Occurrence of Shewanella algae in Danish coastal water and effects of water temperature and culture conditions on its survival

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 degrees 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 degrees C were exposed to cold (2 degrees C) seawater. In contrast, the culturable count of cells exposed to warmer seawater (10 to 25 degrees C) remained constant. Allowing the bacterium a transition period in seawater at 20 degrees C before exposure to the 2 degrees 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 degrees C) winter water temperatures. The culturable counts of samples kept at 2 degrees 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.

FIG. 1

Changes in water temperature and bacterial levels in waters of Køge Bugt from September 1996 to October 1997. TVC, total viable count on iron agar; H₂S, hydrogen sulfide producers on iron agar.

FIG. 2

Changes in culturable counts of S. algae precultured at 37°C and exposed to sterile seawater at 2°C (▴), 10°C (●), or 25°C (■). Arrows indicate counts below the detection limit of 10 CFU/ml.

FIG. 3

Changes in culturable counts of S. algae precultured at 37 or 20°C and exposed to sterile seawater at 2°C with or without a transition period in seawater at 20°C. Symbols: □ and ○, S. algae cultured at 20°C; ▴ and ▾, S. algae cultured at 37°C; □ and ▴, exposure to sterile 20°C seawater for 14 days before exposure to 2°C; ○ and ▾, exposure to 2°C seawater. Arrows indicate counts below the detection limit of 10 CFU/ml. Error bars indicate standard deviations.

FIG. 4

Changes in conductometric detection times (indicating culturable counts) of biofilms of S. algae exposed to chilled (2°C) or temperate (25°C) seawater. Data are from two separate experiments (experiment 1, 2°C [■] and 25°C [□]; experiment 2, 2°C [●] and 25°C [○]). Error bars indicate standard deviations.