Factors Influencing Survival of Legionella pneumophila Serotype 1 in Hot Spring Water and Tap Water

Abstract

ABSTRACT The factors involved in the survival of Legionella pneumophila in the microcosms of both hot spring water and tap water were studied by examining cultivability and metabolic activity. L. pneumophila could survive by maintaining metabolic activity but was noncultivable in all microcosms at 42°C, except for one microcosm with a pH of <2.0. Lower temperatures supported survival without loss of cultivability. The cultivability declined with increasing temperature, although metabolic activity was observed at temperatures of up to 45°C. The optimal range of pH for survival was between 6.0 and 8. The metabolic activity could be maintained for long periods even in microcosms with high concentrations of salt. The cultivability of organisms in the post-exponential phase in a tap water microcosm with a low inoculum size was more rapidly reduced than that of organisms in the exponential phase. In contrast, the loss of cultivability in microcosms of a high inoculum size was significant in the exponential phase. Random(ly) amplified polymorphic DNA analysis of microcosms where cultivability was lost but metabolic activity was retained showed no change compared to cells grown freshly, although an effect on the amplified DNA band pattern by production of stress proteins was expected. Resuscitation by the addition of Acanthamoeba castellanii to the microcosm in which cultivability was completely lost but metabolic activity was maintained was observed only in part of the cell population. Our results suggest that L. pneumophila cell populations can potentially survive as free organisms for long periods by maintaining metabolic activity but temporarily losing cultivability under strict environments and requiring resuscitation by ingestion by amoebas.