Antibiotic Resistance and Genetic Plasticity (2B) Practice Test

A researcher exposes a clonal population of Mycobacterium to isoniazid. After treatment, a few colonies survive. Fluctuation testing across many parallel cultures shows high variability in the number of survivors between cultures (some have none, a few have many). Sequencing reveals a single base substitution in a gene required to activate isoniazid inside the cell. Which mechanism best explains the observed resistance?

A clinician notices that a patient’s Klebsiella isolate becomes resistant to two unrelated antibiotics during therapy. Genome sequencing reveals no new chromosomal mutations but identifies a newly acquired plasmid carrying two resistance genes. The plasmid is readily transferred to a lab strain during co-culture, but transfer is blocked when cell-to-cell contact is prevented using a membrane that allows diffusion of small molecules but not bacteria. Which mechanism best explains the observed resistance?