MLS performed susceptibility testing for E. coli using a 100mm plate and placed 12 antibiotic discs on it. Which outcome would you expect?

MLS performed susceptibility testing for E. coli using a 100mm plate and placed 12 antibiotic discs on it. Which outcome would you expect?

• A. E. coli will not grow
• B. No zones form
• C. Mixed growth
• D. Overlapping zone of inhibition

Answer: (D)

In disc diffusion testing, each antibiotic disc creates a zone where bacterial growth is inhibited, and those zones should be distinct and measurable if the discs are spaced properly. When you place a lot of discs on a single plate, especially on a 100 mm plate, the diffusion areas around neighboring discs can touch and merge. With twelve discs, the chances that the inhibition zones overlap increase, so you’d expect to see zones that merge into a single, continuous clear area rather than separate, well-defined zones. That overlapping pattern makes it difficult to interpret which antibiotic caused which level of inhibition, but it’s the most likely appearance given the dense arrangement. The other options imply scenarios (no inhibition, uniform growth, or complete suppression) that don’t fit the result you’d expect from closely spaced discs on a standard diffusion plate.

In disc diffusion testing, each antibiotic disc creates a zone where bacterial growth is inhibited, and those zones should be distinct and measurable if the discs are spaced properly. When you place a lot of discs on a single plate, especially on a 100 mm plate, the diffusion areas around neighboring discs can touch and merge. With twelve discs, the chances that the inhibition zones overlap increase, so you’d expect to see zones that merge into a single, continuous clear area rather than separate, well-defined zones. That overlapping pattern makes it difficult to interpret which antibiotic caused which level of inhibition, but it’s the most likely appearance given the dense arrangement. The other options imply scenarios (no inhibition, uniform growth, or complete suppression) that don’t fit the result you’d expect from closely spaced discs on a standard diffusion plate.