Graduate Thesis Or Dissertation
 

Some physiological bases for survival of a marine bacterium during nutrient starvation

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  • The physiological responses of populations of a marine Pseudomonas sp. to nutrient starvation were observed for a period of 40 days. During the first 25 days of starvation the viability of the population decreased by 99.9 percent, but thereafter the cultures maintained approximately 5 x 10⁵ viable cells per ml (0.1 percent of the original concentration) for periods in excess of one year. At regular time intervals during the 40 days of starvation the number of viable cells present was determined by spread plating and the total number of cells by direct epifluorescent counts. The number of actively respiring cells was determined by the method of Zimmermann, Iturriaga, and Becker-Birck (1978) using 2-(p iodophenyl )- 3- (p- nitrophenyl) -5- phenyl tetrazolium chloride (INT). Changes in the adenosine triphosphate (ATP) content of the cells were assessed by the firefly bioluminescence assay. Modifications of the uptake systems for glucose and glutamic acid were observed by isotope experiments which determined both the rates of uptake and the percent respiration for each exogenous substrate. The rate of endogenous respiration was determined from the rate of ¹⁴CO₂ evolution from a starving population. Three possible interpretations of the results are presented based on the three different cell counts carried out. It is hypothesized that although the majority of the cells in a starving population die, there is a fraction which somehow adapts to the conditions and survives indefinitely by maintaining its ATP content and respiratory activities at pre-starvation levels.
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