Introduction: One of the key approaches to assessing the quality of bathing waters is the determination of microbiological contamination, primarily through the detection of faecal indicator organisms such as Escherichia coli. These bacteria are part of the normal intestinal microbiota of warm-blooded animals; however, their increased presence in surface waters indicates faecal pollution. Various microbiological methods are used to detect bacteria in surface waters. Purpose: The aim of this study was to assess the microbiological quality of bathing water in Lake Velenje and to compare two alternative methods for determining microbiological contamination of bathing waters with the standard membrane filtration method. In addition, the advantages and limitations of the methods were analysed, and their applicability was evaluated with regard to reliability, time efficiency, and sustainability aspects of analytical procedures. Methods: Water samples were collected from three sampling sites: the bathing area at Lake Velenje, the inflow of the Ljubela River into Lake Velenje, and Lake Škale. Sampling was conducted during the bathing season in accordance with the Regulation on the Management of Bathing Water Quality. The study compared three methods for the detection of Escherichia coli in surface waters: the standard membrane filtration method, the Microbium® MPN analyser, and the Colikat® Rapid method. Results: Physico-chemical measurements revealed seasonal variations in water temperature and the influence of environmental factors on the results of microbiological analyses. The microbiological results showed that the alternative methods generally detected higher concentrations of E. coli compared to membrane filtration; however, the overall trends in bacterial concentration changes were comparable between the methods. The influence of environmental factors on the microbiological quality of water was evident at all sampling sites. Discussion and conclusion: The results indicated that E. coli concentrations at the bathing area of Lake Velenje did not exceed the legally prescribed limit values throughout the entire monitoring period. Consequently, the bathing water was classified as having excellent quality. Higher and more variable concentrations were observed at sampling sites more exposed to catchment influences and precipitation events. Although absolute concentration values obtained by the Microbium® MPN analyser and the Colikat® are not directly comparable, but proved effective in detecting changes in microbiological water quality. These alternative methods can significantly contribute to faster and more operationally feasible monitoring of bathing waters, which is essential for public health protection and the effective management of natural bathing sites.
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