Variation in the activity levels of selected enzymes of Erwinia amylovora 595 in response to changes in dissolved oxygen tension and growth rate of D-glucose-limited chemostat cultures.

Abstract

Chemostat cultures of Erwinia amylovora 595, grown in mineral salts-nicotinic acid medium at 30 degrees C, and limited by D-glucose concentrations in the presence of dissolved oxygen tensions (D.O.T.) greater than about 6mm Hg, became limited by oxygen availability below about 4 mm Hg. This latter limitation was accompanied by a marked increase in acid production as the D.O.T. was depressed. The transition between D-glucose- and oxygen-limitation was also characterized by a maximum in succinate oxidase activity, and a minimum in the in situ respiration. D-Glyceraldehyde-3-phosphate dehydrogenase and D-fructose-1, 6-diphosphate aldolase showed small reductions in specific activity in the region 4-6 mm Hg D.O.T., but further reduction to 2 mm Hg resulted in a marked increase in the specific activity of aldolase. Malate dehydrogenase followed the converse trend, and attained very low activity levels when the D.O.T. decreased beyond the lower limits of detection. The in situ respiration was maximal at 2 mm Hg D.O.T., while potential respiration values were minimal at 2 mm Hg, and maximal at about 8 mm Hg D.O.T. The insitu respiration rate was proportional to dilution rate (D), in presence of excess oxygen, up to 0.18 h-1, after which a marked diminution occurred and continued until the wash-out rate was attained. Succinate oxidase activity decreased with increase in dilution rate, but remained constant above D equals 0.18 h-1. Malate dehydrogenase showed a persistent decline with increase in dilution rate, while D-glyceraldehyde-3-phosphate activity increased somehwat at higher dilution rates. The data are interpreted in terms of two transition points, at 6 and 2 mm Hg D.O.T., and of a change from respiratory to fermentative metabolism at low D.O.T., and at high dilution rates.