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

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