MICROBIOLOGY Study Guide - 2012

CHAPTER 9. MICROBIAL PHYSIOLOGY

9.5. FERMENTATIONS

9.5.4. Propionic Acid Fermentation

Propionic Acid Fermentation is caused by propionic acid Bacteria, which ferment CARBOHYDRATES to produce propionic and acetic acids, as well as carbon dioxide:

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Propionic acid bacteria inhabit the intestinal tract of ruminants and are found in milk, hard cheeses, silage, and fermented olives.

Propionic acid bacteria are grouped into the family Propionibacteriaceae and the genus Propionibacterium. The type species of the genus is Propionibacterium freudenreichii (named after the Swiss bacteriologist Eduard Freudenreich). The group of classical propionic acid bacteria also includes P. jensenii, P. thoenii, P. acidipropionici, and P. acnes.

Propionic acid bacteria are small, pleomorphic rods measuring (1.0–5.0) × (0.5–0.8) µm. Cell Morphology can be coccoid, club-shaped, or branched. Cells typically occur singly, sometimes in pairs, or arranged in Chinese character-like patterns, V, or Y shapes. Propionic acid bacteria are Gram-positive, non-motile, and do not form spores or capsules. Some strains may produce extracellular slime, although it does not form a well-defined capsule.

Regarding atmospheric oxygen, they are facultative anaerobes or microaerophiles; however, most strains grow well under strictly anaerobic conditions.

Propionic acid fermentation is characterized by The addition of carbon dioxide to a Pyruvate molecule, which leads to The formation of a four-carbon Skeleton (i.e., C₃ + C₁ = C₄), as shown in Fig. 30.

Fig. 30. Pathway of propionic acid fermentation.

Enzymes involved in fermentation: E₁ — pyruvate carboxylase; E₂ — malate dehydrogenase; E₃ — fumarase; E₄ — succinate dehydrogenase carboxytransferase; E₅, E₈ — CoA-transferase;

E₆ — methylmalonyl-CoA mutase; E₇ — methylmalonyl-CoA

Propionic acid bacteria are used in baking to prevent The Development of ropy bread disease. The metabolic products of propionic acid bacteria—acetic and propionic acids—inhibit the proliferation of the causative agents of this bread spoilage.

Propionic acid bacteria are also utilized in The production of cheeses with a high second-cooking Temperature (such as Swiss, Soviet, Maasdam, etc.). The propionic and acetic acids accumulating during cheese ripening impart a specific sharp flavor, while carbon dioxide forms the characteristic pattern of large, sparse eyes in the cheese. Another beneficial property of propionic acid bacteria is their ability to synthesize vitamin B₁₂.