Microbiology 12 - Wound, bone and joint infections

Question 1

What are the major pathogens causing surgical site infections?

Answer 1

Staphylococcus aureus (MSSA and MRSA)

Escherichia coli

Pseudomonas aeruginosa

Question 2

At what point is SSI risk increased?

Answer 2

surgical site contaminated with >10^5 microorganisms / gram of tissue

Question 3

What are the 3 levels of wound infection?

Answer 3

Superficial incisional (skin and subcutaneous tissue)
Deep incisional (fasical and muscle layers)
Organ/ space infection

Question 4

What is the likely organism for SSI if there is a Gram +ve, haemolytic coccus visualised?

Answer 4

MRSA

Question 5

What is the treatment for MRSA SSI?

Answer 5

IV linezolid

Question 6

Recall 2 important risk factors for wound/ bone/ joint infection

Answer 6

Diabetes
Obesity (adipose tissue is poorly-vascularised)

age

Rheumatoid arthritis (stoop DMARDs before operation)

Smoking (nicotine delays wound healing and → PVD = poor vascularisation)

Question 7

How does shaving affect risk of SSI?

Answer 7

Shaving ↑ risk (micro-abrasions multiply bacteria)

Question 8

What can be done during orthpaedic procedures to reduce risk of infection?

Answer 8

Laminar flow

Question 9

What is the MOST POWERFUL risk factor for SSI following cardiothoracic surgery?

Answer 9

Staph aureus nasal carriage

Nasal decontamination should be offered if they are found to be carrying S. aureus

Question 10

How does body temperature affect the risk of surgical site infections?

Answer 10

hypothermia → vasoconstriction → ↓O2 → impaired neutrophil function

Question 11

Which patient group is particularly at risk of septic arthritis?

Answer 11

• Rheumatoid arthritis, osteoarthritis, crystal arthritis
• Joint prosthesis
• IVDU
• Diabetes, chronic renal disease, chronic liver disease
• Immunosuppression (e.g. steroids)
• Trauma – intra-articular injection, penetrating injury

Question 12

Where in the joint do organisms adhere to in septic arthritis?

Answer 12

Synovial membrane

Bacterial proliferation in synovial fluid → inflammatory response → joint damage

Joint damage → exposure of host derived protein (e.g. fibronectin) to which bacteria can adhere

Question 13

What are some bacterial factors which may increase the risk of septic arthritis?

Answer 13

S aureus

• fibronectin-binding protein that recognise selected host proteins
• cytotoxin PVL → fulminant infection

Kingella kingae synovial adherence is via bacterial pili

Question 14

What are some host factors which may increase the risk of septic arthritis?

Answer 14

• Leucocyte derived proteases and cytokines → cartilage and bone damage
• Raised intra-articular pressure impedes capillary blood flow → cartilage and bone ischaemia/necrosis
• Genetic deletion of macrophage-derived cytokines → reduce host-response in S. aureus sepsis
• Absence IL-10 increases severity of staphylococcus joint disease

Question 15

Recall some causative organisms for septic arthritis

Answer 15

Staphylococcus aureus 46%

Streptococci: pyogenes, pneumoniae, agalactiae

Gram-negative organisms: Escherichia coli, Haemophilus influenzae, Neisseria gonorrhoea, Salmonella

Coagulase-negative staphylococci 4%

rare: Lyme disease, Brucellosis, Mycobacteria, Fungi Rare

Question 16

What are the symptoms of septic arthritis?

Answer 16

Red, painful and swollen joint with *restricted movement*

Monoarticular in 90%

Knee is involved in 50%

Question 17

What investigations for septic arthritis should be done before starting antibiotics?

Answer 17

1. Blood cultures
2. Synovial fluid aspiration (synovial count >50,000 WBC/mL is used to suggest septic arthritis) - USS guided

Question 18

When is MRI indicated for septic arthritis patients and what would it show?

Answer 18

When osteomyelitis is suspected

joint effusion, articular cartilage destruction, abscess, contiguous osteomyelitis

Question 19

How is septic arthritis managed?

Answer 19

IV cephalosporin or flucoxacillin, up to 6 weeks (outpatient setting)

may need to +vancomycin (if high risk MRSA)

OPAT (outpatient parenteral Abx team)

Drainage - arthoscopic washout

Question 20

What are the symptoms of spinal osteomyelitis/epidural abscess?

Answer 20

Back pain and fever

Neuro impairment

Question 21

Where is Spinal osteomyelitis / epidural abscess most commonly localised?

Answer 21

Lumbar (43.1%)

Cervical (10.6%)

Cervico-thoracic (0.4%)

Question 22

Recall the pathophysiology of spinal osteomyelitis/epidural abscess

Answer 22

Acute haematogenous spread (bacteraemia)

Exogenous (after disc surgery, implant associated)

Question 23

what are the most common causative organisms for spinal osteomyelitis/epidural abscess?

Answer 23

• Staphylococcus aureus (48.3%)
• Streptococcus
• Gram-negative rods
• Coagulase-negative staphylococcus

Question 24

How is vertebral osteomyelitis treated?

Answer 24

At least 6w of antibiotics

Question 25

How is spinal osteomyelitis/epidural abscess diagnosed?

Answer 25

MRI - 90% sensitive Blood cultures CT-guided/open biopsy Chronic - XR first line (screening, changes take about 10 days)

Question 26

Symptoms of chronic osteomyelitis

Answer 26

pain Brodies abscess Sinus tract

Question 27

what is the treatment for chronic osteomyelitis?

Answer 27

Radical debridement to living bone Papineau/Lautenbach Technique:

Question 28

What would you suspect in 76yo man, 4/12 hx of back pain with left leg radiation MRI: discitis of L2/L3 → spinal biopsy → CoNS grown → histology shows vague granuloma No growth on agar of standard organisms and further PCR and IgG

Answer 28

Brucella

Question 29

Management of Brucella

Answer 29

rifampicin, ciprofloxacin and doxycycline

Question 30

symptoms of prosthetic joint infection

Answer 30

Pain Patient complain joint was 'never right' after the operation → early failure Sinus tract

Question 31

What is the most common pathogen implicated in prosthetic joint infections?

Answer 31

**Coagulase negative staphylococcus** (\>S aureus) also strep, enterococci Aerobic Gram-negative bacilli: Enterobacteriaceae, Pseudomonas aeruginosa

Question 32

What would an x ray show in prosthetic joint infections?

Answer 32

loosening (bone loss along cement-bone interface)

Question 33

How many tissue samples are required for the lab when a patient goes to theatre for a suspected joint infection?

Answer 33

5 (If \>3 yield same organism = infection)

Question 34

What are the 2 options for management of prosthetic joint infection?

Answer 34

1. Endo-Klinik - Single stage revision (removal of manky prosthesis and replacement with antibiotic-impregnated material prosthesis) 2. Two stage revision (firstly remove prosthesis and send off samples, put in a spacer, wait a few weeks, then put new prosthesis in)