MICROBIOLOGY Study Guide - 2012

CHAPTER 9. MICROBIAL PHYSIOLOGY

9.5. FERMENTATION

9.5.2. Homofermentative Lactic Acid Fermentation

Characteristics of Lactic acid Bacteria. Lactic acid bacteria (LAB) comprise a specific group of microorganisms whose main distinguishing feature is The production of lactic acid as the primary product of Fermentation.

Lactic acid bacteria have complex nutritional requirements, which is why they are rarely found in natural bodies of Water or soil. They are most commonly found in milk and dairy products, on plants and decomposing plant matter, as well as in the gastrointestinal tract and on the mucous membranes of humans and animals.

Based on Cell Morphology, they are divided into spherical (cocci) and rod-shaped (bacilli) forms. Lactic acid bacteria are Gram-positive, mostly non-motile, and generally do not form spores (with the exception of the atypical species Sporolactobacillus inulinus, isolated from silage, which is motile and spore-forming).

Regarding their DNA nucleotide composition, the group of lactic acid bacteria is highly heterogeneous, with the molar percentage of G+C Base Pairs ranging from 32% to 52%.

Lactic acid bacteria are classified as facultative anaerobes. However, unlike members of the Enterobacteriaceae family, they lack Hemoproteins (Cytochromes and catalase), and Lactic acid fermentation is their sole mechanism for ATP synthesis. Nevertheless, lactobacilli can grow in the presence of atmospheric oxygen, acting as aerotolerant anaerobes. Lactobacilli are the only group of bacteria that lack catalase yet are capable of growing in the presence of air. Catalase is an enzyme that breaks down hydrogen peroxide—formed during substrate oxidation—into water and oxygen. In lactic acid bacteria, this function is performed by peroxidase. The absence of catalase activity combined with The ability to grow under aerobic conditions serves as one of the key diagnostic tests for identifying this group of microorganisms.

Unlike most other microorganisms, lactic acid bacteria are capable of breaking down milk sugar, or lactose. To catabolize lactose, lactobacilli use the enzyme β-galactosidase to cleave it into two hexose sugars:

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Because lactobacilli accumulate lactic acid during their metabolic activity, they are relatively acid-tolerant and capable of growing at low pH values (3.5–3.0).

Depending on their final metabolic products, lactic acid bacteria are divided into homofermentative (which degrade sugars via the Hexose diphosphate pathway) and heterofermentative (which degrade sugars via the Pentose Phosphate Pathway) (Table 7).

Table 7. Selected representatives of lactic acid bacteria grouped by cell morphology and fermentation type

* Lactic acid rods classified as facultatively heterofermentative (subgenus Streptobacterium) ferment hexoses via The Glycolytic Pathway and pentoses via the oxidative pentose phosphate pathway. Consequently, these lactobacilli carry out homofermentative lactic acid fermentation in the first case and heterofermentative fermentation In the second.

Homofermentative lactic acid bacteria can utilize Monosaccharides (glucose, galactose) and Oligosaccharides (lactose, maltose) as their primary energy source. The conversion of glucose to Pyruvate proceeds via the glycolytic pathway (Fig. 27). Here, pyruvate serves as the electron acceptor for the oxidized substrate: it accepts 2 electrons from reduced NADH₂, resulting in The formation of lactic acid:

The energy yield of homofermentative lactic acid fermentation is 2 ATP per mole of glucose fermented.