Question

on which plate do we see evidence of antibiotic resistance?
a. streptomycin-positive plate
b. streptomycin-negative plate
c. both the streptomycin-positive and -negative plates

what is the most likely reason that there are few colonies growing on the streptomycin-positive plate?
a. a few individuals in the original nonresistant culture had a mutation that conferred resistance to streptomycin.
b. streptomycin in the medium caused the bacteria to create mutations that conferred resistance to streptomycin.

Explanation:

First Question (On which plate...):

Antibiotic resistance is seen when bacteria grow on a plate with the antibiotic (streptomycin - positive plate). On the streptomycin - negative plate, growth is expected even without resistance. So evidence of resistance (bacteria growing where antibiotic is present) is on the streptomycin - positive plate.

Mutations conferring resistance are random and pre - existing, not caused by the antibiotic. So a few individuals in the original non - resistant culture had a mutation for streptomycin resistance, leading to few colonies on the streptomycin - positive plate. Option B is incorrect as antibiotics don't cause mutations for resistance (they select for pre - existing resistant mutants).

Answer:

A. Streptomycin - positive plate

Second Question (What is the most likely reason...):