Background. Civilizational changes, including urbanisation and industrialisation, created the
conditions for the development of new pathogens colonising water networks which endanger
human health and life. The aim of the study was to assess the contamination of water supply
systems in public buildings by Legionella spp. a pathogenic group of Gram-negative bacteria.

Material and methods. The material samples were taken from the hot water supply systems
of 41 buildings: school dormitories, boarding schools, educational centres, student dormitories.

Results. In all analysed years, there were samples in which the applicable standards were
exceeded, as specified by the regulation on the quality of water intended for human consumption.
The highest percentage of negative samples was found in 2012, with as many as 81.1%, and the
lowest one in 2011 − 35.3%. Positive samples obtained in the first and subsequent tests were
detected in 20.0% of the tested facilities. The change from positive to negative values was found
in 30.0% of the tested water networks. There were also cases of negative samples both in the
first and subsequent tests, which was detected in 50.0% of the facilities.

Conclusions. 1. An excessive amount of the officially permissible value of Legionella was found
in 53.9% of the examined hot water samples. 2. The obtained results confirm the necessity of
modernising and doing elementary repairs in water supply networks.