The aim of our research was the identification of nosocomial pathogens found in the internal drinking water network of healthcare facilities which can cause infections in hospitals. In this study, the composition of bacterial communities from the internal water network of a health institution in Győr (Hungary) was examined using standard culture-based methods. Identification of the bacterial isolates was performed using Analytical Profile Index (API). Members of 13 bacterial taxa were recovered from a multi-storey healthcare institution. 10 species were identified via API20E and API20NE testing, out of which isolates of genera Aeromonas, Pseudomonas and Sphingomonas were found in the highest proportion on different media. These bacteria can cause nosocomial infections in clinical environments, leading to serious illnesses mainly in patients, as they may have multiresistance genes. In addition, Legionella species were also identified in the water samples, which are also known to be nosocomial pathogens, since they can be spread with aerosols in hospital environments and can cause severe respiratory diseases in immunocompromised individuals.

Using standard and routine methods for drinking water microbiological quality control may lead to uncertainty. In some cases, biochemical tests for confirmation of presumptive colonies have negative results. It can also happen that the target bacteria cannot be found, but the high concentration of the background biota contaminates water sampling points or even the entire water supply system. The aim of the study was to investigate and identify bacterial colonies retrieved from water samples of drinking water supply systems using a relatively cheap, fast and easy-to-use method as well as to acquire information on the presence of pathogens in the background biota. To achieve our goals, water samples from 18 western Hungarian settlements were collected during a two month period. A total of 66 morphologically different bacterial colonies growing on four media were tested for taxonomic identification by three different API (Analytical Profile Index) tests which are suitable for the identification of bacteria at species level. Out of the 66 bacterial colonies involved in the study, 58 could be identified by API tests. During the identification, the presence of 27 different taxa were proved in the water samples. As a result of our research, species (Aeromonas hydrophila, Pseudomonas fluorescens, Burkholderia cepacia) that can cause confusion in the evaluation of water samples by forming morphologically typical colonies on the medium or presenting in high number of colony form units as background contaminants were identified. The presence of species (Pseudomonas aeruginosa, Aerococcus viridans, Aeromonas hydrophila, Gemella morbillorum, Streptococcus constellatus) which originated from the background biota on different agar plates and can cause human diseases, were also confirmed. Even if the organisms which indicate water quality degradation are not present in the water sample, the background biota may contain human pathogenic bacteria that might lead to health problems when enriched in the drinking water supply system.