osteomyelitis

Question 1

define osteomyelitis

Answer 1

infection and inflammation of bone/bone marrow

Question 2

describe the route of infection

Answer 2

• trauma & surgery
  e. g. joint replacement, root canal
• from local infection
  e. g. skin, soft tissue infection
• from blood
  e. g. bacteraemia

Question 3

pathogenesis

Answer 3

1. bacteria infect bone (colonise, proliferate)
2. leukocyte infiltrate, fight bacteria
3. inflammation, pus forms
4. de-vascularisation, dead bone, abscess forms
5. bacteria can invade bone cells and evade immune response
   =chronic osteomyelitis

if it spreads to joints
= septic arthritis

Question 4

risk groups

Answer 4

• diabetics with foot ulcers
• patients with infections following trauma, bone surgery, joint replacement, root canal tx
• patients with skin and soft tissue infections

Question 5

pathogens involved

Answer 5

most common = staph aureus

• strep pyogenes (group A)
• group B strep

Question 6

diagnosis of osteomyelitis

Answer 6

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)

Question 7

describe the lab diagnostic testing

Answer 7

1. gram-staining should change to positive = cocci present (dark purple)
2. 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

Question 8

describe gram staining

Answer 8

• 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

Question 9

virulence factors of staph aureus

Answer 9

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)

Question 10

describe the virulence of spreading factors

Answer 10

• staphylokinase (fibrinolysin) causes fibrinolysis, dissolves clots
• lipases
• DNAases (reduces viscosity of purulent material)
• cytolysins (destroys epithelial cells)

Question 11

describe the virulence of immune evasion factors

Answer 11

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

Question 12

therapy

Answer 12

• 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

Question 13

Beta-lactam resistance penicillin

Answer 13

• can’t be destroyed by B-lactamases
• e.g. flucloxacillin
• can also use penicillin with B-lactase inhibitor
  = augmentin

Question 14

action of beta-lactam antibiotics

Answer 14

• 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)

Question 15

define MRSA

Answer 15

methicillin resistant s. aureus

Question 16

name other diseases caused by s. aureus

Answer 16

• impetigo
• folliculitis
• cellulitis
• septic arthritis
• bacterial pneumonia