Survival, physiological response and recovery of enteric bacteria exposed to a polar marine environment

Abstract

Survival, sublethal injury, and recoverability of Escherichia coli, Enterococcus faecalis, Salmonella typhimurium, and Yersinia enterocolitica were investigated by using diffusion chambers over 54 to 56 days of in situ exposure to a polar marine environment (-1.8 degrees C; salinity, 34.5 ppt) at McMurdo Station, Antarctica. Plate counts were used to assess recoverability and injury, whereas direct viable counts (DVCs) and 5-cyano-2,3-ditolyl tetrazolium chloride (CTC) reduction were utilized to determine substrate responsiveness and respiratory activity, respectively. T90 values (times for 10-fold decreases in numbers of recoverable cells) on nonselective medium were ca. 216 to 259 h for E. coli, S. typhimurium, and Y. enterocolitica and 432 h for E. faecalis. Sublethal injury was greater in populations of indicator bacteria than in pathogens. DVCs, CTC reduction, and plate counts indicated progressive increases in viable but nonculturable cells in E. coli, S. typhimurium, and Y. enterocolitica cultures throughout the 54-day exposure. Forty-eight-day exposure of E. coli, S. typhimurium, and Y. enterocolitica resulted in decreased optimal incubation temperatures for colony formation and inability to form colonies at 37 degrees C. The detection of responsive E. coli, S. typhimurium, and Y. enterocolitica by the DVC and CTC methods remained within 1% of inoculum values during 54 days of exposure, indicating some long-term persistence in the viable-but-nonculturable state. Percentages of respiring E. coli and S. typhimurium increased significantly upon addition of nutrients at all temperatures tested, indicating that nutrient availability rather than temperature limited enteric bacterial activity in this very cold environment. Large nutrient inputs to low-temperature marine environments may thus allow for the long-term persistence of enteric bacteria in a nonrecoverable state.