Propionibacteria

Taxonomy and morphological features

Bacteria of the genus Propionibacterium are gram-positive, non-spore-forming, and non-flagellated (non-motile) rods. The main species include P. acnes (the most clinically significant species), as well as P. acidopropionici, P. avidum, P. granulosum, P. jensenii, and P. thoenii.

These microorganisms are characterized by pronounced polymorphism or pleomorphism. Their shape can vary from coccoid cells to elongated, bifurcated, and even branched forms. Most frequently, they appear as club-shaped rods (diphtheroids). Under microscopy, a structural feature of the cells can be observed: one end of the rod is usually rounded, while the opposite end is cone-shaped or pointed. This pointed end takes up stains significantly less.

In a microscopic smear, the cells are arranged quite specifically. They can occur singly, form pairs, or short chains. Due to their division pattern, the bacteria often form characteristic figures resembling the Latin letters V and Y, as well as clusters visually associated with "Chinese characters".

Cultural properties

Regarding molecular oxygen, propionibacteria are anaerobes or aerotolerant microorganisms. This means that for optimal reproduction they prefer oxygen-free (anaerobic) conditions, although they are capable of surviving in the presence of oxygen.

For successful in vitro cultivation, these microorganisms require nutrient media enriched with peptone, yeast extract, and glucose. The optimal growth temperature is 30–37 °C, and the optimal pH level is 7.0. A distinctive feature is their ability to grow in glucose broth even with the addition of selective inhibitors such as 20% bile or 6.5% sodium chloride (NaCl).

On solid nutrient media, microorganisms form colonies that often exhibit pigmentation. The color of the colonies can be white, gray, pink, red, yellow, or orange, which aids in the macroscopic identification of the culture.

Biochemical activity

The energy metabolism of bacteria of this genus is based on propionic acid fermentation. They utilize various carbohydrates, as well as lactate and pyruvate, as the main substrates for fermentation. During glucose fermentation, acids are produced, along with the release of ammonia and carbon dioxide (CO₂).

The main product of their metabolism is propionic acid, which served as the basis for the name of the entire genus. In addition to it, by-products are formed during their life activity: acetic, isovaleric, formic, succinic, and lactic acids.

An important differential diagnostic feature of propionibacteria is their ability to synthesize the enzyme catalase (they are catalase-positive), which distinguishes them from some morphologically similar microorganisms.

Pathogenicity and ecology

Under natural conditions, propionibacteria are widely distributed. In the external environment, they are frequently found in dairy products of animal origin. In humans, they act as typical representatives of the normal microflora, colonizing the skin, mucous membranes of the oral cavity, respiratory tract, intestines, and female genital organs.

However, these bacteria are capable of producing a number of aggression and invasion enzymes that destroy host tissues and facilitate the spread of infection. Such enzymes include:

• Lipase;
• Protease;
• Neuraminidase;
• Hyaluronidase;
• Phosphatase.

With a decrease in the body's resistance, microorganisms become the etiological factor of inflammatory processes of various localizations. They can cause conjunctivitis, abscesses, peritonitis, parotitis, and pneumonia. Of greatest clinical significance is the species P. acnes. These bacteria actively colonize hair follicles and sweat gland ducts, playing a leading role in the pathogenesis of acne (acne vulgaris).

In clinical practice, the treatment of infections caused by these pathogens takes into account their high susceptibility to antibacterial drugs, such as erythromycin and clindamycin. In addition, in the external environment, bacteria exhibit pronounced susceptibility to standard antiseptics and disinfectants.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanism of propionic acid fermentation and substrates
• Aggression enzymes of P. acnes and their role in tissue destruction
• Pathogenesis of acne vulgaris development
• Cultivation conditions: selective additives
• Differential diagnosis of diphtheroids in a smear

Other topics in the section "Irregular Gram-positive rods (Diphtheria, Mycobacteria/tuberculosis and leprosy, Actinomycetes, Nocardia, Bifidobacteria, Gardnerella, etc.)"

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