Biofilms are found in many water supply systems where they form an environment in which different bacteria can be entrapped for long periods. Besides the aesthetic aspect, biofilm has a major contribution in biocorrosion, disinfection inefficiency and possibly may act as a reservoir for pathogenic and non-pathogenic microorganisms. In the present study, two pathogenic bacteria Legionella pneumophila and Salmonella typhimurium WG-49 were introduced into a biofilm simulation flow system supplied with sterile and non-sterile tap water. The survival of these microorganisms into the biofilm formed on glass and PVC coupons at two temperatures (24°C and 36°C) was compared in this system. On glass supports, under sterile conditions at 36°C, Legionella pneumophila sg3 decreased by 6 logs during 40d continuous recirculation. Under non-sterile conditions, L. pneumophila decreased by only half log <48d. S. typhimurium WG-49 under the same conditions showed an increase of 3 logs in the sterile system for 31d, while in the non-sterile system it dropped by only 0.5 log for 20d. At 24°C, L. pneumophila remained stable for >40d under sterile conditions. In non-sterile conditions, L. pneumophila dropped by 1 log for 35d. S. typhimurium, in a sterile system, remained almost unchanged, while in the non-sterile system an increase of 3 logs was observed for the first 21d and thereafter a decrease of 2 logs for the next 21d of the experiment. L. pneumophila on PVC coupons at 36°C survived better compared with glass support. The experimental data show that survival of pathogenic microorganisms into biofilm is variable and depends on many factors, making the survival prediction a difficult task. However, the survival results of L. pneumophila and S. typhimurium in time terms should raise important questions on their potential threat in water distribution systems.

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