Microbiology
Musculoskeletal, Skin and Soft Tissue Infections
Skin, soft tissue and bone infections for MBBS: Staphylococcus aureus, streptococci, gas gangrene, tetanus, osteomyelitis and dermatophytes, mapped to NMC codes MI4.1 to MI4.3.
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Musculoskeletal, Skin and Soft Tissue Infections
Skin, soft tissue and bone infections for MBBS: Staphylococcus aureus, streptococci, gas gangrene, tetanus, osteomyelitis and dermatophytes, mapped to NMC codes MI4.1 to MI4.3.
This chapter deals with infections of bone, joint, skin and soft tissue. It covers pyogenic and anaerobic organisms, the clostridial diseases of gas gangrene, tetanus and botulism, osteomyelitis and septic arthritis, and the fungal skin infections, together with the laboratory diagnosis of each.
High-yield: Musculoskeletal, Skin and Soft Tissue Infections
- Staphylococcus aureus is the commonest cause of acute osteomyelitis, skin abscesses, impetigo and cellulitis.
- Group A Streptococcus pyogenes causes erysipelas, cellulitis and the rapidly spreading necrotising fasciitis.
- Clostridium perfringens produces gas gangrene through its alpha toxin, a lecithinase that destroys tissue.
- Clostridium tetani causes tetanus by its neurotoxin tetanospasmin, which blocks inhibitory neurotransmitters and gives rigidity and lockjaw.
- Clostridium botulinum toxin blocks acetylcholine release and produces a descending flaccid paralysis.
- Necrotising fasciitis is a surgical emergency, often polymicrobial or due to group A streptococcus, needing urgent debridement.
- Chronic osteomyelitis may harbour Staphylococcus aureus in a dead bone sequestrum that resists antibiotics.
- In sickle cell disease, Salmonella is a characteristic cause of osteomyelitis.
- Prosthetic joint infection is often caused by coagulase-negative staphylococci forming biofilm on the implant.
- Dermatophytes such as Trichophyton cause ringworm and are diagnosed by potassium hydroxide mount showing septate hyphae.
- Candida albicans causes intertrigo and mucocutaneous candidiasis, favoured by moisture and immunosuppression.
- Pseudomonas aeruginosa is an important cause of burn wound and surgical site infection with blue-green pus.
- The tetanus toxoid vaccine and correct wound management prevent tetanus after injury.
- Gram stain of pus guides early empirical therapy in skin and soft tissue infection.
- Anaerobic infections are suspected with foul-smelling discharge, gas in tissues and necrosis.
Clostridial diseases
- **Gas gangrene:** Clostridium perfringens. Alpha toxin (lecithinase), crepitus, myonecrosis, urgent debridement.
- **Tetanus:** Clostridium tetani. Tetanospasmin, rigidity, lockjaw. Prevented by toxoid.
- **Botulism:** Clostridium botulinum. Blocks acetylcholine, descending flaccid paralysis.
- **Necrotising fasciitis:** Group A streptococcus or polymicrobial. Rapid spread, surgical emergency.
NMC competencies in this chapter
- **MI4.1:** Aetiology and pathogenesis of skin, soft tissue, bone and joint infections
- **MI4.2:** Anaerobic and clostridial infections including gas gangrene, tetanus and botulism
- **MI4.3:** Laboratory diagnosis, specimen processing and prevention
Frequently Asked Questions
Which organism most commonly causes acute osteomyelitis?
Staphylococcus aureus is the leading cause of acute osteomyelitis across all age groups, though Salmonella is characteristic in patients with sickle cell disease.
How does tetanus toxin cause its effects?
Tetanospasmin travels up the nerves and blocks the release of inhibitory neurotransmitters in the spinal cord, producing sustained muscle rigidity and spasms such as lockjaw.
Why is necrotising fasciitis a surgical emergency?
It spreads rapidly along fascial planes and destroys tissue faster than antibiotics can act, so urgent surgical debridement alongside antibiotics is life-saving.
How are dermatophyte infections diagnosed in the lab?
A potassium hydroxide mount of skin scrapings dissolves keratin and reveals the septate branching hyphae of dermatophytes, with culture on Sabouraud agar for confirmation.
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