## Epidemiology of Acute Hematogenous Osteomyelitis **Key Point:** Staphylococcus aureus accounts for 80–90% of acute hematogenous osteomyelitis cases in children and adults, regardless of age. ### Organism Characteristics - **S. aureus** is gram-positive, catalase-positive, coagulase-positive - Produces bone-seeking virulence factors (collagen-binding proteins, fibronectin-binding adhesins) - Can survive within osteocytes and osteoblasts - Increasingly includes methicillin-resistant strains (MRSA) in healthcare settings ### Why S. aureus Dominates 1. High affinity for bone matrix proteins 2. Ability to produce biofilms on bone and implants 3. Frequent skin colonization (portal of entry via minor trauma) 4. Hematogenous dissemination to metaphyseal vessels (sluggish blood flow) ### Comparative Organism Frequency | Organism | Frequency | Age/Context | Notes | | --- | --- | --- | --- | | *S. aureus* | 80–90% | All ages | Gold standard causative agent | | *S. pyogenes* | 5–10% | Children, post-streptococcal | Less common, usually group A | | *E. coli* | 2–5% | Neonates, immunocompromised | Gram-negative, less bone tropism | | *H. influenzae* type b | <1% | Now rare (post-Hib vaccine) | Historical significance only | **High-Yield:** In any osteomyelitis question without specific risk factors (neonatal, IV drug use, immunocompromised), the answer is *S. aureus*. **Clinical Pearl:** MRSA prevalence varies by geography and healthcare exposure; empiric coverage should include anti-staphylococcal agents (cephalosporins, clindamycin, or vancomycin depending on local resistance patterns). 
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