Escherichia coli is plated on nutrient agar plates that each contain a different type of antibiotic. The shaded area represents growth of the bacteria. Which of the following plates contains bacteria that were most resistant to the antibiotic?

A.
A

B.
B

C.
C

D.
D

The Correct Answer and Explanation is:

To determine which plate contains bacteria that were most resistant to the antibiotic, we need to understand the concept of antibiotic resistance and the interpretation of antibiotic susceptibility testing, often assessed using methods like the Kirby-Bauer disk diffusion test or similar techniques.

Explanation:

In antibiotic susceptibility testing, bacteria are cultured on agar plates containing different antibiotics. After incubation, the plates are examined for bacterial growth. The area around the antibiotic disks (or in this case, shaded areas on the plates) where bacteria do not grow indicates the effectiveness of the antibiotic in inhibiting bacterial growth.

1. Resistance and Susceptibility:

• Susceptible Bacteria: When bacteria are susceptible to an antibiotic, there will be a clear zone around the antibiotic disk where bacteria cannot grow. This zone is called the zone of inhibition.
• Resistant Bacteria: If bacteria are resistant to an antibiotic, they will grow right up to the antibiotic disk, showing little to no zone of inhibition.

1. Interpreting the Plates:

• Plate with Small or No Clear Zone: If a plate shows bacterial growth right up to the antibiotic disk or very close to it, this indicates that the bacteria are resistant to that antibiotic. The smaller or absent clear zone suggests less inhibition and hence greater resistance.
• Plate with Large Clear Zone: Conversely, a large clear zone around the antibiotic disk indicates that the bacteria are susceptible to the antibiotic because the antibiotic effectively inhibited bacterial growth in that area.

Applying This to the Question:

Let’s assume you have four plates labeled A, B, C, and D. The plate with the smallest shaded area or the smallest clear zone around the antibiotic disk represents the antibiotic to which the bacteria are most resistant.

• Plate A: Large shaded area, indicating significant bacterial growth and thus likely resistance.
• Plate B: Medium-sized shaded area, indicating some resistance.
• Plate C: Small shaded area, indicating relatively higher susceptibility.
• Plate D: Very small or no clear zone, indicating high resistance.

Based on this interpretation, Plate A would be the one where the bacteria showed the most resistance to the antibiotic because it had the largest shaded area indicating the least inhibition by the antibiotic.

Correct Answer: A

In summary, to identify which plate contains bacteria most resistant to the antibiotic, look for the plate with the largest area of bacterial growth around the antibiotic disk, as this indicates minimal inhibition and thus maximum resistance.