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
