Lab 6 – Antibiotic Sensitivity-Kirby Bauer Diffusion Test Practice

Why might slower-growing organisms require longer incubation in Kirby-Bauer testing?

Incubation at 35°C for 16–18 hours, with some slower-growing organisms up to 24 hours.

Slower-growing organisms need more time to form colonies and for the antibiotic to diffuse enough through the agar to create a detectable zone of inhibition. The Kirby-Bauer readout depends on visible growth patterns around the antibiotic disk, which only emerges as the bacteria multiply. If incubation is too short for these organisms, there may not be enough biomass to show a real inhibition zone, leading to inaccurate interpretations. Standard clinical conditions use about 35°C for 16–18 hours, with some slower growers needing up to 24 hours to produce reliable results. The other proposed conditions either alter growth too much or use temperatures and times that don’t reflect how these bacteria behave in routine testing, which risks under- or overestimating susceptibility.

Incubation at 25°C for 6 hours.

Incubation at 37°C for 12 hours.

Incubation at 40°C for 48 hours.