Effets de la température, du pH et du rayonnement solaire sur la survie de différentes bactéries d'intérêt sanitaire dans une eau usée épurée par lagunage
The behaviour of coliforms especially E. coli, pathogenic bacteria (Salmonella typhimurium), and occasionally pathogenic bacteria (Aeromonas hydrophila) in different aquatic environments has often been studied, but rather than a medical than an environmental point of view. However, studies on the affect of various environmental factors such as temperature, pH and solar radiation on their evolution have seldom been carried out. Their action on pathogenic and occasional pathogenic bacteria remains fragmentary. The aim of this survey is analyse the effect (in laboratory experiment) of certain environmental factors such as temperature, pH and solar radiation on the behaviour of E. coli 0126 : B16, S. typhimurium and A. hydrophila, in waste waters treated in a sewage treatment lagoon. The values of these tested factors are comparable to those which exist in aquatic environments at different seasons. Experimentation has been realized in a variable and complex medium in this case water than the basin of lagoon, in order to study the varying effects of differents factors on evolution of abundance bacteria. Hierarchization of effects of environmental factors or of certain of their associations has been proposed. In order to study the action of the temperature and the combined action of temperature-pH on the bacterial evolution, four sterile 1 litre flasks were filled with 500 ml of waste water treated in lagoon. The pH of each flask was adjusted by acid (H2SO4) or alkaline (NaOH) solutions at 6.2; 7.2; 8.4 and 9.6. Each flask was seeded with a bacterial inoculum of each of the tested strains (E. coli, A. hydrophila and S. typhimurium). The temperature used for the incubation of the flasks corresponded to the different seasons : 4 °C, 12 °C and 23 °C. The temporal evolution of the abundance of each tested strain is followed up by the indirect counting on specific media : TTC and Tergitol Lactose Agar (Institut Pasteur Production) and incubation at 44.5 °C for 24 hours (E. coli), Salmonella-Shigella Agar (BioMérieux) and incubation at 37 °C for 24 hours (S. typhimurium 4,5 H 1,2 i), Pril-Xylose-Ampicilin agar (ROGOL et al., 1979) and incubation at 37 °C for 48 hours (A. hydrophila ATCC 7966). The action of the solar radiation and the combined action of solar radiation-pH on E. coli, S. typhimurium and A. hydrophila was carried out as follows : i) 3 series of glass crystallizers were filled with 500 ml of waste water treated in lagoon (height water : 55 mm). Three reactional mediums were thus prepared by adjusting the pH to 7.2; 8.4 and 9.6. Each pH corresponds to two crystallizers which were seeded with a bacterial inoculum of E. coli; S. typhimurium; and A. hydrophila. For each pH one of the crystallizers is placed in front of a window receiving plenty of solar radiation, the other being protected from all radiation by aluminium paper and used as control. The number of bacteria is verified by counting the colony forming units (c.f.u.) through a dilution-spreading technique on the same media as those used to estimate the action of temperature and pH on the survival of E. coli, A. hydrophila and S. typhimurium. ii) In order to understand the night and day variations in the number of E. coli according to the solar radiation, another method was used. A standard E. coli inoculum is added to some water of the outflow of lagoon filtered on 0,45 μm (Millipore) and exposed to solar radiation. The temporal evolution of the number of E. coli is then followed by daily counts at 6:00 AM, at 2:00 PM and at 19:00 PM (GMT hour). Comparison of the different results is made with T90 (the time needed to reduce the initial bacterial population by 90 %) and with an equality test of 2 regression coefficients (FRONTIER, 1981). Obtained results show that temperature and solar radiation could be considered among factors responsible for the variable abundance of E. coli, A. hydrophila and S. typhimurium. At pH 6.2; 7.2 and 8.4 and effect incubation at 23 °C, the survival time (T90) of E. coli and A. hydrophila is almost the same (T = 53 h) and is anyway inferior to the one for S. typhimurrum which survived nearly twice as long (T90 =100 h). On the other hand, at pH 9.6 and with the same incubation temperature of 23 °C, the survival time of different tested bacteria is reduced. The T90 for E. coli is the same as the one for A. hydrophila (T90 = 20 h) and is 24 hours for S. typhimuriun (table 1). It is almost 4,3 times lower than the one with pH 6.2 and 7.2. The evolution in number of the same bacteria with similar pH but at a temperature of 12 °C are shown in figure 2. Compared with results obtained at 23 °C, one notices that the arrival of different tested bacteria is longer. The survival times for E. coli and A. hydrophila are almost the same (T90 = 73 h), but differ than the one for S. typhimurium (T90 =142 h), a bacteria which always survives better than other bacteria under the same experimental conditions. Low temperatures (4 °C) increase the survival et E. coli and Salmonella typhimurium and reduce that of Aeromonas hydrophila. Not only do these low temperatures (4 °C) reduce survival of E. coli and S. typhimurium but they also reduce negative effects of alkaline pH (pH 9.6) which decreases the abundance of these bacteria. Among the alkaline pH values studied, pH 9.6 caused the highest reduction of the survival of E. coli, A. hydrophila and S. typhimurium. We can conclude that at 4 °C the survival time of A. hydrophila significantly decreases in comparison with that of E. coli and S. typhimurium. At 12 and 23 °C, the survival time of E. coli and A. hydrophila is the same. S. typhimurium survived better than both E. coli and A. hydrophila. Solar radiation is also considered an important factor in the decrease of bacterial abundance especially when the pH is alkaline. Under solar radiation and pH 9.6, the survival time of E. coli, A. hydrophila and S. typhimurium is respectively 6, 4 and 6 hours (table 2). The decline in the numbers of these bacteria in effluent lagoon samples was found to be significantly greater in the presence of both pH-temperature or pH-solar radiation than when each of these factors was acting independently. In waste waters treated in lagoon and under sunlight, E. coli 0126 : B16 undergoes a successive evolution (night growth phase-diurnal decrease phase : fig. 5). According to all count values, we can plot the regression straight line (log 10 bacteria = vs (time)) and estimate the death coefficient which is the slope of this regressional straight line. This model expresses the tendency of the linear decrease of E. coli. Predictions of this model are satisfactory for counting values corresponding to periods of 24 hours or their multiple. The decrease phase or stable night phase cannot be predicte by this model owing to the fact that it is linear and presents the negative slope.