Occurrence of Shewanella algae in Danish Coastal Water and Effects of Water Temperature and Culture Conditions on Its Survival

Abstract

ABSTRACT The marine bacterium Shewanella algae , which was identified as the cause of human cases of bacteremia and ear infections in Denmark in the summers of 1994 and 1995, was detected in seawater only during the months (July, August, September, and October) when the water temperature was above 13°C. The bacterium is a typical mesophilic organism, and model experiments were conducted to elucidate the fate of the organism under cold and nutrient-limited conditions. The culturable count of S. algae decreased rapidly from 10 7 CFU/ml to 10 1 CFU/ml in approximately 1 month when cells grown at 20 to 37°C were exposed to cold (2°C) seawater. In contrast, the culturable count of cells exposed to warmer seawater (10 to 25°C) remained constant. Allowing the bacterium a transition period in seawater at 20°C before exposure to the 2°C seawater resulted in 100% survival over a period of 1 to 2 months. The cold protection offered by this transition (starvation) probably explains the ability of the organism to persist in Danish seawater despite very low (0 to 1°C) winter water temperatures. The culturable counts of samples kept at 2°C increased to 10 5 to 10 7 CFU/ml at room temperature. Most probable number analysis showed this result to be due to regrowth rather than resuscitation. It was hypothesized that S. algae would survive cold exposure better if in the biofilm state; however, culturable counts from S. algae biofilms decreased as rapidly as did counts of planktonic cells.