15A05: Exam Report
Classify gram positive bacteria with examples (20% of marks). Outline the pharmacology of vancomycin (80% of marks).
75% of candidates passed this question.
The classification should have demonstrated a framework that covered relevant gram positive pathogens. Examples should have included both genus and species. More detail than “strep” or “Staph” was expected.
Knowledge of vancomycin was expected to include an outline of pharmaceutics, pharmacodynamics, pharmacokinetics, dosing and adverse effects. In particular, pharmacokinetics should be well understood as there are significant implications for dosing.
Common errors included incorrect examples such as “clostridium” or classifications lacking detail with respect to examples.
T1i / 15A05: Classify Gram Positive bacteria with examples (20 marks). Outline the pharmacology of Vancomycin (80 marks)
Definitions
Bacteria = Prokaryote – single cell organism lacking a nucleus or any membrane-bound organelles
Gram Stain
- Bacterial slides are first stained w Crystal Violet and Iodine
- Then destained w alcohol
- Then counter-stained w safranin
Gram +ve
- Blue purple
- Large peptidoglycan wall
- Crystal violet precipitates in wall and not affected by alcohol wash
Gram +ve
Rods
Cocci
Anaerobic
Clostridium
Staphylococci
Streptococci
Enterococci
Aerobic
Listeria
Peptococcus
Additional Classification
Catalase
- Hydrogen peroxide added to bacterial sample
- Presence of catalase produced oxygen
- Catalase+ = Staphylococci
- Catalase- = Streptococci & Enterococci
Coagulase
- Performed on Staphylococcals
- Coagulase is an enzyme wh cleaves fibrinogen → fibrin
- Rabbit plasma added to bacterial sample
- If coagulase is present, fibrin will form
- Coag+ = S Aureus
- Coag- = S Epidermidis
Hemolytic
- Performed on Streptococcal
- Streptococcal colonies are added to blood agar
- Colour change = hemolysis
- α haemolytic
- dark green agar
- due to metHb produced by organism
- Strep pneumonia
- β haemolytic
- yellow agar
- complete hemolysis
- Strep pyogenes
- y haemolytic
- Agar unchanged
- E faecalis
- E faecium
- α haemolytic
Vancomycin
Class
Glycopeptide
Dose
25-30mg/kg (load)
+
1.5mg bd-tds (normal renal fn)
Mechanism
Cell Wall Inhibitor
MoA
Binds D-Ala-D-Ala sequence on bacterial cell wall
Interrupts peptidoglycan synthesis → Bactericidal
Time v Concentration
Time > MIC
Post dose effect
Short
Spectrum
Covers
Doesn't Cover
G+ including MRSA
C diff (oral only)
C perfringens
G -ves
VRE/VRSA
Anaerobes
Indications
Suspected/proven MRSA
C diff
Penicillin allergy
PK
A
Poor. Only po for C diff
D
PPB 50%. Vd 2L/kg
M
None
E
Renally excreted unchaged. ∴needs dose adjustment in renal failure
Critical Illness
A/E
Red man syndrome; mass histamine release (related to rate of infusion not an allergy. Cease and recommence at a slower rate)
Ototoxicity: related to peak dose
Drug fever
Resistance
Bacteria mutate to D-Ala-D-lactate
∴ no binding site for Vanc
Monitoring
Renal Function
Levels required; as per hospital protocol eg trough target 15-20mg/dL