Metabolic Behavior of Lactococcus lactis MG1363 in Microaerobic Continuous Cultivation at a Low Dilution Rate

Abstract

Minute amounts of oxygen were supplied to a continuous cultivation of Lactococcus lactis subsp. cremoris MG1363 grown on a defined glucose-limited medium at a dilution rate of 0.1 h ؊1. More than 80% of the carbon supplied with glucose ended up in fermentation products other than lactate. Addition of even minute amounts of oxygen increased the yield of biomass on glucose by more than 10% compared to that obtained under anaerobic conditions and had a dramatic impact on catabolic enzyme activities and hence on the distribution of carbon at the pyruvate branch point. Increasing aeration caused carbon dioxide and acetate to replace formate and ethanol as catabolic end products while hardly affecting the production of either acetoin or lactate. The negative impact of oxygen on the synthesis of pyruvate formate lyase was confirmed. Moreover, oxygen was shown to down regulate the protein level of alcohol dehydrogenase while increasing the enzyme activity levels of the pyruvate dehydrogenase complex, ␣-acetolactate synthase, and the NADH oxidases. Lactate dehydrogenase and glyceraldehyde dehydrogenase enzyme activity levels were unaffected by aeration.Homofermentative lactic acid bacteria (LAB) are used primarily in the dairy industry but may also prove to be advantageous hosts for production of lactate as a bulk chemical or of food additives such as bacteriocins or amino acids using recombinant DNA technology (10, 15). The metabolism of these bacteria is constrained by the requirement for a balance between NADH-producing and -consuming reactions. In the absence of external electron acceptors (e.g., oxygen), the carbon fluxes of catabolism are tightly coupled. In anaerobic culture, glucose may be redox-neutrally converted to lactate or, alternatively, to the mixed acid products formate, acetate, and ethanol in a molar ratio of 2:1:1 (12, 26). However, when oxygen is present in the growth medium, the catabolic carbon fluxes are uncoupled from the redox metabolism due to the NAD ϩ -regenerating activity of NADH oxidases (NOX). This oxygenrelated potential for redirecting fluxes is used industrially in connection with the formation of other metabolites such as diacetyl in fermented milk products. Even submillimolar concentrations of diacetyl have a strong impact on the taste of buttermilk and cheese (22). Hence, minute amounts of oxygen may influence an industrial process greatly. In this light, it is surprising that, to our knowledge, no investigations have been carried out on the microaerobic physiology of starter cultures. A number of studies on the effect of oxygen in fully aerated cultures have been published, but little light has been shed on the window ranging from anaerobic to fully aerobic conditions of growth (4,7,8,20).Interest in the microaerobic physiology of LAB is furthermore spurred by reports on the extreme sensitivity of pyruvate formate-lyase (PFL) to oxygen (1, 25) and the negative effect of oxygen on pfl gene expression (2,19). Similarly, it has recently been reported that vigorous aeration reduces the...

Bet-hedging during bacterial diauxic shift

“…S5). When L. lactis grows on glucose, it mainly produces lactate (homolactic fermentation), whereas upon slower growth on cellobiose, it shifts to the energetically more-efficient heterolactic fermentation (17,18). An essential enzyme in the latter pathway is acetate kinase AckA.…”

Enantioselective lactic acid production by an Enterococcus faecium strain showing potential in agro-industrial waste bioconversion: Physiological and proteomic studies

“…The induction of PDH during growth in aerobiosis has already been reported for other enterococci and LAB (Snoep et al, 1992;Jensen et al, 2001). PDH competes with other enzymes such as lactate dehydrogenase (LDH) for intracellular pyruvate.…”

Trophic interaction of the aerotolerant anaerobe Clostridium intestinale and the acetogen Sporomusa rhizae sp. nov. isolated from roots of the black needlerush Juncus roemerianus

“…RC's tolerance to highly oxic conditions illustrates that clostridial species in the rhizosphere of estuarine plants can have extraordinary tolerance to O 2 . Under highly oxic conditions, the catabolism of RC shifted to a nearly homolactate fermentation, a form of anaerobic catabolism utilized by a variety of aerotolerant fermenters (Jensen et al, 2001;van Niel et al, 2002;Matthies et al, 2004;Madigan & Martinko, 2005). Butyrate fermentation necessitates the use of O 2 -sensitive hydrogenase (Buckel, 2005), and RC's capacity to engage homolactate fermentation is likely the primary mechanism for its ability to cope with high amounts of O 2 .…”