Staph aureus abscess

Question 1

List some DDx for a presentation of:

• productive cough
• fever
• tachycardia
• dyspnoea
• tachypnoea
• R base coarse crepitations

Answer 1

PDx. CAP
DDx.
• Respiratory
o Infective- CAP, HAP, TB, lung abscess, acute bronchitis, infective exacerbation of COPD/asthma, bronchiectasis, pharyngitis
o Malignancy- primary lung ca (small cell, non-small cell- adenocarcinoma, squamous cell, large cell, bronchial)
o Vascular- PE
o Genetic- CF
• Cardiac- APO (from sudden HF)

Question 2

What investigations would you do for CAP?

Answer 2

Diagnostic:
o Sputum MCS- staph aureus (GP cocci, Horse blood agar, catalase positive, coagulase positive) 
o Acid-fast bacilli test
o CXR- multi-lobar, consolidation, complicating lesions (pulmonary cavitation, abscess, empyema, pleural effusion, pneumothorax), most right lung (65%)
Bedside:
o ABG- resp failure
o ECG- APO cardiac failure
Labs:
o FBC- neutrophilia
o CRP/ESR
o Blood culture- sepsis 
o Bronchoalveolar lavage + PCR

Question 3

Differentiate between staph a strep upon culture?

Answer 3

Both staph and strep are GP cocci
• Staph:
o Morphology: grape-like, irregular clusters, golden colonies
o Catalase positive
o Coagulase then to differentiate staph aureus (positive) and Staph epidermidis/Staph saprophyticus (negative)
• Strep: rows
o Catalase negative

Question 4

Describe the pathogenesis of a lung abscess formation?

Answer 4

• staph aureus infiltration -> immune response
• > neutrophils
• > fibrinosuppurative exudate
• > staph aureus coagulase activates prothrombin (-> thrombin -> fibronogen to fibrin)
• > fibrin encloses organism to protect from phagocytosis -> abscess

Question 5

List some staph aureus virulence factors?

Answer 5

Evade immune defence:
- coagulase (fibrin production to wall off organism)
- catalase (protects against hydrogen peroxide)
- protein A (binds IgG inhibiting opsonisation)
- penicillinase (breaks down B lactam ring -> penicillin resistance)
- mutated PBP (MRSA)
- haemolysins (destroys RBCs, macrophages, neutrophils, platelets)
- leukocidins (destroys leukocytes)
Local destruction:
- hyaluronidase (CT breakdown, inavsion)
- staphylokinase (plasminogen, plasmin, breakdown fibrin clot)
- DNAse (breaks down DNA, spread)
- protease (breaks down protein)
- lipase (fat breakdown, cutaneous colonisation)

Question 6

What else could cause a cavitating lesion on CXR?

Answer 6

• Neoplasm- primary (e.g. SCC), mets
• AI- Wegner’s Granulomatosis, RA nodules, Sarcoidosis
• Vascular- septic PE
• Infection- necrotising/cavitating pneumonia, TB, post-pneumonia pneumatocoee (air-filled cyst), lung abscess
• Congenital- bronchogenic cyst, congenital malformation
• Aspiration

Question 7

What organisms would you find in a throat swab? 


Answer 7

Normal oropharyngeal flora:
• Strep viridans
• Staphylococci 
• HACEK- Haemophilus, Aggregatibacter (previously Actinobacillus), Cardiobacterium, Eikenella, Kingella.
• Moraxella catarhalis 
• Candida

Question 8

How would you treat a staph a abscess?

Answer 8

MSSA
- Flucloxacillin (B-lactamase resistant penicillin due to bulkier R group)
- If penicillin hypersensitivity: Cephazolin (1st gen cephalosporin) (covers strep, staph/B-latamase producing staph, E.coli, Klebsiella, PECK)
- If penicillin anaphylaxis: Clindamycin (Lincosamide, B-lactam abx), due to 5% chance Cephalosporin allergy
MRSA
- Vancomycin

Question 9

Describe the MA and SE of Flucloxacillin?

Answer 9

Flucloxacillin (B-lactamase resistant penicillin due to bulkier R group)
o MA: beta lactam ring selectively, irreversibly binds to PBP -> inhibits peptidoglycan cross-linking -> accumulation of cell wall precursors -> autolytic enzyme activation -> cell lysis
o SE: hypersensitivity, GI distress, C diff, interstitial nephritis, encephalopathy, haem toxicity

Question 10

Describe the MA and SE of Cephazolin?

Answer 10

Cephazolin (1st gen cephalosporin)
- covers strep, staph/B-latamase producing staph, E.coli, Klebsiella, PECK
o MA: inhibits peptidoglycan crosslinking -> inhibits cell wall synthesis
o SE: similar to penicillin

Question 11

Describe the MA and SE of Clindamycin?

Answer 11

Clindamycin (Lincosamide, B-lactam abx)
o MA: inhibits peptidoglycan crosslinking -> inhibits cell wall synthesis
o SE: hypersensitivity, nephrotoxicity, ototoxicity, pseudomembranous colitis, neutropenia