L14: Osteomyelitis Flashcards
Routes of infection
Osteomyelitis = infection and inflammation of bone/bone marrow
Trauma: joint replacement, root canal treatment
Spreading from local area of infection: SSTI, diabetic ulcer
Haematogenous route: bacteria
Pathogenesis of osteomyelitis
- Bacteria infect bone = colonise and proliferate
- Leukocytes infiltrate and fight bacteria
- Inflammation and formation of pus
- Devascularisation, dead bone, abscess formation
- Bacteria can invade bone cells and evade immune response and drugs -> chronic osteomyelitis
- Bacteria can spread to joint -> septic arthritis
What area of the body is affected by osteomyelitis?
Adults: can be vertebral (rare in children), prosthetic joint infection, diabetic foot infection or post-traumatic infection (anywhere)
Children: mostly long bones (tibia, femur) and less common (humerus, fibula, calcaneus)
Risk groups for osteomyelitis
- Diabetics with foot ulcers
- Patients with infections following trauma, bone surgery, joint replacement, root canal
- IV drug user
- Patients with skin and soft tissue infections
- Children with chicken pox infection (infrequent)
Causative pathogens of osteomyelitis
- S. aureus most common (>80% of cases)
- S. pyogenes (group A strep)
- Group B strep (mainly infants at delivery)
- Coagulase-negative staph
- Haemophilus influenzae
- Enterobacter species
- Fungi
Diagnosis of osteomyelitis
Specific symptoms: pain/weakness of specific bone, redness, fever
Radiology: xray, CT
Bone biopsy: invasive, avoided if possible
Blood sample: high WBC count, bacteria present when associated with bacteraemia
MRI: to confirm diagnosis
Lab diagnostics - gram staining
- Differentiation of cell wall type (gram negative = thin peptidoglycan layer, gram positive = much thicker)
- Crystal violet stained (water soluble) taken up by both gram neg and pos then iodine to crystalise dye (trapped in gram positive)
- Decolourise
- Gram positive = violet (staph grape-like, strep chains)
- Gram negative = wash out then counter stain with safranin (pink)
Lab diagnostics - catalase test
- Staph positive and strep negative
- Bacteria on glass slide with drop of hydrogen peroxide = oxygen bubbles with staph (conversion to oxygen and water)
- Staph has catalase enzyme
Lab diagnostics - haemolysis and coagulase test
Haemolysis: on blood agar to distinguish alpha, beta and gamma strep
Coagulase test:
- For staphylococci
- Positive for S. aureus (clot forms - converts fibrinogen to fibrin)
- Negative for coagulase negative staphylococci (CONS)
Staphylococci aureus
- Opportunistic pathogen
- Colonises anterior nares (20% of people are asymptomatic carriers), transient carriage on skin
- Transmitted human to human
- Source of infection from community, hospital
- Diseases: SSTIs, invasive disease, toxic shock
S. aureus virulence factors
Adhesions (e.g. MSCRAMMS)
Immune evasion factors (neutralise parts of immune response)
Spreading factors:
- Staphylokinase (fibrinolysin) causes fibrinolysis (dissolves clots)
- Lipases
- DNAases (decrease viscosity of purulent material)
- Cytolysins (destroy epithelial cells)
Immune evasion factors
- Cytolysins: (haemolysis, leukocidin) kills RBCs, leukocytes, tissue cells
- Capsule: prevents opsonisation by C3b or Ig
- Slime layer (biofilm): microbial community with bacteria attached to substrate embedded in a matrix of EC polymeric substances, prevents antibiotics and immune components getting to bacteria (e.g. catheter)
- Protein A: binds IgG via Fc region, prevents opsonisation and phagocytosis
- Cell bound coagulase (clumping factor): binds prothrombin and induces fibrin polymerisation, fibrin surrounds bacteria preventing opsonisation and phagocytosis
Superantigens
- Family of heat-resistant secreted proteins (>20 members)
- Non-specific, highly potent T cell mitogens
- Trigger strong pro-inflammatory immune responses (binds to T cells and stimulates mitosis)
- Synergistic effect with endotoxin
- Systemic inflammation with tissue destruction, vascular leakage, multi-organ failure, toxic shock
Therapy for osteomyelitis
Prolonged antibiotic treatment (weeks-months)
90% of S. aureus resistant to penicillin, 30% resistant to methicillin (MRSA)
Vancomycin often last option for MRSA
Possibly surgical debridement
Beta-lactam antibiotics
- Penicillins, cephalosporins, carbapenems
- Only work against gram positive
- Extended spectrum penicillins e.g. amoxycillin
- Irreversible inhibition of transpeptidase (=penicillin binding protein)
Resistance:
- By beta-lactamases e.g. penicillinase
- Plasmid-encode enzymes transferred between bacterial strains and species
Beta-lactamase-resistant penicillin
- Cannot be destroyed by beta-lactamase
- e.g. oxacillin, flucloxacillin (not methicillin)
- Or use penicillin with beta-lactamase inhibitor e.g. amoxycillin + clavulanic acid (gentamicin)
Impetigo
- Purulent infection of skin
- Affects mostly young children (tropical climate, low hygiene)
- Pus-filled vesicles on face and limbs
- Rupture and drying out causing crusting
- Prevent with good hygiene
- Treat with washing with soap water, topical antibiotics e.g. mupirocin
Folliculitis
- Pyogenic infection of hair follicles
- Base of follicle is raised, reddened, pus beneath epithelial surface
- Furuncle = extension of folliculitis, boil (large painful, pus-filled, raised cutaneous nodules)
- Carbuncles = progress of infection into deeper tissues, sometimes systemic symptoms
- Treatment with antibiotic cream, drainage, oral antibiotics (flucloxacillin, augmentin)
Ritter’s disease
= staphylococcal scalded skin syndrome
- Abrupt onset of localised perioral erythema over entire body
- Formation of large bullae, cutaneous blisters
- Desquamation of epithelium
- No leukocytes in blisters (toxigenic disease, exfoliative toxins - ET)
- ET are proteases which cleave cell connecting proteins
- Primarily neonates, young children
Cellulitis
- Rapidly spreading pyogenic inflammation of dermis/subcutaneous tissue
- Often after mild traumas, burns, surgical incisions
- Area tender, warm, erythematous, swollen, no sharp demarcation
- Risk factors = old age, immunodeficiency, diabetes
- Treatment by oral or IV antibiotics (augmentin, flucloxacillin)
Acute infectious endocarditis
- Infection of inner tissue of heart including valves
- S. aureus most common cause for acute IE
- Sub-acute IE can be caused by other organisms (e.g. streptococci)
Bacterial pneumonia
- Inflammation of lung by colonisation of alveoli
- Aspiration of oral secretion or by hematogenous spread
- Consolidation and abscess formation in lung
- Necrotising pneumonia is severe and caused by S. aureus
- S. aureus common cause but most common is strep pneumoniae
Necrotising fasciitis
- Infection in subcutaneous tissue and fascia, extensive and rapid spreading necrosis
- Can develop after wound infection
- Deep tissue infection supported by spreading factors, often linked with bacteraemia and toxic shock syndrome
- Treatment: IV antibiotic (flucloxacillin, clindamycin), surgical debridement
Toxic shock syndrome
Menstrual TSS from S. aureus only
Non-menstrual TSS from superantigens, bacteria enter through wounds/small cuts, systemic inflammation, high mortality
Therapy: IV antibiotics, supportive
