No abstract

Advanced disinfection processes (peracetic acid, UV irradiation and ozonation) have been tested and evaluated through bench and pilot scale studies. 3 log removals of total coliforms, faecal coliforms and faecal streptococci were achieved by 10mg/L peracetic acid at a 10min contact time, by UV radiation at 35mW.s/cm2 and by ozone at 5mg/L for 10min contact time. Higher doses are required for virus removal by UV and PAA and especially for highly resistant viruses such as F-specific bacteriophage MS2. Ozonation has the advantage of having a strong effect on all types of bacteriophages and protozoa cysts even when low treatment doses and short contact times are applied. The results of this study demonstrated that evaluation of disinfection efficiency of ozone, UV and PAA depends on the criteria and methods employed. Standard method (plate count) results showed an important disinfection effect on culturability, while results from non-standard methods (respiratory activity and β-galactosidase activity assay) indicated less reduction of viable cells. Moreover, the results confirm that disinfectants act on bacteria in different ways. It has been clearly demonstrated that b-galactosidase activity is affected by PAA while UV treatment has no or very limited effect on the enzyme activity. Even without sunlight reactivation, bacterial regrowth in seawater was observed after disinfection of sewage effluents. This study also shows that the biodegradability of sewage effluent for an E coli strain was affected differently by the oxidative disinfectants ozone and PAA. Biodegradability should therefore be considered when evaluating the total disinfection efficiency.

Differential decreases over time of two bacterial species, Escherichia coli and Enterococcus faecalis, in a freshwater and a marine ecosystem were observed and explained by a differential rate of digestion of these bacteria by phagotrophic flagellates and ciliates. For this purpose, fluorescence-labeled bacteria (FLB) were used and prepared from the two species cited above. The number of FLB was observed for 5 days in fresh and marine waters in the presence or absence (0.2-,unm-pore-size-filtered water) of natural microbiota. These experiments showed a longer persistence of Enterococcusfaecalis FLB as opposed to Escherichia coli FLB in the presence of natural microbiota. Removal of FLB was due to protozoan grazing because no decrease of FLB number was observed in the absence of natural microbiota. In short-term (about 40 min) ingestion experiments, we found similar clearance rates of Escherichia coli and Enterococcus faecalis FLB by assemblages of flagelates from the freshwater and the marine ecosystem and by cultured assemblages of ciliates from the marine ecosystem. Clearance rates of Enterococcus faecalis FLB were greater than those of Escherichia coli FLB for assemblages of ciliates from the freshwater ecosystem. Comparison of rates of ingestion and digestion of FLB by protozoa showed that Escherichia coli FLB were digested and ingested at similar rates. However, Enterococcusfaecalis FLB were digested slower than they were ingested. These results suggest that a longer persistence of Enterococcus faecalis as opposed to Escherichia coli can be explained by a differential digestion by flagellates and ciliates in aquatic ecosystems. Moreover, rates of ingestion and digestion were strongly correlated for both FLB types.

Changes in the outer membrane subproteome of Escherichia coli along the transition to the viable but nonculturable state (VBNC) were studied. The VBNC state was triggered by exposure of E. coli cells to adverse conditions such as aquatic systems, starvation, suboptimal temperature, visible light irradiation and seawater. The subproteome, obtained according to Molloy et al., was analysed at the beginning of exposure (inoculum, phase 1), after a variable exposure time (95% of population culturable, phase 2) and when populations were mainly in the VBNC state (95% of cells VBNC, phase 3). Proteome changes were dependent on adverse conditions inducing the transition and were detected mainly in phase 2. The permanence of E. coli cells in seawater under illumination conditions entailed a dramatic rearrangement of the outer membrane subproteome involving 106 new spots, some of which could be identified by peptide fingerprinting. However, proteins exclusive to the VBNC state were not detected.

The influence of biotic and abiotic factors on plasmid transfer between Escherichia coli strains in terms of the variation in the number of transconjugants formed and the variation in transfer frequency was investigated. The density of parent cells affected the number of transconjugants, reaching a maximum when the cell density was on the order of 108 CFU ml-'. As the donor-to-recipient ratios varied from 104 to 104, the number of transconjugants varied significantly (P < 0.001), reaching a maximum with donor-to-recipient ratios between 1 and 10. The concentration of total organic carbon in the mating medium affects both the number of transconjugants and the transfer frequency, being significantly higher (P < 0.001) when the total organic carbon concentration was higher than 1,139 mg of C liter-'. However, the transconjugants were detected even with less than 1 mg of C liter-. Linear regression of loglo transconjugants versus mating temperature showed a highly significant regression line (P < 0.001). Neither the transfer frequency nor the transconjugant number varied significantly in the range of pHs assayed. We can conclude that plasmid transfer by conjugation can take place within a wide range of conditions, even in such adverse conditions as the absence of nutrients and low temperatures.