilms, we compared the proteomes of a rpoS mutant and the wild-type strain. Experiments were performed on planktonic cells (in exponential or stationary growth phase) and biofilms developed on glass wool. Spot-by-spot comparison of gels obtained from biofilm and planktonic wild-type organisms showed that the intensity of between 22 and 30% of detected spots was affected by the growth mode, depending of the control used. Principal component analysis, used to interpret the variations in protein spot densities, discriminated exponential-phase cells (wild-type and mutant) from the other incubation conditions and secondarily 72-old cultures. The statistical analysis demonstrated that the rpoS mutation did not significantly modify the proteome of exponential-growth phase cells, the differences involving only 3% of the proteome. However, increasing the incubation time from 8 to 72 h noticeably increased the number of changed proteins. A cluster analysis showed that RpoS plays a role in the special nature of the gene expression of biofilm cells but lower than in stationary-phase bacteria. We identified 35 rpoS-regulated proteins that were already or not described as controlled by this sigma factor. For some of them, the mode of regulation by RpoS was obviously dependent on the culture condition (planktonic vs biofilm).

Specific growth rate determines the sensitivity of Escherichia coli to thermal, UVA, and solar disinfection

Knowledge about the sensitivity of the test organism is essential for the evaluation of any disinfection method. In this work we show that sensitivity of Escherichia coli MG1655 to three physical stresses (mild heat, UVA light, and sunlight) that are relevant in the disinfection of drinking water with solar radiation is determined by the specific growth rate of the culture. Batch- and chemostat-cultivated cells from cultures with similar specific growth rates showed similar stress sensitivities. Generally, fast-growing cells were more sensitive to the stresses than slow-growing cells. For example, slow-growing chemostat-cultivated cells (D = 0.08 h(-1)) and stationary-phase bacteria from batch culture that were exposed to mild heat had very similar T(90) (time until 90% of the population is inactivated) values (T(90, chemostat) = 2.66 h; T(90, batch) = 2.62 h), whereas T(90) for cells growing at a mu of 0.9 h(-1) was 0.2 h. We present evidence that the stress sensitivity of E. coli is correlated with the intracellular level of the alternative sigma factor RpoS. This is also supported by the fact that E. coli rpoS mutant cells were more stress sensitive than the parent strain by factors of 4.9 (mild heat), 5.3 (UVA light), and 4.1 (sunlight). Furthermore, modeling of inactivation curves with GInaFiT revealed that the shape of inactivation curves changed depending on the specific growth rate. Inactivation curves of cells from fast-growing cultures (mu = 1.0 h(-1)) that were irradiated with UVA light showed a tailing effect, while for slow-growing cultures (mu = 0.3 h(-1)), inactivation curves with shoulders were obtained. Our findings emphasize the need for accurate reporting of specific growth rates and detailed culture conditions in disinfection studies to allow comparison of data from different studies and laboratories and sound interpretation of the data obtained.

mRNA detection by reverse transcription-PCR for monitoring viability and potential virulence in a pathogenic strain of Vibrio parahaemolyticus in viable but nonculturable state

AIMS

This work investigates the maintenance of viability and potential virulence of Vibrio parahaemolyticus in a viable but nonculturable population (VBNC) state by reverse transcription-polymerase chain reaction (RT-PCR).

METHODS AND RESULTS

Housekeeping genes, 16S-23S rDNA and rpoS, as well as virulence genes, tdh1 and tdh2, were selected and detected by PCR in a pathogenic strain of V. parahaemolyticus (Vp4). Their expression was then studied by RT-PCR in V. parahaemolyticus Vp4 cultivated in rich medium at 37 degrees C. The 16S-23S rDNA and rpoS, tdh1, tdh2 genes were transcripted at the mid-logarithmic, stationary and late stationary phases, corresponding to various physiological states. The expression of these genes was also studied by RT-PCR in a VBNC population of V. parahaemolyticus Vp4 in artificial seawater (ASW). The effect of temperature (washing of bacterial culture and microcosms) on the attaining VBNC bacteria was first considered. Washing of V. parahaemolyticus Vp4, collected at the mid-logarithmic phase, at 10 or 4 degrees C before inoculation in ASW at 4 degrees C allowed bacteria entered the VBNC state between 22 and 31 days. The 16S-23S rDNA and rpoS gene were expressed in the VBNC bacteria whereas no expression of the tdh1 and tdh2 genes was observed in the same populations.

CONCLUSION

The two selected housekeeping genes, 16S-23S rDNA and rpoS, proved to be good viability markers for V. parahaemolyticus Vp4 in culturable and VBNC states. These first data indicated that the pathogenic strain Vp4 would not maintain the expression of the virulence genes, tdh1 and tdh2, in VBNC state.

SIGNIFICANCE AND IMPACT OF THE STUDY

Use of RT-PCR for investigating the maintenance or not of viability and potential virulence in VBNC V. parahaemolyticus will facilitate further study to evaluate the potential risk presented by this pathogen in the environment.

Integral strategy for evaluation of fecal indicator performance in bird-influenced saline inland waters

Wild birds are an important nonpoint source of fecal contamination of surface waters, but their contribution to fecal pollution is mostly difficult to estimate. Thus, to evaluate the relation between feces production and input of fecal indicator bacteria (FIB) into aquatic environments by wild waterfowl, we introduced a new holistic approach for evaluating the performance of FIB in six shallow saline habitats. For this, we monitored bird abundance, fecal pellet production, and the abundance of FIB concomitantly with a set of environmental variables over a 9-month period. For estimating fecal pellet production, a new protocol of fecal pellet counting was introduced, which was called fecal taxation (FTX). We could show that, over the whole range of investigated habitats, bird abundance, FTX values, and FIB abundance were highly significantly correlated and could demonstrate the good applicability of the FTX as a meaningful surrogate parameter for recent bird abundances and fecal contamination by birds in shallow aquatic ecosystems. Presumptive enterococci (ENT) were an excellent surrogate parameter of recent fecal contamination in these saline environments for samples collected at biweekly to monthly sampling intervals while presumptive Escherichia coli and fecal coliforms (FC) were often undetectable. Significant negative correlations with salinity indicated that E. coli and FC survival was hampered by osmotic stress. Statistical analyses further revealed that fecal pollution-associated parameters represented one system component independent from other environmental variables and that, besides feces production, rainfall, total suspended solids (direct), and trophy (indirect) had significant positive effects on ENT concentrations. Our holistic approach of linking bird abundance, feces production, and FIB detection with environmental variables may serve as a powerful model for application to other aquatic ecosystems.

Influence des conditions de culture préalables et de la présence du gènerpoSpour la survie deSalmonella typhimuriumen eau de mer exposée à la lumière solaire

The effect of sunlight exposure on Salmonella typhimurium isogenic strains harboring an rpoS gene functional (rpoS+) or not functional (rpoS–) was investigated in microcosms of sterile sea water at 20 °C. The two strains rapidly lost their ability to produce colonies on solid culture media. The detrimental action of sunlight was more important when the salinity of sea water increased. The survival of stationary phase cells was influenced by RpoS. Bacteria grown in media with high salinity or osmolarity and transferred to sea water in stationary phase were more resistant to irradiation than those grown in media with low salinity. Prior growth under oxidative (0.2 mmol/L of H2O2) or amino acid starved (minimal medium) conditions did not modify the survival of either strain when they were exposed to sunlight. Bacteria were more resistant when cells were incubated in sea water in the dark prior to being exposed to sunlight. The resistance to sunlight irradiation was also greater in clones of both strains isolated from microcosms exposed to sunlight for 90 min, then further inoculated into sea water and reexposed to sunlight.Key words: Salmonella typhimurium, sea water, sunlight, rpoS, σs.

Survival of enteric bacteria in seawater

Enteric bacteria exposed to the marine environment simultaneously encounter a variety of abiotic and biotic challenges. Among the former, light appears to be critical in affecting seawater survival; previous growth history plays a major part in preadaptation of the cells, and stationary phase cells are generally more resistant than exponentially growing ones. Predation, mostly by protozoa, is probably the most significant biotic factor. Using Escherichia coli as a model, a surprisingly small number of genes was found that, when mutated, significantly affect seawater sensitivity of this bacterium. Most prominent among those is rpoS, which was also dominant among genes induced upon transfer to seawater.