In this paper we focus on the evaluation of toxicity and phototoxicity of phenothiazine dye methylene blue on selected bacteria (Escherichia coli CCM 3988, Pseudomonas aeruginosa CCM 3955, Salmonella enterica ssp. Enterica serovar Enteritidis CCM 4420 and Bacillus subtilis CCM 1999), which are part of the activated sludge. The results obtained demonstrate that the induction of photodynamic effect is showed. Increased photoantibacterial activity may lead to inhibition of bacterial metabolic activity in the sludge. This effect may represent a potential risk to the ongoing processes in wastewater treatment.