What are Bacterial Skin and Soft Tissue Infections?
Bacterial skin and soft tissue infections (SSTIs) is a broad clinical term that encompasses a diverse group of illnesses. It refers to any infection that arises when pathogenic bacteria successfully invade and multiply within the skin or the underlying soft tissues. The specific diagnosis within this category is often determined by the depth of the infection—that is, which layer of tissue is primarily involved, from the superficial epidermis down to the subcutaneous fat, muscle, and fascia.
This classification includes some of the most common human infections. It covers relatively minor surface conditions like impetigo and folliculitis, more extensive infections of the deeper skin layers like cellulitis, and contained pockets of pus known as abscesses. The most frequent bacterial culprits responsible for the vast majority of community-acquired SSTIs are Staphylococcus aureus and Streptococcus pyogenes.
Causes:- Physical Disruption of the Skin's Surface: The most direct cause is a break in the skin. This can be a significant traumatic injury like a cut or puncture, but also includes less obvious entry points such as insect bites, minor abrasions, or cracks in dry, chapped skin that provide a gateway for bacteria.
- Impairment of Local Circulation: Conditions that reduce blood flow to an area, such as peripheral vascular disease or significant swelling (edema), starve the tissue of oxygen and immune cells. This creates a weakened environment where bacteria can multiply with little resistance from the body's defenses.
- Introduction of a High Bacterial Inoculum: An infection is more likely to take hold when a very large number of bacteria are introduced into the tissue at once. This can occur with a bite wound (human or animal), which injects oral flora deep into the tissue, or with injuries heavily contaminated by soil or organic matter.
- Creation of a Favorable Microenvironment: Any situation that traps heat and moisture against the skin can foster bacterial overgrowth. This includes wearing occlusive clothing, improperly managed wound dressings, or conditions that block skin follicles, all of which create an ideal warm, moist environment for bacteria to thrive.
- Systemic Immunocompromise and Metabolic Disease: Individuals with chronic health issues that disrupt body-wide functions are highly susceptible. Conditions like uncontrolled diabetes mellitus create a high-sugar environment conducive to bacterial growth, while diseases or medications that suppress the immune system limit the body's ability to police its own tissues.
- Disruption of the Lymphatic System: This is a major predisposing factor, particularly for cellulitis. People with lymphedema—a condition involving chronic swelling due to compromised lymphatic drainage, often seen after certain surgeries or radiation therapy—have a significantly increased risk because the impaired fluid clearance hinders local immune surveillance.
- Persistent Inflammatory Skin Conditions: Chronic dermatological diseases create an ongoing vulnerability. Ailments such as eczema (atopic dermatitis) or psoriasis result in a chronically impaired skin barrier, with microscopic cracks and inflammation that serve as constant potential entry points for pathogenic bacteria.
- Injection Drug Use: This behavior carries an exceptionally high risk for severe SSTIs, including abscesses. The practice directly bypasses the skin's protective layer, often with non-sterile needles, introducing a high concentration of skin and environmental bacteria deep into the soft tissues and bloodstream.
