Osteomyelitis microbiology
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"osteomyelitis"[MeSH Terms] AND "microbiology"[MeSH Terms]
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Osteomyelitis - Microbiology
Overview
Osteomyelitis is bone and marrow inflammation virtually always caused by infection. Although viruses, parasites, fungi, and bacteria can all produce osteomyelitis, pyogenic bacteria and mycobacteria account for the vast majority of cases. The causative organism depends strongly on age, route of infection, host risk factors, and anatomical location.
- Robbins & Kumar Basic Pathology, p. 779
- Rockwood and Green's Fractures in Adults 10e, p. 955
Routes of Infection and Their Microbiology
| Route | Typical Microbiology |
|---|---|
| Hematogenous | Usually monomicrobial; S. aureus predominates |
| Contiguous focus / direct spread | Often polymicrobial (up to 29% after trauma) |
| Post-traumatic / open fracture | Mixed gram-positive + gram-negative |
| Post-surgical / prosthetic | Coagulase-negative staphylococci (S. epidermidis) |
General Principle: Age-Stratified Pathogens
This is one of the most tested aspects of osteomyelitis microbiology:
| Age Group | Most Common Organisms |
|---|---|
| Neonates | Group B Streptococcus (GBS / S. agalactiae), E. coli & gram-negative coliforms, S. epidermidis |
| Infants < 4 years | S. aureus dominant; Kingella kingae now recognized as causing up to 82% of musculoskeletal infections in this group |
| Children | S. aureus (most common), Streptococcus spp.; H. influenzae type b (now rare with vaccination) |
| Adolescents / Adults | S. aureus (60-80% of culture-positive cases) |
| Older adults / Diabetics | S. aureus still #1, but gram-negative rods account for a higher proportion (15-35%) |
| Sickle cell disease | Salmonella spp. and other gram-negative organisms (bone necrosis + asplenia) |
| IV drug users | S. aureus most likely, then Pseudomonas aeruginosa (especially cervical/lumbar spine) |
- Rosen's Emergency Medicine, 9e, p. 3728
- Firestein & Kelley's Textbook of Rheumatology, p. 2450
- Robbins & Kumar Basic Pathology, p. 779 (sickle cell pattern)
The Dominant Pathogen: Staphylococcus aureus
S. aureus is the leading cause of osteomyelitis across all age groups (except neonates) and is isolated in 30-60% of culture-positive cases. Key mechanisms:
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Cell wall proteins (fibronectin-binding proteins, collagen adhesins) bind bone matrix, facilitating adherence
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MRSA is increasingly isolated from chronic osteomyelitis, associated with more surgical reinterventions and prolonged hospitalization
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Panton-Valentine Leukocidin (PVL)-producing strains cause leucocyte destruction; though uncommon in adults, they can cause invasive life-threatening disease - rapid identification is essential
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Rockwood and Green's, p. 956
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Robbins & Kumar, p. 779
Pathogen Table by Osteomyelitis Type (Rockwood & Green 2025)
Post-Traumatic Osteomyelitis
| Pathogen | Frequency |
|---|---|
| Staphylococcus aureus | 35-50% |
| Coagulase-negative staphylococci | 9-25% |
| Enterococcus faecalis | 5-8% |
| Streptococcus spp. | 3-6% |
| Propionibacterium acnes | 6% |
| Gram-negatives (Bacteroides, E. coli, Enterobacter, Klebsiella) | 6-15% each |
| Pseudomonas aeruginosa | 4% |
| Polymicrobial | 10-29% |
| Culture-negative | 11-35% |
Vertebral Osteomyelitis
| Pathogen | Frequency |
|---|---|
| Staphylococcus aureus | 33-55% |
| Coagulase-negative staphylococci | 2-22% |
| Streptococcus spp. | 2-16% |
| Enterococcus faecalis | 2-18% |
| E. coli, Pseudomonas | Notable gram-negative component |
| Mycobacterium tuberculosis | Up to 31% in some endemic regions |
- Rockwood and Green's Fractures in Adults 10e, Table 29-4, p. 956
Special Clinical Scenarios
Vertebral / Spinal Osteomyelitis
- S. aureus accounts for ~40-66% of acute cases
- Gram-negative bacilli (~20%): E. coli (9%), P. aeruginosa (6%)
- Subacute vertebral osteomyelitis: Mycobacterium tuberculosis (Pott's disease) or Brucella species - Harrison's Principles, 22e
Diabetic Foot Osteomyelitis
- Often polymicrobial: S. aureus, streptococci, gram-negatives, anaerobes
- Direct spread from soft tissue / pressure ulcers
Skull Base Osteomyelitis ("Malignant Otitis Externa")
- Pseudomonas aeruginosa is the most common organism (opportunistic, only pathogenic when host defences are defective)
- Scott-Brown's Otorhinolaryngology, Vol. 2
Puncture Wound Osteomyelitis (e.g., nail through shoe)
- Pseudomonas aeruginosa is the classic pathogen
Animal Bite Wounds
- Pasteurella multocida (dogs and cats)
Human Bite Wounds (hand)
- Oral flora: Streptococcus anginosus, Fusobacterium nucleatum, Eikenella spp.
Water Exposure (freshwater)
- Aeromonas hydrophila
Coagulase-Negative Staphylococci
- S. epidermidis is the dominant pathogen in periprosthetic / implant-associated infections; forms biofilm on prosthetic surfaces
Immunocompromised Patients / Endemic Areas
- Fungi: Candida, Aspergillus, Coccidioides, Blastomyces, Cryptococcus
- Mycobacteria (TB and non-tuberculous)
- Spirochetes (Treponema pallidum - syphilitic osteomyelitis)
- Brucella spp. in endemic areas (livestock contact, unpasteurized dairy)
Kingella kingae - Emerging Pathogen (Clinically Important)
- Part of normal nasopharyngeal flora in young children; spreads hematogenously
- Now recognized as causing up to 82% of musculoskeletal infections in children under 4 years
- Fastidious organism; may be misidentified as Haemophilus or Neisseria
- Often requires PCR for diagnosis (culture misses it)
- Rosen's Emergency Medicine, p. 3728; Firestein & Kelley's Rheumatology, p. 2450
Culture-Negative Osteomyelitis
Nearly 50% of patients have no organism identified on culture (Robbins). Among specific types:
- Post-traumatic: up to 32% culture-negative
- Vertebral: up to 41% culture-negative
- The proportion of culture-negative cases is increasing
Clinical implications: empirical antibiotic therapy required, surgical debridement may be less targeted, and local antibiotic delivery systems may be ineffective.
Summary Mnemonic Aid
| Scenario | Key Organism |
|---|---|
| Any age (most common overall) | S. aureus |
| Neonates | GBS, E. coli |
| Children < 4 years | S. aureus + Kingella kingae |
| Sickle cell disease | Salmonella spp. |
| IV drug users | S. aureus > Pseudomonas |
| Diabetic foot | Polymicrobial |
| Prosthetic / implant | S. epidermidis |
| Puncture wound / nail | Pseudomonas |
| Skull base | Pseudomonas |
| Vertebral (subacute, endemic) | M. tuberculosis, Brucella |
| Animal bite | Pasteurella multocida |
| Human bite (hand) | Eikenella, oral anaerobes |
Sources: Robbins & Kumar Basic Pathology | Rosen's Emergency Medicine 9e | Rockwood and Green's Fractures in Adults 10e | Firestein & Kelley's Rheumatology | Bailey & Love's Surgery 28e | Harrison's Principles of Internal Medicine 22e | Scott-Brown's Otorhinolaryngology
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