Aerobic Respiration in Lactic Acid Bacteria: Current and Future Applications in Dairy Starter Culture

Aerobic Respiration in Lactic Acid Bacteria: Current and Future Applications in Dairy Starter Culture

Sarang Dilip Pophaly, Manorama Chauhan, Jitesh Tarak, Shekhar Banala Bashetty, Tejinder Pal Singh, Sudhir Kumar Tomar
Copyright: © 2018 |Pages: 14
DOI: 10.4018/978-1-5225-5363-2.ch005
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Abstract

Lactic acid bacteria (LAB) are used as food-grade microorganisms for production of a variety of fermented milk products. They are also the most common probiotic organisms used for making functional foods. Lactic acid bacteria are well known for their fermentative metabolism wherein they convert simple carbohydrates to organic acids and other end products. Fermentation helps the bacteria to generate ATP required for various cellular activities via substrate level phosphorylation reaction. Fermentation results in incomplete oxidation of substrate and hence is an inefficient process with a low ATP yield. However, some LAB are genetically capable of activating an auxiliary respiratory metabolism in which a quinol oxidase serves as the final electron acceptor and high ATP production is achieved due to oxidative phosphorylation. The respiratory process is associated with high biomass production of LAB and more robust bacterial cells, which are essentially required for manufacture of high viability starter culture. This chapter explores LAB's current and future applications in dairy starter cultures.
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Introduction

Lactic acid bacteria (LAB) consist of a group of Gram-positive bacteria that are united by certain morphological, metabolic and physiological characteristics. LAB are the workhorses of most food fermentation processes, and have been extensively studied for their metabolic and genetic properties. LAB are primarily used as starter cultures in manufacturing of diverse range of fermented products from milk, cereals etc. LAB are also used as probiotic bacteria for their physiological stress resistance and health promoting attributes (Ljungh & Wadstrom, 2006). LAB metabolism is fermentative in nature which means energy production in these bacteria is depended on the substrate level phosphorylation. Fermentation involves anaerobic breakdown of carbohydrates with an organic molecule acting as the final electron acceptor. Fermentative metabolism has peculiar disadvantages with regard to bacterial growth. First, since fermentation does not involve an electron transport system, it provides little energy due to partial breakdown of glucose. Secondly, fermentation results in high amount of lactic acid production in the medium which limits the growth of cells and do not allow to attain high biomass concentration. These hindrances limit the use of fermentation in manufacturing of commercial starter cultures and probiotics, which require high cell densities. These problems can be overcome by switching over from fermentative to respiratory metabolism (Pedersen et al., 2012).

The use of alternative terminal electron acceptor such as cytochrome oxidase enzyme allows the mounting of a conditional respiration system in LAB. Respiration is a high energy yielding process in which organic or reduced inorganic compounds are oxidized by inorganic compounds. Aerobic respiration involves glycolysis, citric acid cycle (Kreb’s Cycle) and an electron transport chain (ETC) in model organisms such as E. coli. However, LAB lack a functional Kreb’s Cycle and ETC. Instead, the heme dependent cytochrome quinol oxidase can act as a terminal electron acceptor for supporting respiration. This conditional respiration system generates a high biomass and also improves stress resistance in LAB. These advantages can be used for enhanced technological and physiological performance of LAB during industrial processing conditions and in-situ settings.

Fermentation

In biological systems, energy is produced or conserved through reduction-oxidation (redox) reactions. Redox reactions involve transfer of electrons between molecules. Oxidation refers to the removal of electrons and ‘reduction’ refers to the addition of electrons. Fermentation is basically an energy production reaction-using breakdown of organic molecules such as glucose to pyruvic acid. In this process, glucose is actually oxidized (stripped of electrons) and pyruvate is produced. The actual fermentative step involves conversion of pyruvate to lactate.

In fermentation an organic molecule serves as the final electron acceptor. It does not involve an electron transport chain and hence results in low ATP yield. Glycolysis is the initial stage of fermentation. In glycolysis, a six carbon glucose molecule is partially broken down into two, three carbon molecules of pyruvate, 2NADH, 2 H+ and 2 ATP as a result of substrate level phosphorylation. If NADH is not oxidized back to NAD+, there will be no further catabolism. Thus NADH produced during glycolysis should be oxidized by transferring its electron to an electron acceptor. Therefore, a suitable terminal electron acceptor must be used. The NADH donates its electrons to the pyruvate molecule formed during glycolysis and regenerates NAD+. In this process the pyruvate molecule is in turn reduced and results in formation of lactic acid. This step is catalyzed by lactate dehydrogenase enzyme.

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