The ability of microbes to survive in hospital water reservoir was described more than 30 years ago, and numerous studies have confirmed hospitals water as a source of nosocomial infection 19. The bacterial contamination level related to the tap water was a point of dispersion of bacterial species, including potentially pathogenic organisms 20. Modes of transmission for waterborne infections include direct contact, ingestion of water, indirect contact, inhalation of aerosols dispersed from water sources, and aspiration of contaminated water 21.Several factors could be attributed to the contamination of surface water such as the age of the distribution system, the quality of the delivered water, defective chlorination, and improper maintenance 22. Contamination can occur at the source, between source and storage points or in storage tanks 3. The bacterial contamination occurs because of regrowth of microorganisms in biofilms which are formed on interior surfaces of water pipes. Biological activity in biofilms is controlled by nutrient content of water, temperature, and residual chlorine 23.Microorganisms are widely distributed in nature and their abundance and diversity may be used as indicators for suitability of water 24. Certain types of microorganisms are characteristics of human fecal contamination 25. In the present study we used three bacteria (E. coli, P. aeruginosa, and L. pneumophila) to evaluate quality of studied water supplies because river and underground waters are main sources of tap water in the area and these bacteria are main human pathogens with water sources. Actually in this study, E. coli was used as a representative of the enteric bacterial pathogens. We also used L. pneumophila as ubiquitous waterborne microorganism and P. aeruginosa as ubiquitous waterborne opportunistic pathogen.E. coli is a type of fecal coliform bacteria commonly found in the intestines of animals and humans and the presence of it in water is a strong indication of recent sewage or animal waste contamination 26. We did not find E. coli in cold water and incubator water samples and just two warm water samples showed presence of E. coli. In some similar studies water distribution systems of examined hospitals were totally free of Gram-negative bacteria belonging to family Enterobacteriaceae 27. Although coliforms are known as the best index for monitoring water microbial quality, in some cases this index is not very efficient. Some believe that the excess growth of heterotrophic bacteria results in suppression of the coliforms. Therefore, it is recommended to use other bacteria as an alternative index in water microbial quality control and Pseudomonas can be one of these indicators 28.P. aeruginosa is part of a large group of free-living bacteria that are ubiquitous in the environment. P. aeruginosais of major concern in hospitals, since it can cause severe secondary infections, such as pneumonia in burn victims or immunocompromised persons 29. Its occurrence in drinking water is probably related more to its ability to colonize biofilms in plumbing fixtures (i.e., faucets and showerheads) than its presence in the distribution system or treated drinking water. P. aeruginosa can survive in deionized or distilled water; hence, it may be found in low nutrient or oligotrophic environments such as distilled water which is used for incubators of newborn ward 30. In the present study contamination degree of warm water with P. aeruginosa was higher (37.5%) than cold water (16.6%) and incubator water (6.8%). High temperatures of water can be favorable for the growth of P. aeruginosa 30. It also can be isolated even in the presence of 3.00?ppm residual free chlorine 30.A number of laboratory-based studies have demonstrated the colonization of heterogeneous biofilms developed from tap water microorganisms by pathogens such as E. coli 31, L. pneumophila 32, and P. aeruginosa 33. A number of studies showed that these bacteria are important opportunistic pathogens which can be involved in biofilm-associated contamination of domestic plumbing systems 34–37.De Abreu et al. presented that biofilm tap water was the major environmental source of pseudomonads in the healthcare facility 20. In the present study the frequency of P. aeruginosa with Metallo-?-lactamases was 37%. It should be noted that Metallo-?-lactamases producing strains have been increasingly reported worldwide and have been responsible for large outbreaks in several Asian, European, and American hospitals 38–42. Genes encoding Metallo-?-lactamases are generally located within class 1 integrons, together with those encoding aminoglycoside-modifying enzymes that confer multidrug resistance. In addition, the integrons harboring Metallo-?-lactamases determinants are frequently located on plasmids, certainly facilitating their intra- and interspecies spread 43. High prevalence of Metallo-?-lactamases producing epidemic or endemic strains is also found in other Mediterranean countries, such as Italy and Greece 44.Presence of L. pneumophila is associated with biofilms in warm water plumbing systems 45. This bacterium can also survive at lower temperatures in drinking water environments 46, 47. L. pneumophila, particularly strains within serogroups 1 and 6, is known as cause of both community acquired and hospital associated pneumonia 48, 49. Transmission to humans occurs through inhalation of aerosols generated from an environmental source. Several reports have demonstrated that the major sources for Legionnaires’ disease are the potable water systems of large buildings including hospitals, nursing home, and hotels 50.In a similar study, Yaslianifard et al. found considerable contamination rate of L. pneumophila and P. aeruginosain hospital water samples in Tehran, Iran 51. Shamabadi and Ebrahimi found low rate contamination with Gram-negative flora, moderate concentrations of Legionella, and potentially pathogenic nonenterobacterial species 28.In the present study we found the degree of presence of L. pneumophila in cold water was lower than in warm water and incubator water. It seems that high loads of P. aeruginosa can mask the presence of Legionella, leading to an underestimation in some cases. Many authors agree that different microbe species as well as high microbial loads can play an important role in limiting or even inhibiting the growth of Legionella 52, 53. In our study the total bacterial contamination of cold water samples was very high (93.3%). Actually overall pollution of hospital water systems with Legionella found in the present work was smaller compared to earlier studies made in Germany and Italy 54–57. In a USA national surveillance study of 20 hospitals in 13 states, 14 hospitals were colonized with Legionella in the water systems in which 43% of hospitals had environmental positive rates for Legionella 58. In a study in Spain on 12 hospitals, the environmental positive rates forLegionella were ?30% 59. L. pneumophila was also found in 63% of hospital water systems in Taiwan 52.The present study showed that care should be taken concerning cleanliness and decontamination of water supplies for pathogenic organisms. Chlorine has been widely used as disinfectant due to its low cost and effectiveness in many countries but it is not sufficient to ensure the safety of water. The maintenance of chlorine residue is needed at all points in distribution system supplied with chlorine as a disinfectant 60. The failure of conventional treatment processes to eliminate critical waterborne pathogens in drinking water demands that improved and/or new disinfection technologies be developed. Recent research has disclosed that nanotechnology may offer solutions in this area, through the use of nanosorbents, nanocatalysts, bioactive nanoparticles, nanostructured catalytic membranes, and nanoparticle-enhanced filtration 4.