Antibiotic Resistance and Genetic Plasticity (2B) Practice Test

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?

A bacterium becomes resistant to rifampin, which targets bacterial RNA polymerase. Sequencing shows a single nucleotide change in rpoB (RNA polymerase beta subunit). In vitro transcription assays show normal transcription rate but rifampin no longer inhibits the enzyme. Based on the data, which genetic change is most consistent with the resistance observed?