Yeasts possess a range of environmental adaptations that allow them to colonize soil and sand. They can circulate seasonally between different components of lake ecosystems, including beach sand, water, and the coastal phyllosphere. The accumulation of people on beaches promotes the development and transmission of yeasts, posing an increasing sanitary and epidemiological risk. The aim of this study was to determine the species and quantitative composition of potentially pathogenic and pathogenic yeasts for humans present in the sand of supervised and unsupervised beaches along the shores of lakes in the city of Olsztyn (north-eastern Poland).

The study material consisted of sand samples collected during two summer seasons (2019; 2020) from 12 research sites on sandy beaches of 4 lakes located within the administrative boundaries of Olsztyn. Standard isolation and identification methods used in diagnostic mycological laboratories were applied.

A total of 259 yeast isolates (264, counting species in two-species isolates separately) belonging to 62 species representing 47 genera were obtained during the study. Among all the isolates, 5 were identified as mixed (two-species). Eight isolated species were classified into biosafety level 2 (BSL-2) and risk group 2 (RG-2). The highest average number of viable yeast cells was found in sand samples collected in July 2019 (5,56 x 10² CFU/g), August, and September 2020 (1,03 x 10³ CFU/g and 1,94 x 10³ CFU/g, respectively). The lowest concentrations were in samples collected in April, September, and October 2019, and October 2020 (1,48 x 10² CFU/g, 1,47 x 10² CFU/g, 1,40 x 10² CFU/g, and 1,40 x 10² CFU/g, respectively).