Abstract
Lactobacillus panis strain PM1 is an obligatory heterofermentative and aerotolerant microorganism that also produces 1,3-propanediol from glycerol. This study investigated the metabolic responses of L. panis PM1 to oxidative stress under aerobic conditions. Growth under aerobic culture triggered an early entrance of L. panis PM1 into the stationary phase along with marked changes in end-product profiles. A ten-fold higher concentration of hydrogen peroxide was accumulated during aerobic culture compared to microaerobic culture. This H2O2 level was sufficient for the complete inhibition of L. panis PM1 cell growth, along with a significant reduction in end-products typically found during anaerobic growth. In silico analysis revealed that L. panis possessed two genes for NADH oxidase and NADH peroxidase, but their expression levels were not significantly affected by the presence of oxygen. Specific activities for these two enzymes were observed in crude extracts from L. panis PM1. Enzyme assays demonstrated that the majority of the H2O2 in the culture media was the product of NADH: H2O2 oxidase which was constitutively-active under both aerobic and microaerobic conditions; whereas, NADH peroxidase was positively-activated by the presence of oxygen and had a long induction time in contrast to NADH oxidase. These observations indicated that a coupled NADH oxidase - NADH peroxidase system was the main oxidative stress resistance mechanism in L. panis PM1, and was regulated by oxygen availability. Under aerobic conditions, NADH is mainly reoxidized by the NADH oxidase - peroxidase system rather than through the production of ethanol (or 1,3-propanediol or succinic acid production if glycerol or citric acid is available). This system helped L. panis PM1 directly use oxygen in its energy metabolism by producing extra ATP in contrast to homofermentative lactobacilli.
Highlights
Lactobacillus panis PM1 is an aerotolerant and obligatory heterofermentative microorganism isolated from bioethanol thin stillage, and has been the focus of attention due to its ability to produce 1,3-propanediol (1,3-PDO) during the fermentation of glycerol under anaerobic conditions (Khan et al 2013)
Influence of oxygen on the physiology of L. panis PM1 The rates of growth during the first 24 hours of culture were similar between aerobic and microaerobic L. panis PM1; the aerobically cultured L. panis PM1 entered stationary phase earlier than the microaerobic culture (Figures 1a and b)
This early entry into stationary phase was associated with a halt in production of end-products, but not with glucose depletion (Figure 1a), indicating that glucose concentration was not a critical cause of the cellular growth interruption
Summary
Lactobacillus panis PM1 is an aerotolerant and obligatory heterofermentative microorganism isolated from bioethanol thin stillage, and has been the focus of attention due to its ability to produce 1,3-propanediol (1,3-PDO) during the fermentation of glycerol under anaerobic conditions (Khan et al 2013). Lactobacillus panis belongs to the group III heterofermentative lactobacilli, which includes L. brevis, L. buchneri and L. reuteri, where the 6-phosphogluconate/phosphoketolase In theory, when one glucose molecule is consumed, three NADH and one ATP molecules are generated. One pyruvic acid and one acetyl phosphate molecules accept protons from one and two NADH molecules, respectively, and regenerate NAD+. End-products of this metabolism are lactic acid and ethanol, respectively. Overall heterolactic fermentation of glucose through the 6-PG/PK pathway results in 1 mol each of lactic acid, ethanol, and CO2 and 1 mol ATP per mol glucose consumed (Kandler 1983)
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