Effects of pH and trace minerals on long-term starvation of Leuconostoc mesenteroides

Laboratory experiments have definitively shown that exopolymer-producing ba cteria have the potential to modify the flow of fluids in oil reservoirs to enhance oil production. Once injected into the reservoir, they will be sub jected to a wide range of pH values and to starvation resulting from nutrie nt depletion. For successful field implementation it is necessary to have a fundamental understanding of these effects on the viability of bacteria. T his paper addresses the effects of pH and trace minerals on cell viability of Leuconostoc mesenteroides during carbon source depletion. Two different carbon sources were used to grow cells before transferring the cells to sta rvation conditions: sucrose and a combination of glucose and fructose. Thes e substrates were chosen because L. mesenteroides produces a significant am ount of water-insoluble exopolymers (dextran) under sucrose-fed conditions, which may enhance cell survival under harsh conditions. The effects of dex tran on the cell viability were tested at different pH values with and with out trace minerals. The rate of cell death followed an exponential-decay la w for different values of the solution pH. The optimal solution pH for surv ival was pH 5, whereas cells died rapidly at pH 3 and below and at pH 13 an d above. The sucrose-fed cells showed a greater viability than cells fed gl ucose and fructose for all pH ranges tested. The results indicated that wat er-insoluble exopolymers help cells survive for longer periods of time unde r starvation conditions. The effects of trace minerals on cell culturabilit y were tested at two pH values, 4.5 and 7. For both cases, cells showed a g reater culturability (smaller decay rate constant) in the presence of trace minerals than without trace minerals. It was also found that the effects o f trace minerals on cell culturability were greater for glucose-fructose-fe d cells than for sucrose-fed cells. The Michaelis pH function theory was us ed for comparing the relationships between the cell decay rate and pH.