Black Pigmented Biofilms Occurring on Water Taps – Characterization, Causes and Countermeasures

Abstract

In recent years the occurrence of slimy black biofilms on individual water taps, shower heads and toilet tanks was reported from various drinking water supply areas in Germany. In the course of the investigations presented here, the herpotrichiellaceous black yeast Exophiala lecanii-corni could be identified as the main constituent of these biofilms. This species could not be detected in any sample taken from corresponding public drinking water production, treatment and distribution facilities, but was found to be common in sinks of sanitary installations. Thus, retrograde contamination of the water taps as it is known for Pseudomonas aeruginosa appears to be likely. Volatile organic compounds from indoor air, which serve the fungi as carbon and energy source, seem to be an important cofactor in the formation of the respective biofilms. Comparative nephelometrically recorded growth kinetics with E. lecanii-corni showed that acetic acid and ethanol, common components of cleaning agents and disinfectants, can be utilized by this fungi already at low concentrations. Lab experiments under otherwise optimal nutrient conditions revealed a growth-promoting effect of phosphate at a concentration of 0.75 mg/L PO43- and higher. SDS turned out to be a very effective alternative to more aggressive conventional disinfectants for biofilm control and elimination. A comprehensive evaluation concerning the risk arising from the fungi identified in the biofilms revealed that health hazards to humans are rather unlikely.