Bacterial communities associated with slow sand filtration and closed plant growth systems
1999
Maes, M. | Jamart, G. (Agricultural Research Centre, Gent (Belgium). Dept. Crop Protection)
Nutrient solutions recirculating in a closed plant culture system and passing over a slow sand filter were sampled. The filter was installed in a commercial greenhouse and was connected to a variety of ornamental plant cultures. Bacterial concentration, diversity and distribution were followed in the influent and effluent and down to 40 cm in the sand filter by means of growth tests and molecular DGGE typing. Bacterial titers counted by plating were nearly the same in the influent and effluent (2 to 500 cfu per ml), raised sharply with a factor of 1000000 in the liquid just above the sand and in the upper 2 cm layer of the filler, and dropped again from 10 cm filter depth on to 80000 cfu/g of sand. At a 40 cm depth still 26000 cfu per g of sand was counted. DGGE in combination with bacterial culturing showed that the bacterial composition of the influent and effluent were different. Competitive-growing bacterial types were mostly present in both samples, whereas a non-competitive bacterial flora was enriched during sand filtration. This suggests that some bacterial types present in the filter and in the effluent were retained during passage of the nutrient solution in the plant cultures. DGGE patterns also indicated that certain bacteria were present in the sand filter, while absent from influent and effluent. A range of bacterial types dominantly present in diluted plating of sand filter extracts were isolated. None of them showed a phytopathogenic nature when infiltrated in tobacco leaves. The bacterial isolates showed variable in vitro antagonism towards a range of plant pathogenic Phytophthora, Pythium, Rhizoctonia and Fusarium strains. Two bacteria were identified. One belonged to the Bacillus cereus group, the other to the Pseudomonas fluorescens complex. Based on DGGE interpretation one of the non-competitive bacteria, present in high concentrations deeper in the sand filter and in the effluent, could be identified as a bacterium from the Bacillus cereus group.
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