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1 Swiss Federal Institute for Aquatic Science and Technology (Eawag), Überlandstrasse 133, PO Box 611, CH-8600 Dübendorf, Switzerland
2 Institute of Biogeochemistry and Pollutant Dynamics, ETH Zürich, 8092 Zürich, Switzerland
Correspondence
Thomas Egli
egli{at}eawag.ch
Growth of Vibrio cholerae O1 Ogawa Eltor was studied with a growth assay in which autoclaved and filtered (0.22 µm) freshwater was inoculated at low cell density (5x103 cells ml–1) and proliferation was followed with flow cytometry. Against the common view, V. cholerae was able to grow extensively in different kinds of freshwater. The bacterium multiplied in river water, lake water and effluent of a wastewater treatment plant up to a cell density of 1.55x106 cells ml–1. In these samples, apparent assimilable organic carbon (AOCapp) concentrations ranged from 52 up to 800 µg l–1 and the results demonstrate a positive trend between the AOCapp concentration and final cell concentration, suggesting that AOC was a key parameter governing growth of V. cholerae. No growth was observed in waters (tap and bottled drinking water) containing less than approximately 60 µg AOCapp l–1. When pure cultures of V. cholerae were grown on identical lake water at different temperatures (20, 25 and 30 °C) the maximum specific growth rates (µmax) achieved were 0.22 h–1, 0.32 h–1 and 0.45 h–1, respectively. In addition, growth was characterized in lake water samples amended with different concentrations of NaCl. The highest µmax of V. cholerae was recorded at moderate salinity levels (5 g NaCl l–1, µmax=0.84 h–1), whereas at 30 g NaCl l–1 (µmax=0.30 h–1) or 0 g NaCl l–1 (µmax=0.40 h–1) specific growth rates were significantly reduced. In the water tested here, µmax of V. cholerae was always around 50 % of that exhibited by a freshwater community of indigenous bacteria enriched from the water sampling site. Direct batch competition experiments between V. cholerae and the lake water bacterial community were performed at different temperatures in which V. cholerae was enumerated in the total community using fluorescent-surface antibodies. In all cases V. cholerae was able to grow and constituted around 10 % of the final total cell concentration of the community. No significant effect of temperature was observed on the outcome of the competition. Mathematical modelling of the competition at the different temperatures based on the calculated µmax values confirmed these experimental observations. The results demonstrate that V. cholerae is not only able to survive, but also able to grow in freshwater samples. In these experiments the bacterium was able to use a large fraction (12–62 %) of the AOCapp available to the bacterial AOC-test community, indicating that V. cholerae has the ability to gain access to the substrates present in freshwater even in competition with an autochthonous bacterial lake water consortium.
Two supplementary figures are available with the online version of this paper.
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