A cyst is a closed sac under the skin that contains fluid or semi-solid material. While most cysts are benign, when one drains or ruptures, it can release a strong, foul odor. This unpleasant symptom results from the material trapped inside the sac and the microscopic processes that occur when that material breaks down.
The Internal Contents of a Cyst
The most common type of skin growth that produces this smell is the epidermoid cyst, which is lined with epidermal cells. These cells continually shed, filling the sac with keratin, a protein found in hair, skin, and nails. This keratinous material is often described as thick, pasty, and cheese-like.
Keratin is rich in sulfur-containing amino acids. Even when not infected, the slow decomposition of these substances releases mildly odorous compounds. This mild odor is distinct from the severely offensive smell that occurs when the cyst actively drains or ruptures.
The Mechanism Behind the Foul Odor
The offensive, “rotten” smell is caused by the metabolic waste products of bacteria, not the keratin itself. When a cyst ruptures or drains, the contents are exposed to the bacteria naturally living on the skin’s surface. The closed, oxygen-poor environment inside the cyst provides an ideal habitat for specific microorganisms.
Anaerobic bacteria, which thrive without oxygen, colonize the cyst material and break down proteins and dead tissue. This microbial breakdown, or putrefaction, generates highly volatile molecules. The primary culprits for the extreme odor are Volatile Sulfur Compounds (VSCs), including hydrogen sulfide and methyl mercaptan.
Bacteria also metabolize amino acids, creating polyamines like putrescine and cadaverine. These compounds are named for their association with decomposing flesh and are responsible for the strong, decaying odor. The combination of VSCs and polyamines creates the intense stench associated with a draining or infected cyst.
Common Cysts Prone to Odor
While any cyst can become infected and produce an odor, certain types are more susceptible due to their contents and location. Epidermoid cysts are the most frequent source of this smell because their keratin contents provide a rich protein source for odor-producing bacteria. They commonly appear on the face, neck, chest, and upper back.
Pilonidal cysts are another common source of foul smell, often draining a musty, thick fluid. These cysts form near the tailbone in the cleft of the buttocks, a location prone to friction, moisture, and poor ventilation. This environment allows anaerobic bacteria to flourish, leading to deep-seated, odorous infections.
When to Seek Medical Attention
A foul smell or discharge indicates that a cyst has ruptured or become infected, requiring professional evaluation. Immediate medical attention is necessary if the cyst is accompanied by signs of a spreading infection. These signs include fever, red streaks radiating away from the bump, or a significant increase in pain and warmth.
Healthcare providers typically manage an infected or draining cyst with incision and drainage (I&D) to fully evacuate the purulent contents. Attempting to squeeze or drain an infected cyst at home is strongly discouraged, as this can push the infection deeper into the tissue or lead to cellulitis.
Depending on the severity and spread, a provider may also prescribe oral antibiotics. These medications target the polymicrobial nature of the infection and are often chosen specifically to combat the problematic anaerobic bacteria.