In disk diffusion, what is the effect of increasing inoculum size on the zone diameter?

In disk diffusion, what is the effect of increasing inoculum size on the zone diameter?

• A. Larger zones
• B. Smaller zones
• C. No effect
• D. More variable, but average unchanged

Answer: (B)

Disk diffusion measures how far the antibiotic can prevent growth as it diffuses from the disk. When you increase the number of bacteria on the plate, more cells must be inhibited, so a higher local antibiotic concentration is needed to stop growth. The diffusion gradient from the disk is fixed, so the distance at which the antibiotic concentration drops below inhibitory levels occurs closer to the disk, producing a smaller zone of inhibition. This inoculum effect is why standardized inocula are used; a higher inoculum than standard tends to yield smaller zones and can mislead toward resistance. So the zone diameter becomes smaller with a larger inoculum.

Disk diffusion measures how far the antibiotic can prevent growth as it diffuses from the disk. When you increase the number of bacteria on the plate, more cells must be inhibited, so a higher local antibiotic concentration is needed to stop growth. The diffusion gradient from the disk is fixed, so the distance at which the antibiotic concentration drops below inhibitory levels occurs closer to the disk, producing a smaller zone of inhibition. This inoculum effect is why standardized inocula are used; a higher inoculum than standard tends to yield smaller zones and can mislead toward resistance. So the zone diameter becomes smaller with a larger inoculum.