Glucose Metabolism in Lactococcus lactis MG1363 under Different Aeration Conditions: Requirement of Acetate To Sustain Growth under Microaerobic Conditions (original) (raw)

2003, Applied and Environmental Microbiology

Lactococcus lactis subsp. lactis MG1363 was grown in batch cultures on a defined medium with glucose as the energy source under different aeration conditions, namely, anaerobic conditions, aerobic conditions, and microaerobic conditions with a dissolved oxygen tension of 5% (when saturation with air was used as the reference). The maximum specific growth rate was high (0.78 to 0.91 h ؊1) under all aeration conditions but decreased with increasing aeration, and more than 90% of the glucose was converted to lactate. However, a shift in by-product formation was observed. Increasing aeration resulted in acetate, CO 2 , and acetoin replacing formate and ethanol as end products. Under microaerobic conditions, growth came to a gradual halt, although more than 60% of the glucose was still left. A decline in growth was not observed during microaerobic cultivation when acetate was added to the medium. We hypothesize that the decline in growth was due to a lack of acetyl coenzyme A (acetyl-CoA) needed for fatty acid synthesis since acetyl-CoA can be synthesized from acetate by means of acetate kinase and phosphotransacetylase activities. MATERIALS AND METHODS Strain and growth conditions. The bacterium used throughout this study was the homofermentative laboratory strain L. lactis subsp. lactis MG1363 (14). This microorganism was grown in in-house bioreactors with a working volume of 1.0 liter under anaerobic and aerobic conditions or in an MBR bioreactor (MBR Bio Reactor AG, Wetzikon, Switzerland) with a working volume of 1.5 liters under microaerobic conditions. All cultures were incubated at 30°C, and the baffled bioreactors were fitted with four-blade Rushton turbines rotating at 350 rpm. A constant pH of 6.6 was maintained by automatic addition of 10 M KOH. Unless stated otherwise, the cells were grown in defined MS10 medium (7) supplemented with the following components to allow growth under aerobic conditions: MnSO 4 (1.25 ϫ 10 Ϫ5 g ⅐ liter Ϫ1), thiamine (1 mg ⅐ liter Ϫ1), and DL-6,8-thioctic acid (2.5 mg ⅐ liter Ϫ1). The glucose concentration was 10 g ⅐