osteomyelitis Flashcards
define osteomyelitis
infection and inflammation of bone/bone marrow
describe the route of infection
- trauma & surgery
e. g. joint replacement, root canal - from local infection
e. g. skin, soft tissue infection - from blood
e. g. bacteraemia
pathogenesis
- bacteria infect bone (colonise, proliferate)
- leukocyte infiltrate, fight bacteria
- inflammation, pus forms
- de-vascularisation, dead bone, abscess forms
- bacteria can invade bone cells and evade immune response
=chronic osteomyelitis
if it spreads to joints
= septic arthritis
risk groups
- diabetics with foot ulcers
- patients with infections following trauma, bone surgery, joint replacement, root canal tx
- patients with skin and soft tissue infections
pathogens involved
most common = staph aureus
- strep pyogenes (group A)
- group B strep
diagnosis of osteomyelitis
symptoms
- pain/ weakness of specific bones
- redness
- fever
blood sample
- high WBC count
- bacteria present if associated with bacteraemia
MRI = confirm diagnosis
bone biopsy (highly invasive but specific)
describe the lab diagnostic testing
- gram-staining should change to positive = cocci present (dark purple)
- catalase test to confirm staph (+ve) or strep (-ve)
if +ve
= then a coagulase to get a positive result for S. aureus
if -ve
= hemolysis to get beta and therefore s. pyogenes
describe gram staining
- differentiates cell wall type (gram negative = thin peptidoglycan layer)
- crystal violet stained, soaks into cell wall; washed with iodine
- counter stain safranin to show gram-negative
- staph and step both positive
chain = strep
grape-like cluster = staph
virulence factors of staph aureus
adhesions
- for binding to host tissue
- e.g. MSCRAMMs
immune evasion factors
- neutralize certain parts of the immune response
spreading factors
- allow bacteria to spread from local infection into deeper tissue or blood (bacteraemia)
describe the virulence of spreading factors
- staphylokinase (fibrinolysin) causes fibrinolysis, dissolves clots
- lipases
- DNAases (reduces viscosity of purulent material)
- cytolysins (destroys epithelial cells)
describe the virulence of immune evasion factors
capsule
- prevents opsonisation by C3b or Ig
cytolysins
- kills erythrocytes, leukocytes, tissue cells
protein A
- binds IgG in wrong orientation
- prevents opsonisation and phagocytosis
cell bound coagulase (clumping factor)
- binds prothrombin & induces fibrin polymerisation
- fibrin surrounds bacteria preventing opsonisation and phagocytosis
therapy
- prolonged antibiotic tx
- 90% of s. aureus resistant to penicillin, 30% resistant to MRSA
tx usually requires the use of B-lactamase resistant derivatives of penicillin
Beta-lactam resistance penicillin
- can’t be destroyed by B-lactamases
- e.g. flucloxacillin
- can also use penicillin with B-lactase inhibitor
= augmentin
action of beta-lactam antibiotics
- includes penicillin
- penicillin works only against gram-positives
- extended spectrum penicillins e.g. amoxycillin
irreversibly inhibits transpeptidase, which catalyses final step in cell wall and inhibits cross linking (breakdown of cell wall)
define MRSA
methicillin resistant s. aureus
name other diseases caused by s. aureus
- impetigo
- folliculitis
- cellulitis
- septic arthritis
- bacterial pneumonia
