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Rétention de Escherichia coli d'un eau d'infiltration en sol équatorial au Cameroun (Afrique Centrale) : Rôle de divers horizons de sol

par Nola, M.;Nijne, T.;Boutin, Claude;Messouli, M.;Servais, Pierre ;Foto Menbohan, S.;Ngo Bidjeck, L.M.;Zébazé Togouet, Serge Hubert;Kemka, N.
Référence Journal of the Cameroon Academy of Sciences, 4, 2, page (107-116)
Publication Publié, 2004

Article révisé par les pairs

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Résumé :

La capacité de rétention des sols et la composition chimique de l’eau d’infiltration peuvent influencer le transfert dans le sol des espèces microbiennes contenues dans ces eaux. Une étude a été réalisée au laboratoire sur le rôle de divers horizons d’un sol tropical dans la rétention d’Escherichia coli contenu dans une eau d’infiltration. Il en ressort qu’en l’absence de tout composé organique biodégradable dans l’eau, les particules de sol adsorbent les cellules bactériennes, cette adsorption se poursuivant durant toute la durée de l’incubation au laboratoire (300 minutes). La vitesse de rétention de E. coli augmente des horizons supérieurs de sol aux horizons profonds. En présence d’un substrat organique biodégradable (la peptone), il se produit en plus de l’adsorption des cellules de E. coli sur les particules de sol, une multiplication cellulaire qui entraîne un accroissement du nombre de cellules après la première phase d’adsorption. Le taux de croissance apparent varie d’un horizon de sol à l’autre, et est relativement plus élevé dans les horizons supérieurs de sol. Les eaux souterraines devraient plus pauvres en bactéries indicatrices de contamination fécale dans les régions où les eaux d’infiltration contiennent peu de substrats biodégradables et/ou les horizons profonds du sol ont des épaisseurs relativement élevées.

The adsorption capacity of soils and the chemical composition of seeping water may significantly influence the transfer in soils of contaminating bacteria present in the seeping water. The present study was carried out in the laboratory to investigate the role of different tropical soil horizons in the retention of Escherichia coli found in seeping water. The soil horizons were characterised by the carbon concentration, total nitrogen concentration and phosphorus concentration varying respectively from 3.7 to 18.1 mg/g of soil, from 0.1 to 3.2 mg/g of soil and from 17.4 to 26.1 mg/g of soil. Carbon and total nitrogen concentration decrease from upper horizons to the deeper horizons. Total phosphorus concentration undergoes large fluctuations from one horizon to another. The technique used to study E. coli adsorption on soil particles was as follows: for each test, 500 mg of each tested soil was incubated for 300 minutes in the presence of 50 ml physiological solution containing 106 to 107 E.coli/ml and the abundance of E. coli was followed in the water phase by enumeration on plate count. In a second series of experiments with the same soils; peptone was added to the physiological solution (final concentration 20 mg/l) in order to study the impact of the presence of a biodegradable substrate in the seeping water. In the absence of biodegradable compound, a rapid adsorption of bacterial cells was observed and this adsorption process takes place throughout the incubation. Bacterial cells adsorption characterised by first order rates increased from upper horizons to the deeper soil horizons. In the presence of the biodegradable organic compound, we observed first a rapid decrease of bacterial abundance due to adsorption and than an increase in E. coli number due to growth. This cellular growth was visible only with a delay with respect to the beginning of adsorption. The delay varied, depending on soil horizon and it was relatively more pronounced in deeper horizons. The apparent growth rate of E. coli was calculated as slopes of regression lines obtained when the logarithm of the cell number was plotted versus time for the period corresponding to the increase in the number of planktonic bacterial cells. The apparent growth rate varied from 0.55 to 1.39/h and was more pronounced in the upper soil horizons. It was of 1,34/h with the horizon in contact with soil surface (close to the maximum growth rate of E. coli at 22-24 °C the temperature t which the experiments were performed) and of 0,81/h with soil horizon in contact with groundwater table. The capacity of bacterial retention in soils can thus be counterbalanced by a possible bacterial growth when seeping water are rich in biodegradable organic compounds, as it is the case of seeping domestic wastewaters. Groundwater would contain less bacteria in region where seeping water contain less or no biodegradable substrate and/or the nature and environment of soil horizon favours more the bacterial cells adsorption than their growth. It seems important to elaborate a predictive model considering physico-chemical and trophic properties of different soil horizons, which would predict the level of contamination of groundwater (usually used in tropical countries for drinking water) by seeping waters.