Environ Monit Assess (2009) 158:9–14 DOI 10.1007/s10661-008-0560-7 Comparison of the microbial load of incoming and distal outlet waters from dental unit water systems in Istanbul Irfan Türetgen · Duygu Göksay · Aysin Cotuk Received: 29 April 2008 / Accepted: 11 September 2008 / Published online: 9 October 2008 © Springer Science + Business Media B.V. 2008 Abstract This is a cross-sectional study of the incoming and distal outlet water quality from 41 dental units in Istanbul, carried out to compare the total microbial loads using traditional cul- ture method versus epifluorescence microscopy. The possible presence of Legionella pneumophila using traditional culture method was also ana- lyzed. One hundred and twenty three samples were taken from the high-speed handpiece lines, air–water syringe lines and source (incoming) wa- ter supplies from 41 dental units. The samples were assayed for live/dead bacteria, heterotrophic bacterial counts and presence of L. pneumophila bacteria. Thirty nine out of 41 dental units (91%) were not able to meet the standard limit of 200 CFU/ml in dental unit waters. The live bacte- rial counts were 1–1.5 orders of magnitude higher than aerobic mesophilic heterotrophic bacteria. L. pneumophila (serogroup 2–14) was isolated from five out of 41 units. Some dental units were using commercially bottled (19 l) drinking water as a source. The source water of eight dental unit was heavily contaminated which were fed up by com- mercially bottled drinking water. I. Türetgen (B ) · D. Göksay · A. Cotuk Department of Biology, Faculty of Science, Istanbul University, 34134 Vezneciler, Istanbul, Turkey e-mail: turetgen@istanbul.edu.tr Keywords Dental units · DUWS · Legionella · DAPI-CTC · Biofilm · Istanbul Introduction The existence of high concentrations of micro- bial accumulations in dental unit water systems (DUWSs) was first reported by Blake (1963). The microbial quality of dental unit water is of consid- erable importance since patients and dental staff are regularly exposed to water and bioaerosols generated from dental units. It was reported fre- quently that DUWSs are an ideal place for mi- crobial growth (Burke et al. 2004; Göksay et al. 2008; Sacchetti et al. 2006; Szymanska et al. 2004). Consequently, efforts are under the way to im- prove the performance of existing anti-microbial agents or to develop new ones (Montebugnoli et al. 2004; O’Donnell et al. 2007; Schel et al. 2006; Smith et al. 2002). Heterotrophic bacteria persist in dental units (DUs) growing as a multispecies biofilm on the inner surfaces and can continuously enter DUWSs (Coleman et al. 2007; Franco et al. 2005; Walker and Marsh 2007). Therefore, the mi- crobiological quality of DU water used in general dental practice can be poor compared to that from drinking water sources. Furthermore, some den- tists may not clean, disinfect the system or monitor the microbial load of their DUWSs. The Amer- ican Dental Association (ADA) recommended