The bacterium Legionella pneumophila commonly inhabits artificially made water systems and causes respiratory illnesses. In contaminated systems, L. pneumophila forms biofilm which increases bacterial resistance to most commonly used methods for water disinfection such as thermal processing, chlorination, disinfection by UV radiation and ozone. Photodynamic inactivation is one of the potential methods for water disinfection and its antimicrobial effect is achieved by combining nontoxic components: photosensitizers, oxygen, and light. In this research three cationic porphyrinic photosensitizers were used, amphiphilic compound TMPyP3-C17H35 and two hydrophilic compounds TMPyP3-CH3 and TMPyP3. LED lamp that emits purple light was used as the light source. The focus of this thesis was to determine the minimal effective concentration of photosensitizers and to test the anti-adhesion and anti-biofilm properties of photosensitizers in waters of natural origin and different water hardness. All three photosensitizers were efficient in inhibiting the adhesion of L. pneumophila to polystyrene and against the formation of biofilm in concentrations that correspond to MEK values of 0.25×MEK and 0.5×MEK in soft water. The highest dark toxicity and the best efficiency of photodynamic inactivation were observed in demineralized water which suggests that concentration of dissolved ions in water i.e., water hardness, could have affected the efficiency of photodynamic inactivation.