Bioenergetic consequences of lactose starvation for continuously cultured Streptococcus cremoris

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Abstract
Streptococcus cremoris cells that had been grown in a chemostat were starved for lactose. The viability of the culture remained essentially constant in the first hours of starvation and subsequently declined logarithmically. The viability pattern during starvation varied with the previously imposed growth rates. The death rates were 0.029, 0.076, and 0.298 h-1 for cells grown at dilution rates of 0.07, 0.11 and 0.38 h-1, respectively. The proton motive force and the pools of energy-rich phosphorylated intermediates in cells grown at a dilution rate of 0.10 h-1 fell to zero within 2 h of starvation. The culture, however, remained fully viable for at least 20 h, indicating that these energy-rich intermediates are not crucial for survival during long-term lactose starvation. Upon starvation, the intracellular pools of several amino acids depleted with the proton motive force, while large concentration gradients of the amino acids alanine, glycine, aspartate, and glutamate were retained for several hours. A quantitative analysis of the amino acids released indicated that nonspecific protein degradation was not a major cause of the loss in viability. The response of the energy metabolism of starved S. cremoris cells upon refeeding with lactose was monitored. Upon lactose starvation, the glycolytic activity and the rate of proton motive force generation decreased rapidly but the steady-state level of the proton motive force decreased significantly only after several hours. The decreasing steady-state level of the proton motive force and consequently the capacity to accumulate amino acids after the addition of lactose correlated well with the loss of viability. The response of the energy metabolism of starved S. cremoris cells upon refeeding with lactose was monitored. Upon lactose starvation, the glycolytic activity and the rate of proton motive force generation decreased rapidly but the steady-state level of the proton motive force decreased significantly only after several hours. The decreasing steady-state level of the proton motive force and consequently the capacity to accumulate amino acids after the addition of lactose correlated well with the loss of viability. It is concluded that a regulatory loss of glycolytic capacity has pivotal role in the survival of S. cremoris under the conditions used.