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Investigation of mature biofilm populations in the distribution of drinking water with attention to bacteria of hygienic relevance
Citation key Conradi2011
Author Conradi, Bianca
Year 2011
School Technische Universität Berlin, FG Umweltmikrobiologie
Abstract In the present study, mature drinking water biofilms were investigated in reactor systems and distribution pipes. The aim of the study was to characterize the microbial community with emphasis on bacteria of hygienic relevance. In the reactor in Berlin and the Ruhrgebiet middle aged biofilms (6 to 24 months exposed) were grown on coupons of the materials glass, copper, PE, stainless steel, and PVC. The experimental setup takes place in between the actual distribution system and inhouse installations. In addition, biofilms of another reactor system were investigated by cooperation with the DTU in Lyngby, Denmark. Furthermore, pipe biofilms were obtained from pipes cut out of the actual drinking water distribution system. The investigations of the pipes focussed on the materials cast iron, grey cast iron, and cement lined cast iron as well as PE and PVC. Investigation of the culturable population in biofilm samples revealed 454 isolates in total and were phylogenetically analysed by sequencing or a combined RFLP-sequencing approach. The culturable bacteria from the biofilms of the German and Danish reactor systems clustered in the phyla Firmicutes, Bacteroidetes, and predominately Proteobacteria (79 %). Only two gram-positive Bacillus and four Flavobacteriaceae were identified. In contrast to this, in the old biofilm samples nearly as much gram-positive (49 %) as gram-negative (51 %) bacteria were found. Some genera with pathogenic potential were identified but not tested for virulence factors or infectivity. In the bulk water phase of the reactor systems the genera Acinetobacter, Aeromonas and P. aeruginosa were found. Additionally, in the biofilms of the Berlin reactor system Aeromonas and in the pipes Mycobacterium was detected. Detection of total cell counts in the middle aged biofilms on glass, PE, PVC, copper, and steel showed an approximation with increased exposition times. This effect is related to less impact of the materials to growth of the middle aged biofilms. In contrast to this is the long lasting effect of the alkalinity of cement to total cell counts of the old biofilms. Moreover, it was shown that scraping and homogenization of coupon biofilms reduced total cell counts. The statistical analysis of heterotrophic plate counts of old biofilms resulted in a ranking of colony forming units on the used media and incubation conditions: R2A+starch > R2A+Tween > GDWR 20°C and 36°C. Furthermore, standard R2A and modified R2A are able to detect quantitative differences in colony forming units between the metallic materials, PVC, and cement of the old biofilms. A significant correlation was found between standard R2A and modified R2A. The results of this study showed limitations of selected molecular techniques in mature drinking water biofilms. Detection of bacterial DNA was hindered by adsorption of DNA and/or inhibition of PCR by the biofilm extracts. Moreover, neither the different commercially available column systems nor the traditional phenol/chloroform extraction were able to supply bacterial DNA detectable by PCR out of selected biofilm samples. Regarding the health risk of consumers the results of this study allow the conclusion that there is a pathogenic potential depending on the environmental parameters but no acute risk.
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