Microbiology Cheat Sheet
Bone, Joint & Surface Infections
Bone, joint and skin infections for NEET-PG: osteomyelitis organisms, sickle cell Salmonella, septic arthritis, PJI biofilm, and erysipelas vs cellulitis.
MedNext Academy | 3 min read
Bone, Joint & Surface Infections
Bone, joint and skin infections for NEET-PG: osteomyelitis organisms, sickle cell Salmonella, septic arthritis, PJI biofilm, and erysipelas vs cellulitis.
This topic covers the microbiology of osteomyelitis, septic arthritis, prosthetic joint infection, and skin and soft tissue infections, organised by anatomical depth and clinical setting.
High-yield lines
- Staphylococcus aureus is the most common cause of acute haematogenous osteomyelitis in all age groups except neonates.
- In sickle cell disease, Salmonella species surpass S. aureus as the leading cause of osteomyelitis.
- In children, osteomyelitis affects the metaphysis of long bones, while in adults the vertebral bodies are the most common site.
- Sequestrum is dead avascular bone harbouring bacteria, and involucrum is new bone enveloping it; both are hallmarks of chronic osteomyelitis.
- Bone biopsy or deep tissue is the gold-standard specimen for osteomyelitis, and sinus tract swabs correlate poorly and should not guide therapy.
- MRI is the most sensitive and specific imaging modality for early osteomyelitis, whereas plain films may be normal for 10 to 14 days.
- Septic arthritis is a medical emergency because articular cartilage is avascular and bacterial proteases cause irreversible destruction within 24 to 48 hours.
- Synovial fluid analysis is the cornerstone of septic arthritis diagnosis, with a white cell count above 50,000 per microlitre and neutrophil predominance being highly suggestive.
- The presence of crystals in synovial fluid does not exclude septic arthritis, as gout and infection can coexist.
- Neisseria gonorrhoeae causes septic arthritis in sexually active young adults, presenting as a migratory dermatitis-arthritis syndrome with often culture-negative synovial fluid.
- Prosthetic joint infection is a biofilm disease; growth of the same organism from at least two independent deep samples is the threshold for diagnosis.
- Erysipelas has a raised, sharply demarcated border and is almost exclusively caused by Streptococcus pyogenes, whereas cellulitis has a flat, poorly defined margin.
- Bullous impetigo is caused exclusively by S. aureus exfoliative toxins that cleave desmoglein 1 in the epidermis.
- Dermatophytes invade keratinised tissue and show septate branching hyphae on KOH mount; specimen must be taken from the active scaly edge of the lesion.
Mapped competency codes
- MI4.2
- MI4.3
Continue into the full chapter
This summary maps to MI4-musculoskeletal-skin-and-soft-tissue-infections.
Frequently Asked Questions
Which organism causes osteomyelitis in sickle cell disease?
Salmonella species are the classic cause of osteomyelitis in sickle cell disease, surpassing S. aureus in this setting.
What is the gold-standard specimen for osteomyelitis diagnosis?
Bone biopsy or deep tissue at surgery, because sinus tract swabs correlate with deep culture in only 40 to 50 percent of cases.
Does synovial fluid crystal presence exclude septic arthritis?
No, gout and septic arthritis can coexist, particularly in elderly patients with pre-existing joint disease.
How is erysipelas distinguished from cellulitis?
Erysipelas has a raised, sharply demarcated border and is caused by S. pyogenes, while cellulitis has a flat, poorly defined margin.
Continue reading
Cheat SheetsAll Microbiology cheat sheets
Browse the complete authored set for this subject.
Open the complete revision pathway
Explore clinician-written learning resources, structured revision and practice across the MedNext platform.
Open in the MedNext appSee plans