Salt mines are often used for halotherapy against lung and skin diseases. In addition to salt, they also contain various types of microorganisms, which remain poorly characterised. Here, we examined culturable halophilic and non-halophilic microbial populations in relation to the physico-chemical characteristics in the air of four different sites of the Bochnia Salt Mine, a popular halotherapy resort in Southern Poland. At the mine entrance, the temperature was highest (20.8°C) and decreased with increasing distance from the entrance (15.5°C at 2671 m from entrance), while humidity increased from 55.9% to 77.0%, as did the NaCl concentration. At the entrance, non-halophilic microorganisms prevailed, especially fungi that grew at 21°C. Halophiles gradually dominated with distance from the entrance, including halophilic archaea that grew at 28°C or 37°C on medium containing
15%, 20%, or 25% NaCl. Seven halophilic archaeal species were identified by 16S rRNA gene sequencing.

The frequency of non-halophiles was inversely related to distance from the entrance, humidity, and presence of ions, while the reverse was seen for halophiles.

An exception was the site used for halotherapy, where non-halophilic bacteria dominated. Thus, natural salt mines contain a wide variety of non-halophilic and halophilic microorganisms, including archaea, which may contribute to the halotherapeutic effects.