Lactic acid translocation: terminal step in glycolysis by Streptococcus faecalis

Area of Science:

• Microbiology
• Cell Biology
• Biochemistry

Background:

• Streptococcus faecalis primarily uses glucose fermentation to lactic acid for metabolic energy.
• Understanding lactic acid translocation across the cytoplasmic membrane is crucial for comprehending bacterial energy metabolism.

Purpose of the Study:

• To elucidate the mechanism of lactic acid translocation across the cytoplasmic membrane in Streptococcus faecalis.
• To investigate the role of pH gradients and ionophores in lactate transport.

Main Methods:

• Utilized radiolabeled [(14)C]lactate to study efflux from and influx into Streptococcus faecalis cells.
• Employed ionophores (nigericin and valinomycin) to manipulate and assess pH gradients.
• Investigated lactate transport under varying external pH conditions and in nonmetabolizing cells.

Main Results:

• Lactate efflux was enhanced by increased external pH and ionophore treatment, suggesting electroneutral lactic acid movement.
• Cellular uptake of [(14)C]lactate was limited, with the membrane being impermeable to lactate anions but permeable to lactic acid.
• Accumulation of lactate against a concentration gradient occurred when the cytoplasm was alkaline, driven by the pH gradient and abolished by ionophores.

Conclusions:

• Lactic acid translocation across the Streptococcus faecalis cytoplasmic membrane is an electroneutral process.
• The distribution of lactic acid is dependent on the pH difference between the cytoplasm and the external medium.
• Evidence suggests a low-specificity carrier mediates lactic acid translocation.