Lactic Acid Bacteria: Biochemistry, Pathways, and Food Preservation
Lactic Acid Bacteria (LAB)
Gram-positive, non-motile, and non-spore forming bacteria that are strict fermenters.
They are catalase and oxidase negative and cannot synthesize cytochromes.
LAB can be either homofermentative or heterofermentative. The type of fermentation is determined by the enzymes produced by the organism.
Homofermentative: Converts glucose into two lactic acid molecules, with 85–95% of the glucose carbon ending up as lactate. This pathway uses the glycolytic (Embden-Meyerhof) pathway. Examples include Streptococcus and Pediococcus.
Heterofermentative: Converts glucose into one lactate molecule, one ethanol molecule, and one carbon dioxide molecule. Only 50% or less of the glucose carbon is converted to lactate. This pathway uses the phosphoketolase pathway. An example is Leuconostoc.
Lactobacillus species can be either homofermentative or heterofermentative.
A 2020 taxonomic note reclassified the genus Lactobacillus into 25 new genera, including an emended genus Lactobacillus and the union of the families Lactobacillaceae and Leuconostocaceae. The term “lactobacilli” remains useful to refer to organisms previously classified in the genus Lactobacillus.
Fermentation Biochemistry and Pathways
Fermentation is an anaerobic process that does not use oxygen as the final electron acceptor. However, oxygen does not have to be absent from the environment.
Reactions occur in the cytoplasm and involve partial oxidation.
Fermenting cells must convert NADH back to NAD+ to continue making ATP via glycolysis. This is done by passing electrons from NADH to pyruvate.
Glycolysis (Embden-Meyerhof pathway): The primary pathway for glucose catabolism in both respiration and fermentation. It produces ATP, NADH, and pyruvate from glucose.
Products: The specific end products of fermentation depend on the organism and include a variety of acids, alcohols, and gases.
Lactic Acid: Produced by Streptococcus and Lactobacillus.
Ethanol and CO₂: Produced by yeasts like Saccharomyces.
Fermentation is the least efficient metabolic pathway for ATP production, yielding only 2 ATP per glucose molecule, compared to up to 38 ATP from aerobic respiration in eukaryotes.
Fermented Foods and Microbial Ecosystems
Fermentation is a natural process that has been used by humans for thousands of years for food preservation and transformation.
Fermented foods and their microbial communities (MCoFFs) provide excellent models for studying microbial ecosystems because they are accessible, reproducible, and easy to manipulate.
These communities can be composed of bacteria, fungi, or a mix of both. They form in various ways, such as in biofilms on surfaces (e.g., cheese rinds), suspended in liquids (e.g., kefir, kombucha), or dispersed in semi-solid substrates (e.g., kimchi).
SCOBY (Symbiotic Community Of Bacteria and Yeast): A self-reproducing, rubbery mass of bacteria and fungi that forms a biofilm. These communities are often used as starter cultures for products like kombucha and kefir and can contain up to 30 different microbial types, including Lactobacilli, Leuconostoc, Acetobacter, and Saccharomyces.
The “three A’s” of fermentation—Acidity, Alcohol, and Anaerobic/Aerobic conditions—are key to food preservation.
Acidity: The production of acids lowers the pH, inhibiting the growth of spoilage microbes and pathogens.
Alcohol: Ethanol acts as an antimicrobial agent that prevents the growth of undesirable fungi and bacteria.
Aerobic/Anaerobic Conditions: Fermentation often happens in oxygen-free conditions, which stops the growth of aerobic spoilage organisms while promoting the growth of beneficial fermenting microbes.