Wound Infections

Skin Infections

• Breaks in skin or mucous membranes allow microorganisms to enter.
• Infection results depending on:
  • Virulence of microbes
  • Number of microbial cells in wound
  • Host’s immune system
  • Type of wound

Common Bacterial Infections of Wounds

Staphylococcal Wound Infections

• Signs and Symptoms:
  • Staphylococcus are pyogenic, causing pus production.
  • Inflammatory reaction (redness, swelling, pain, heat).
  • Fever if infection is large or spreads to blood and lymph.
  • Toxic shock syndrome from toxin-producing strain: high fever, muscle aches, life-threatening drop in blood pressure, shock.
• Causative Agents:
  • Staphylococcus aureus, Staphylococcus epidermidis
  • Facultative anaerobes; thrive on dry salty skin
  • S. aureus most common cause of wound infections
  • S. epidermidis can cause device-associated infections
• Transmission:
  • Survive well in environment and are easily transferred via direct and indirect contact.
  • S. epidermidis usually part of normal microbiota; opportunist in immunocompromised individuals.
  • S. aureus carriers have increased risk for surgical wound infection.
  • Found in microbiota of some individuals: skin, upper respiratory tract
• Treatment and Prevention:
  • Often difficult due to antibiotic resistance.
  • Methicillin-resistant S. aureus (MRSA) resists multiple antibiotics.
  • Vancomycin-intermediate S. aureus (VISA) and vancomycin-resistant S. aureus (VRSA) have emerged.
  • Thorough cleaning of wounds reduces infections.

Group A Streptococcal “Flesh-Eating Disease”

• Severe infections can progress rapidly and are very invasive.
• Spread into tissues and organs, causing:
  • Pneumonia
  • Meningitis
  • Streptococcal toxic shock syndrome
  • Necrotizing fasciitis (“flesh-eating disease”)
• Signs and Symptoms:
  • Severe pain at wound site.
  • Rapid progression, sometimes within hours, with swelling, stretching, and discoloring of the skin.
  • Fever and confusion develop.
  • Without immediate treatment: shock and death may follow quickly.
• Causative Agent:
  • Streptococcus pyogenes
  • Enzymes destroy subcutaneous fatty tissue and fascia.
  • M protein causes neutrophils to release inflammatory molecules; also helps avoid phagocytosis.
  • Virulent strains produce damaging streptococcal pyrogenic exotoxins.
  • Superantigens inducing inflammation by non-specific activation of helper T cell, release cytokines.
  • Protease causing tissue destruction and breakdown.
• Transmission:
  • Bacteria enter through abrasions and immune system can’t fight it off.
  • Risk factors: skin wounds (trauma, burns, surgery), chronic diseases (diabetes, alcoholism), weakened immune system (cancer, AIDS, injected-drug abuse).
• Treatment and Prevention:
  • Generally easy to treat: susceptible to penicillin.
  • Toxins spread rapidly: immediate debridement often essential to remove dead tissue and reduce pressure; amputation may be necessary to remove source of toxins.
  • Combination of intravenous antibiotics given with supportive care.
  • Vaccines against M proteins being tested in animals.

Pseudomonas aeruginosa Infections

• Opportunistic pathogen and significant cause of healthcare-associated infections.
• Signs and Symptoms:
  • Characteristic greenish pus.
  • After entering bloodstream: chills, fever, skin lesions, shock.
• Causative Agent:
  • Pseudomonas aeruginosa: Gram-negative rod with polar flagellum.
• Transmission:
  • Pseudomonas aeruginosa widespread in nature, grows in most places with moisture.
  • Sources: Soaps, ointments, eye drops, contact lens solutions, cosmetics, disinfectants, swimming pools, hot tubs, even distilled water.
  • Introduced into hospitals on ornamental plants, flowers, fruit baskets.
  • Not allowed in burn wards or intensive care units.
  • Also found on hospital equipment, soles of shoes, illegal injectable drugs.
• Treatment and Prevention:
  • Resistant to many antibiotics; form biofilms that increase resistance to treatment.
  • Only a few antimicrobial medications effective, and not against all strains.
  • Sensitivity tests determine most appropriate medication.
  • Intravenously high dosed medication necessary for systemic infections.
  • Eliminate possible sources of infection; treat wounds promptly; remove dead tissue from burn wounds; apply antibacterial cream.

Diseases Due to Anaerobic Bacterial Wound Infections

• Many wounds are relatively anaerobic (lacking O2), allowing growth of obligate anaerobes such as Clostridium tetani.
• Due to extensive tissue damage, small but deep punctures, or contamination with dirt.

Tetanus (Lockjaw)

• Signs and Symptoms:
  • About a week after infection: irritability, difficulty swallowing, muscle stiffness (often starting in the jaw).
  • Then continuous, painful, uncontrollable muscle spasms.
  • Breathing becomes difficult, abnormal heart rhythms may occur, bones can fracture; severe pain.
  • Death often follows lung damage from pneumonia or regurgitation of stomach contents.
• Causative Agent:
  • Clostridium tetani
  • Anaerobic, Gram-positive, rod-shaped; forms spherical endospores at one end of cell.
  • Exotoxin tetanospasmin (AB toxin) released from cells.
  • Binds to motor neurons and blocks release of inhibitory neurotransmitters that allow muscle relaxation.
  • Causes spastic paralysis (uncontrolled contraction).
• Transmission:
  • C. tetani widespread in soil and dust, also in gastrointestinal tract and feces of those who have eaten endospore-contaminated food.
  • Anaerobic puncture wounds: stepping on a nail, body piercing, tattooing, animal bites, splinters, injected-drug abuse, insect stings, also surgery; abrasions, and burns if anaerobic due to dirt, dead tissue.
  • Individuals who get tetanus have inadequate immunity, either not immunized, or missed scheduled booster shot.
• Treatment and Prevention:
  • Thoroughly cleaning all wounds of dead tissue and foreign material that could cause anaerobic conditions.
  • Vaccine (DTaP) contains inactivated tetanospasmin.
  • Also includes diphtheria toxoid and acellular pertussis (whooping cough) vaccine.
  • Booster given every 10 years.
  • Human tetanus immune globulin (TIG) injection
  • Antibodies bind to toxin molecules, provide passive immunity, but do not neutralize attached tetanospasmin or repair nerve damage.
  • Muscle relaxants and supportive care necessary.
  • Antibacterial kills actively growing C. tetani cells.

Clostridial Myonecrosis (“Gas Gangrene”)

• Signs and Symptoms:
  • Severe pain, swelling, and fluid leaks from wound; may look frothy (gas production).
  • Incubation period: 1–5 days.
  • Tight skin becomes mottled with black as tissue dies.
  • Near death, victim becomes delirious and goes into coma.
  • Rare but life-threatening.
• Causative Agent:
  • Clostridium perfringens most common.
  • Encapsulated, Gram-positive, spore-producing, anaerobic, rod-shaped.
  • Releases gases as a product of fermentation.
  • Gases increase pressure and aid spread of infection.
• Transmission:
  • C. perfringens widespread in soil, common in intestinal tract of animals and humans; sometimes in vagina of healthy women.
  • Gas gangrene occurs in neglected wounds and occasional surgical wounds; in uterus after self-induced abortions, occasionally after miscarriages or childbirth.
  • Blood vessel disease or diabetes (poor oxygenation of tissue) are predisposing factors.
• Treatment and Prevention:
  • Prompt debridement of all dead and infected tissue; perhaps amputation.
  • Antibiotics, perhaps hyperbaric oxygen treatment.
  • No available vaccine.

Fungal Wound Infections

Sporotrichosis (“Rose Gardener’s Disease”)

• Signs and Symptoms:
  • Resemble those of a bacterial skin infection, often leading to misdiagnosis.
  • Chronic localized forms, usually within 1 to 3 weeks.
  • Series of ulcerating nodules that develop toward center of body.
  • Lymph nodes enlarge, but patients generally do not become ill unless immunocompromised.
  • Death is rare; painless.
• Causative Agent:
  • Sporothrix schenckii
  • Dimorphic fungus that switches between single cellular (yeast-like) to multicellular (mold) life stages.
  • Found in soil and moss.
• Transmission:
  • Conidia (reproductive spore) enters through injury.
  • Mostly associated with puncture wounds from plant material; farmers, carpenters, gardeners.
  • Can be transmitted by infected animals.
• Treatment and Prevention:
  • Antifungal medications.
  • Prevention includes wearing protective gloves and clothing.

Review Questions

• Pseudomonas aeruginosa causes:
  • Greenish pigment in wounds
• Popular name for tetanus is:
  • Lockjaw