Estilo de Liderazgo

In aquatic environments, the survival of bacteria is strongly dependent on the presence of nutrient sources. Nutrients allow the bacterial cells to build up necessary material for cell biomass and for the production of biologically utilizable energy and are required for some functions such as cellular motility (Dills et al., 1980). It has been reported by several researchers that the addition of organic and inorganic nutrient supplements to water enables coliform bacteria to reduce their rate of disappearance from the aquatic environments (Carlucci and Pramer, 1960; Lim and Flint, 1989). Carlucci et al. (1986) showed that only 15% of the bacteria capable of growing on a rich medium were able to grow on unsupplemented seawater agar. Chai (1983) noted that nutrient supplementation of natural waters delayed the reduction in the number of viable cells but numbers did decrease even though direct count revealed can increase in count. He also demonstrated that rich medium grown cells survived for a longer period than those grown in a basal medium. Flint (1987) pointed out that E.coli could grow better in a sewage-polluted river water than at less polluted site.

In natural aquatic environments most bacteria exist under nutrient-limited conditions. Therefore microbial growth in natural aquatic environment must occur only under the stress of starvation in most aquatic environments (Veldkamp and Jannasch, 1972; Tempest and Neijssel, 1981).

There have been many reports of starvation-induced changes in bacterial cells in aquatic environments (Dawes, 1976; Morita, 1982; Kjelleberg et al., 1987). These changes appear as morphological and physiological alterations in bacterial cells (Morita, 1982; Kjelleberg et al., 1982). A significant reduction in

cell size is generally accepted as a general phenomenon when cells are subjected to nutrient deprivation (Novitsky and Morita, 1977; Smigielsky et al., 1989, 1990). Such morphological changes in cell size has been termed 'miniaturisation' (Morita, 1982). MacDonell and Hood (1982) showed that

Vibrio, Pseudomonas, Aeromonas and Alcaligenes spp. were found as very

small cells and considered this to be an adaptation to the nutrient-poor environment.

Many bacteria produce specialised forms such as endospores of Bacillus spp and the exospore of Streptomyces spp. in response to starvation stress. Non­ spore forming bacteria can also survive for a long-time under conditions of nutrient depletion by using endogenous energy supplies, reducing cell size ratio and having a low metabolic activity (Morita, 1982). Flint (1987) reported that

E.coli could survive for 260 d in river water without the addition of any nutrient

sources at 25 °C when the natural microbial flora had been eliminated by autoclaving. To survive for this period of time without addition of any nutrient

E.coli must enter a dormant state similar to that described by Morita and his co­

workers for a marine Vibrio under nutrient-limited conditions (Morita, 1982). Extreme starvation stress on bacteria including E.coli causes cells to enter a dormant state, that is they can become viable but non-culturable which is very important for the detection of indicator bacteria (Novitsky and Morita, 1977; Dawson et al., 1981; Colwell et al., 1985). This phenomenon in E.coli and S.

typhimurium examined in water by Roszak and Colwell (1987) who showed that

these bacteria were not able to grow on standard culture media but the cells were detectable by direct viable count techniques.

Other common alterations in starved bacterial cells include an increased tendency for adhesion and change in cell hydrophobicity. This might be important during substrate changes (Dawson et al., 1981; Kjelleberg and Hermans son, 1984; 1987). Because surface-active organic molecules are accumulated at surfaces (Hunter, 1980) these nutrients can be utilised by

surface-attached bacteria (Hermansson and Marshall, 1985). Dawson et al. (1981) showed that during the initial phase of starvation some fimbriae were formed by the marine Vibrio DW1 and starvation increased adhesion to a glass surface. Kjelleberg and Hermansson (1984) found that the degree of hydrophobicity of cells was increased in Vibrio by starvation stress and suggested that this was due to increased fimbriation or due to change in other cell surface components such as lipopolysaccharide.

Some physiological and metabolic alterations have been investigated, mostly in marine bacteria in response to starvation stress. Generally these alterations occur in transport mechanisms and nutrient uptake, the proton motive force, the stringent response, the degradation of reserve polymers and endogenous macromolecules, the release of catabolite repression which may be required in order that different substrates from the environment may be utilised (Kjelleberg and Hermansson, 1987). Copiotrophic bacteria must undergo a number of physiological and metabolic adaptations in the nutrient-depleted environments in order to meet their energy requirements and improve their chances of long-term survival. These include a low respiration rate (Novitsky and Morita, 1977), the regulation and induction of starvation specific proteins (Reeve et al., 1984; Groat and Matin, 1986; Nystrdm et al., 1986; Jouper-Jaan

et al., 1986) and the use of endogenous reserve material (Jones and Rhodes-

Roberts, 1981). Furthermore the ability to scavenge nutrients efficiently from the environment and transport them into the cells are also important under nutrient-limited conditions (Morita, 1984). The most significant changes occur during the initial transient phase of starvation. These include a rapid decline in the total amount of carbohydrate and lipid (Hood et al., 1986) and polyhydroxybutyrate (Malmcrona-Friberg et al., 1986) in the the cell, an increase in the ratio of unsaturated to saturated fatty acids in the membrane which may increase fluidity of the membrane (Malmcrona-Friberg et al., 1986),

and a temporary increase in amino acid uptake and incorporation (Nystrflm et al., 1986).