Question

in the first part of the experiment, there were only a few colonies on the streptomycin positive plate. in this second part, there are a large number of colonies on the streptomycin positive plate that was inoculated with the antibiotic-resistant strain. what is the most likely explanation for this difference?
a. the antibiotic-resistant strain was composed of a large number of individuals with resistance to streptomycin.
b. the presence of streptomycin on the agar plate caused the bacteria to mutate to a form that had resistance to the antibiotic.
c. a few individuals in the antibiotic-resistant strain had a mutation that provided them with resistance to streptomycin.

Explanation:

To solve this, we analyze each option:

• Option A: Initially, the antibiotic - resistant strain couldn't have a large number of resistant individuals as the first plate had few colonies. So A is incorrect.
• Option B: Antibiotics like streptomycin don't cause bacteria to mutate to a resistant form. Mutations are random, not induced by the antibiotic to gain resistance. So B is incorrect.
• Option C: In a population, a few individuals can have random mutations that provide antibiotic resistance. When the antibiotic - resistant strain is used to inoculate the plate, these pre - existing resistant individuals (from the mutation) can grow and form many colonies. This matches the scenario where the first plate (with non - resistant or less - resistant population) had few colonies and the second (with resistant strain) had many.

Answer:

C. A few individuals in the antibiotic - resistant strain had a mutation that provided them with resistance to streptomycin.