The effect of sunlight and other factors on the survival of starved enteric bacteria in natural waters
Results are reported from a series of experiments to determine the effects of certain environmental factors, namely, sunlight, temperature and , salinity on the survival of selected enteric bacteria which are of public health importance either as indicators of faecal pollution or The survival of the as pathogens. starved organisms in seawater and in freshwater microcosms exposed to artificial and natural sunlight, and in the dark, at a range of temperatures was investigated. An acridine orange direct viable count (AODVC) using an epifluorescent microscope was employed in addition to selective and non-selective cultural methods for estimation of decay rates of the test bacteria. The use of the AODVC allowed enumeration of those bacteria which respond to the stresses of the natural environment by entering into a viable but non-culturable form. These otherwise would not be detected, as they are, by definition, non-culturable using traditional enumeration techniques based upon the production of visible signs of growth. Two strategies were employed by the organisms in response to the unfavourable conditions. Prior growth of those members of the Enterobacteriaceae in a nutrient-poor medium before inoculation into the microcosms allowed adaptation of the cells to low nutrient concentrations, thus extending survival and resulting in higher resistance to other stresses such as visible light. This extension was, however, only temporary, the ultimate fate of the bacteria being death. Enterococci were particularly sensitive to low nutrient concentrations and died very rapidly in the light. All bacteria tested were able to adopt the viable but non-culturable strategy in the dark as a temporary measure, though it was only a matter of time before viability as well as culturability was also lost. This included enterococci, for which an AODVC using the antibiotic ciprofloxacin was developed by modifying the original nalidixic acid method. Responses of bacteria in the dark were influenced by temperature. Bright natural sunlight produced rapid death in bacteria exposed in seawater microcosms, a result of the synergistic interaction of salinity, UV light and possibly temperature too, whereas the presence of humic acids in freshwater afforded some protection to the cells by absorbing the damaging UV component of sunlight. Low intensities of UV and visible light typical of those found below the surface of water may induce bacteria to evolve towards a viable but non-culturable form.