Survival of protozoa in cooling tower biocides

Abstract

Protozoa from cooling towers may serve as hosts for legionellae, but such protozoa have not been examined with respect to effects of cooling tower biocides. In this study, two ciliate species, Tetrahymena sp and Colpoda sp, and two amoebae species, Vannella miroides and Acanthamoeba hatchetti, were isolated from a cooling tower and tested for survival in the presence of four cooling tower biocides. The protozoa were exposed for 24 h to a thiocarbamate compound, an isothiazolone compound, quaternary ammonium compounds (QAC), and tributyltin neodecanoate with quarternary ammonium compounds (TBT/QAC). After exposure, cells were examined for viability. The highest concentration of each biocide in which cells could survive was compared to the manufacturers' recommended maintenance dosage (MRMD) of the biocides. Tetrahymena and Colpoda survived concentrations within the range of the MRMD of thiocarbamate and QAC. Vannella and Acanthamoeba survived concentrations within the MRMD of thiocarbamate, isothiazolone, and QAC. Colpoda cysts and Acanthamoebae cysts remained viable after exposure to concentrations much greater than the MRMD of thiocarbamate, isothiazolone, and QAC. None of the protozoa in any stage could survive the MRMD of TBT/QAC. These results show that protozoa indigenous to cooling towers may survive the recommended concentration of certain biocides, and this information may be important in devising procedures for eradicating hosts for legionellae.