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Microbial Fermentation01:23

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Fermentation is a crucial anaerobic metabolic process that enables microbes to derive energy from sugar without relying on oxygen or an electron transport chain. This process is fundamental to various biological and industrial applications and is classified based on the metabolic products generated.Role of Pyruvate in FermentationPyruvate and its derivatives serve as key electron acceptors in fermentative pathways. The oxidation of NADH to regenerate NAD+ is essential for the continuation of...
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Pyruvate is the end product of glycolysis, where glucose is oxidized to pyruvate, simultaneously reducing NAD+ to NADH. Two molecules of ATP are also produced by substrate-level phosphorylation.
In aerobic organisms, pyruvate is metabolized via the citric acid cycle to produce reduced coenzymes NADH and FADH2. These coenzymes are then oxidized in the electron transport chain to produce ATP and, in the process, regenerate the NAD+ and FAD. As seen in some cell types and organisms, fermentation...
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Loss of Carboxy Group as CO2: Decarboxylation of Malonic Acid Derivatives01:35

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Most eukaryotic organisms require oxygen to survive and function adequately. Such organisms produce large amounts of energy during aerobic respiration by metabolizing glucose and oxygen into carbon dioxide and water. However, most eukaryotes can generate some energy in the absence of oxygen by anaerobic metabolism.
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Microorganisms rely on proteins as an essential carbon and energy source, particularly in environments with limited polysaccharides or lipids. However, proteins are too large to cross the plasma membrane unaided, necessitating enzymatic degradation. Microbes secrete extracellular proteases and peptidases that hydrolyze proteins into peptides, which can then be transported across the membrane. Once inside the cell, intracellular proteases degrade these peptides into free amino acids, which...
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Related Experiment Video

Updated: Nov 15, 2025

Purification of a High Molecular Mass Protein in Streptococcus mutans
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Lactic acid excretion by Streptococcus mutans.

Stuart G Dashper1, Eric C Reynolds1

  • 1Biochemistry and Molecular Biology Unit, School of Dental Science, Faculty of Medicine, Dentistry and Health Sciences, The University of Melbourne, 711 Elizabeth Street, Melbourne, Victoria 3000, Australia.

Microbiology (Reading, England)
|March 4, 2021
PubMed
Summary

Streptococcus mutans excretes lactic acid via an electroneutral process, independent of cellular energy. This mechanism, crucial for understanding bacterial metabolism and acid production, involves lactic acid transport across the cell membrane.

Keywords:
Streptococcus mutans,dental cariesglycolysisintracellular pHlactate

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Area of Science:

  • Microbiology
  • Bacterial Physiology
  • Biochemistry

Background:

  • Lactic acid is the primary glycolysis end-product of Streptococcus mutans, especially under sugar excess or low pH.
  • The specific mechanism for lactic acid excretion by S. mutans has not been previously elucidated.
  • Understanding lactic acid transport is vital for comprehending S. mutans's role in cariogenesis and its metabolic adaptations.

Purpose of the Study:

  • To characterize the transmembrane movement and excretion mechanism of lactic acid in Streptococcus mutans.
  • To investigate whether lactic acid transport is an energy-dependent or independent process.
  • To determine the form in which lactate crosses the bacterial cell membrane.

Main Methods:

  • Monitoring the transmembrane movement of radiolabeled lactate in de-energized S. mutans cells.
  • Utilizing transmembrane pH gradients (ΔpH) and electrical potentials (Δψ) to analyze lactate flux.
  • Measuring intracellular and extracellular lactate concentrations enzymatically under varying pH conditions.

Main Results:

  • Lactate equilibrates across the cell membrane driven by the transmembrane pH gradient, not the electrical potential.
  • Lactate efflux from cells creates a ΔpH but not a Δψ, indicating electroneutral transport of lactic acid.
  • High extracellular lactate inhibits glycolysis by altering intracellular pH and electrical potential, impacting the proton motive force.

Conclusions:

  • Lactate is transported across the S. mutans cell membrane as undissociated lactic acid in an electroneutral process.
  • This transport mechanism is independent of cellular metabolic energy, relying solely on passive diffusion driven by pH gradients.
  • The findings have significant bioenergetic implications for S. mutans, influencing its survival and acid production capabilities in dental plaque environments.