1. When antibiotic-sensitive bacteria are spread on a plate containing antibiotic,
colonies of resistant bacteria grow. The best explanation for this is:
a. the antibiotic is mutagenic
b. the antibiotic induces resistance through epigenetic effects
c. cells only become resistant once they sense the antibiotic
d. some cells in the original population were already resistant
Antibiotic Resistance in Bacteria: Pre-Existing Mutants Explained
Antibiotic-sensitive bacteria on an antibiotic plate yield resistant colonies due to natural selection of pre-existing resistant mutants. The correct answer is option d: some cells in the original population were already resistant.
Option Analysis
a. The antibiotic is mutagenic
Antibiotics rarely cause mutations directly at rates sufficient for observable resistance during plating; resistance arises from rare spontaneous mutations present before exposure. Mutagenic effects, if any, occur post-exposure but do not explain immediate colony growth.
b. The antibiotic induces resistance through epigenetic effects
Epigenetic changes like DNA methylation influence gene expression but do not confer heritable resistance matching genetic mutants in standard plating; true resistance requires stable genetic changes. Epigenetics plays minor roles in persistence, not colony formation.
c. Cells only become resistant once they sense the antibiotic
This adaptive mutation hypothesis was disproven by the Luria-Delbrück experiment, showing resistance mutations occur randomly before selection, not in response to the antibiotic. Sensing induces no directed mutations.
d. Some cells in the original population were already resistant
Pre-existing mutants at low frequency (e.g., 10^-6 to 10^-9) survive and form colonies under selection, as confirmed by fluctuation tests where variance in resistant colonies exceeds Poisson expectations.
