SURVIVAL OF ALLOCHTHONOUS BACTERIA IN AQUATIC SYSTEMS - A BIOLOGICAL APPROACH

The survival of allochthonous bacteria in aquatic systems is affected by biotic and abiotic environmental factors. Grazing by protozoa is on e of the main biological processes that control allochthonous bacteria l density. Its extent depends on the concentration of bacteria and the digestion capacity of the grazer. The physiological state of bacteria is affected by multiple physicochemical stresses, to which they respo nd by entering a dormant, viable but non-culturable state. Starved bac teria show a tendency to shrink, and a generally enhanced resistance t o heat, oxidative and osmotic shock is observed. Nutrient scarcity, te mperature, osmotic stress and visible light seem to be the abiotic fac tors that most negatively influence survival. The negative effect of l ight upon the culturability of enterobacteria in aquatic systems has l ong been recognized. In relation to the influence of plasmids on bacte rial survival, heterogeneous and contradictory results have been repor ted. Some authors reported that plasmid-bearing strains can survive as well as their wild-type counterparts or even better, whereas in other reports the effects of various plasmids on the survival of their host s were very variable. Plasmid transfer could be affected by the physio logical status of donors and recipients during survival. Flow cytometr y is a recent approach with great potential, especially for assessing the heterogeneity of cell size, metabolic state and molecular content in the population.