2018-01-16-Acid stress in lactic acid bacteria

There are several mechanisms regulate the homeostasis of the intracellular pH (pHi) and the proton-translocating:

• F0F1-ATPase proton pumps
• Amino acid decarboxylation/catabolism
• The expression of general stress proteins (GSPs)
• Chaperones that repair or degrade damaged DNA and proteins
• The synthesis of alkaline compounds, and the modification of cell membrane composition.

Arginine deiminase (ADI)

Three enzymes: arginine deiminase (EC 3.5.3.6), ornithine cabamoyl transferase (cOTC, EC 2.1.3.3) and carbamate kinase (CK, EC 2.7.2.2)
Arginine can produce Orithine, ammonina, carbon dioxide and ATP
The resulting NH3 reacts with H+ and helps to alkalize the environment, and in addition, the extra energy (ATP) produced via the ADI pathway enables the extrusion of cytoplasmic protons by the F0F1-ATPase and may help cells to survive longer after depletion of the primary energy source.
An arginine/ornithine antiporter completes the system and allows the exchange of these two molecules at no energy cost.
Moreover, the factors involved in ADI regulation appeared to be a combination of arginine availability, energy depletion, catabolite repression and oxygenation rather than low pH. Therefore, although ADI activity can alkalize the environment, its importance for the acidurance of LAB may vary between species.

F0F1-ATPase

F0F1-ATPase is the most important for fermentative bacteria. This enzyme was characterized from L. casei and L. plantarum, and its activity was optimal at values of pH 5.0–5.5 lower than those (pH 7.0–7.5) found for Streptococcus thermophilus and Lactococcus lactis subsp. lactis. The overall proton permeability of the plasma membrane also contributes to the regulation of the pHi. The minimal membrane permeability of L. casei and L. plantarum was recorded at pH 4.0, while that in the acid-sensitive organism Acinomyces viscosus was found at pH 6.0. This difference could explain the rapid decrease of pHi in lactobacilli since they may not actively regulate the pHi until the extracellular pH reaches very low values.

Glutamate

Gutamate decarboxylase (GAD) [EC 4.1.1.15] activity: The expression of GAD genes is assumed to control the acidification of the cytosolic environment by decarboxylating an acid substrate (glutamate) into a neutral compound (g-aminobutirric acid, GABA) via the incorporation of H+. GABA would then be exported into the extracellular environment, thereby contributing to alkalinization. GABA-producing strains of L. paracasei, L. delbrueckii subsp. bulgaricus, and L. plantarum survived and synthesized GABA under simulated gastrointestinal conditions.