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Bacterial Skin Infection (Cellulitis)

What is Bacterial Skin Infection (Cellulitis)?

Cellulitis is an acute, bacterial infection of the deeper layers of the skin (the dermis and subcutaneous tissue). It typically presents as a painful, red, warm, and swollen area that spreads rapidly. The infection most often results from bacteria entering the skin through a break—such as a cut, scrape, surgical wound, or insect bite—and can affect any part of the body, though the legs and arms are the most common sites.

While cellulitis is usually caused by Streptococcus or Staphylococcus aureus species, other organisms can be involved, especially in people with compromised immune systems or chronic skin conditions. Without prompt treatment, the infection can spread to the bloodstream (bacteremia) or deeper structures, leading to serious complications.

Common Causes

Cellulitis rarely occurs without an entry point for bacteria. The following conditions and situations increase the risk of developing cellulitis:

• Breaks in the skin: cuts, abrasions, surgical incisions, puncture wounds, or animal bites.
• Chronic skin disorders: eczema, psoriasis, athlete’s foot, or dermatitis.
• Venous insufficiency or lymphedema: stagnant fluid creates a breeding ground for bacteria.
• Diabetes mellitus: high blood sugar impairs immune response and wound healing.
• Immune suppression: chemotherapy, HIV/AIDS, corticosteroids, or biologic drugs.
• Obesity: skin folds can become moist and fissured, facilitating bacterial entry.
• Recent trauma or surgery: especially orthopedic procedures on the lower extremities.
• Insect bites or stings: especially when the site is scratched vigorously.
• Foot ulcers: common in patients with peripheral arterial disease or diabetic neuropathy.
• Exposure to contaminated water: e.g., swimming in lakes with high bacterial load.

Associated Symptoms

Cellulitis is usually accompanied by a constellation of local and systemic signs. Commonly reported symptoms include:

• Intense redness that expands outward from the original site.
• Swelling that can feel “tight” or “full‑bodied.”
• Heat to the touch compared with surrounding skin.
• Pain or tenderness that may worsen with movement.
• Skin that appears glossy or stretched.
• Fever, chills, or malaise (especially in more extensive infections).
• Flu‑like symptoms such as headache, muscle aches, or nausea.
• Red streaks (lymphangitis) that follow the path of lymphatic vessels.
• Increased white blood cell count on laboratory testing.

When to See a Doctor

Cellulitis can progress quickly. Seek medical attention promptly if you notice any of the following:

• Rapid spreading of redness (>2‑3 cm per hour) or swelling.
• Fever ≥ 38 °C (100.4 °F) or chills.
• Severe pain out of proportion to the size of the rash.
• Red streaks radiating from the lesion (possible lymphangitis).
• Sudden shortness of breath, chest pain, or rapid heartbeat (signs of sepsis).
• Swelling, redness, or pain in the face, neck, or eye area.
• Underlying conditions that impair healing (e.g., diabetes, immune deficiency).
• History of recent antibiotic use or known MRSA colonisation.

Diagnosis

Diagnosing cellulitis is primarily clinical, but doctors may use additional tools to confirm the diagnosis, assess severity, and rule out mimicking conditions.

Physical Examination

• Visual inspection of the affected area for redness, edema, warmth, and tenderness.
• Palpation to assess the extent of induration and fluctuation (which may suggest an abscess).
• Evaluation of lymph nodes and distal pulses.

Laboratory Tests

• Complete blood count (CBC): often reveals leukocytosis.
• C‑reactive protein (CRP) and erythrocyte sedimentation rate (ESR): markers of inflammation.
• Blood cultures: recommended if the patient is febrile, has systemic signs, or is immunocompromised.
• Wound culture: if an abscess or open wound is present.

Imaging

• Ultrasound: distinguishes cellulitis from an underlying abscess.
• CT or MRI: reserved for deep tissue involvement, necrotizing fasciitis suspicion, or spread to bone (osteomyelitis).

Differential Diagnosis

Conditions that can mimic cellulitis include deep vein thrombosis, contact dermatitis, venous stasis dermatitis, gout, necrotizing fasciitis, and erysipelas. Accurate diagnosis prevents unnecessary antibiotics and ensures proper treatment.

Treatment Options

Management combines antimicrobial therapy, supportive care, and addressing any underlying risk factors.

Antibiotic Therapy

• First‑line oral agents (uncomplicated cellulitis):
  • Cephalexin 500 mg q6h for 5‑7 days
  • Cloxacillin 500 mg q6h
  • Dicloxacillin 500 mg q6h
• If MRSA is suspected or confirmed:
  • Trimethoprim‑sulfamethoxazole (TMP‑SMX) 160/800 mg bid
  • Clindamycin 300 mg q6h
  • Doxycycline 100 mg bid (adults)
• Intravenous therapy (severe infection, systemic signs, or inability to take PO meds):
  • IV cefazolin or nafcillin
  • IV vancomycin for MRSA
  • IV clindamycin or linezolid as alternatives

Therapy is typically continued for 7‑14 days, with a minimum of 48‑72 hours after clinical improvement. Always complete the prescribed course even if symptoms resolve sooner.

Supportive Care

• Elevation: Keep the affected limb above heart level to reduce edema.
• Analgesia: Acetaminophen or ibuprofen for pain and fever.
• Hydration: Adequate oral fluids; IV fluids if the patient is febrile or hypotensive.
• Wound care: Clean and dress any open lesions; avoid further trauma.

Adjunct Measures

• Treat underlying conditions (e.g., optimize glycemic control in diabetes).
• Consider decolonisation strategies for recurrent MRSA (mupirocin nasal ointment + chlorhexidine washes).
• Physical therapy after resolution to restore range of motion if swelling limited movement.

Prevention Tips

Most cases of cellulitis are preventable by maintaining skin integrity and managing chronic health problems.

• Skin hygiene: Wash daily with mild soap; keep nails trimmed to prevent scratching.
• Prompt wound care: Clean cuts with water and mild antiseptic, apply a sterile dressing, and seek medical care for deep or contaminated wounds.
• Moisturize: Use emollients on dry, cracked skin, especially on the feet and lower legs.
• Manage chronic conditions: Control diabetes, treat venous insufficiency (compression stockings), and address lymphedema with compression therapy.
• Avoid tight clothing or shoes: Reduces friction and skin breakdown.
• Protect against insect bites: Use repellents, wear long sleeves when appropriate, and avoid scratching bites.
• Vaccinations: Stay up to date on tetanus and influenza vaccines, which can indirectly lower infection risk.
• Hand hygiene: Wash hands before touching wounds; use alcohol‑based hand rubs when soap isn’t available.
• Regular skin checks: Individuals with diabetes or peripheral neuropathy should inspect feet daily for cracks or sores.

Emergency Warning Signs

Key Take‑aways

• Cellulitis is a bacterial infection of the deep skin layers that spreads quickly and can become life‑threatening.
• Common triggers are skin breaks, chronic dermatoses, diabetes, lymphedema, and immune suppression.
• Typical symptoms include redness, warmth, swelling, pain, and sometimes fever.
• Early medical evaluation is essential; look for rapid spread, fever, red streaks, or systemic signs.
• Treatment involves antibiotics (chosen based on likely organisms and local resistance patterns) plus elevation, analgesia, and wound care.
• Prevention focuses on skin integrity, chronic disease management, and proper hygiene.
• Emergency warning signs—especially signs of sepsis or necrotizing fasciitis—require immediate care.

For the most current guidelines, consult reputable sources such as the Mayo Clinic, the CDC, and the NIH.

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