Roles of L-malic acid on growth and glucose utilization of Leuconostoc oenos

Abstract

L-Malic acid was added to the culture medium and its effects on growth and metabolism of Leuconostoc oenos strains ML34 and A25 were observed. The level of pH in the culture solution during batch cultures was not controlled while it was maintained constant in continuous cultures. For the continuous cultures the amount of glucose in the culture medium was regulated to limit the growth. Samplings from the continuous cultures were made when each culture reached steady state. The batch culture results showed distinct stimulation of L-malic acid on the specific growth rates of both strains at all levels of initial pH``s with optimum around 4.5. The change of medium pH during the culture due to the malo-lactic fermentation did not correlate with the growth stimulation. It was also shown that L-malic acid did not serve as energy source in this case. Studies, therefore, were directed to explore mechanisms involved in the growth stimulation of L-malic acid. Carbon dioxide liberated from the malolactic fermentation was found to be partially responsible to the stimulation of growth of L. oenos cells by L-malic acid. The presence of L-malic acid in culture medium increased substrate affinity and reduced maintenance energy in these organisms substantially. These roles of L-malic acid could help to explain the growth stimulation of the compound. In addition to these mechanisms, the growth of L. oenos strains seems to be strongly accelerated by the rapid utilization of glucose stimulated by L-malic acid. The following evidence obtained from the present study explains L-malic acid stimulation on glucose utilization of the organisms. When 15 mM of L-malic acid was added to the culture medium having excess glucose, the synthesis of D-lactic dehydrogenase (D-LDH) in ML34 increased 5.5 fold and 2.3 fold in A25. L-malic acid stimulated the activity of D-LDH also ; the presence of L-malic acid in the reaction mixture almost doubled the enzyme reaction in both strains. It was also...