Taxonomy and Morphological Features
Bacteria of the genus Propionibacterium are Gram-positive, non-spore-forming, and non-motile bacilli. Major 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 exhibit marked polymorphism or pleomorphism. Their shape can vary from coccoid cells to elongated, bifurcated, and even branched forms. Most commonly, they appear as club-shaped bacilli (diphtheroids). Under microscopy, a structural cellular feature can be observed: one end of the bacillus is typically rounded, while the opposite end is cone-shaped or pointed. This pointed end takes up stains significantly weaker.
In a microscopic smear, cells are arranged quite specifically. They may occur singly, in pairs, or in short chains. Due to their pattern of division, the bacteria often form characteristic figures resembling the Latin letters V and Y, as well as clusters visually reminiscent of "Chinese characters".
Cultural Properties
Regarding molecular oxygen, propionibacteria are anaerobes or aerotolerant microorganisms. This means they prefer oxygen-free (anaerobic) conditions for optimal reproduction, 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 temperature optimum for their growth is 30–37 °C, and the optimal medium pH is 7.0. A distinctive feature is their ability to grow in glucose broth even when selective inhibitors such as 20% bile or 6.5% sodium chloride (NaCl) are added.
On solid nutrient media, the microorganisms form colonies that are often pigmented. Colony color 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 in this genus is based on propionic acid fermentation. They utilize various carbohydrates, as well as lactate and pyruvate, as the main substrates for fermentation. The fermentation of glucose produces acids along with the release of ammonia and carbon dioxide (CO₂).
Their primary metabolic product is propionic acid, which served as the basis for naming the entire genus. In addition, byproducts such as acetic, isovaleric, formic, succinic, and lactic acids are formed during their metabolic activity.
An important differential-diagnostic feature of propionibacteria is their ability to synthesize the enzyme catalase (they are catalase-positive), which distinguishes them from certain morphologically similar microorganisms.
Pathogenicity and Ecology
Under natural conditions, propionibacteria are widely distributed. In the environment, they are frequently found in dairy products of animal origin. In humans, they act as typical members of the normal microbiota, colonizing the skin, mucous membranes of the oral cavity, respiratory tract, intestines, and female reproductive organs.
However, these bacteria are capable of producing a number of aggressive and invasive enzymes that destroy host tissues and facilitate the spread of infection. These enzymes include:
- Lipase;
- Protease;
- Neuraminidase;
- Hyaluronidase;
- Phosphatase.
When host resistance decreases, these microorganisms become the etiological factor in inflammatory processes of various localizations. They can cause conjunctivitis, abscesses, peritonitis, parotitis, and pneumonia. The species P. acnes is of greatest clinical significance. These bacteria actively colonize hair follicles and sweat gland ducts, playing a leading role in the pathogenesis of 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 environment, the bacteria exhibit marked susceptibility to standard antiseptics and disinfectants.