Antimicrobial Susceptibility Test
What type of agar is used for the Kirby-Bauer test?
Mueller-Hinton agar, because it allows uniform diffusion of antibiotics.
What is the selective agent in MSA, and what does it inhibit?
7.5% NaCl, which inhibits most bacteria except halotolerant organisms like Staphylococcus.
What is the acceptable range of colonies for a countable plate?
30–300 colonies.
What is the purpose of the McFarland standard?
To ensure standardized bacterial inoculum density (usually 0.5 McFarland ≈ 1.5 × 10⁸ CFU/mL).
What does yellow growth indicate on MSA?
Mannitol fermentation, producing acid and lowering pH (e.g., Staphylococcus aureus).
What does TNTC mean, and why is it problematic?
Too Numerous To Count; cannot accurately estimate CFU.
How is the zone of inhibition measured?
In millimeters, across the diameter of the clear zone around the antibiotic disk.
What is the pH indicator in MSA?
Phenol red.
How do you calculate original cell density (OCD)?
OCD = CFU ÷ (dilution factor × volume plated).
Why must plates be incubated for a specific time and temperature?
To maintain standardized conditions for accurate comparison
What does pink or red growth indicate?
Growth without mannitol fermentation (e.g., Staphylococcus epidermidis).
What is the unit used to express bacterial counts in this method?
CFU/mL (Colony Forming Units per milliliter).
Why is Mueller-Hinton agar used for this test?
It is non-selective, non-differential, and supports most pathogens while allowing proper diffusion.
Why is MSA useful for identifying Staphylococcus aureus?
Because S. aureus ferments mannitol, turning the medium yellow.
Why is the spread plate method preferred over pour plates for enumeration?
Spread plates avoid heat damage to microbes and allow better colony isolation.