A filter capable of preventing bacteria from passing is placed

A filter capable of preventing bacteria from passing is placed at the bottom of a U tube to separate the two sides. Streptomycin-resistant cells of a bacterial strain are placed on one side of the filter and streptomycin-sensitive cells are placed on the other side. After incubation, the side of the tube that originally contained only streptomycin-sensitive cells now contains some streptomycin-resistant cells. Give three possible reasons for this observation. What further experiments would you do to determine the correct explanation?
Three possible reasons:

  1. The streptomycin sensitive cells mutated to resistance.
  2. The streptomycin sensitive cells were transformed to streptomycin resistance by DNA- mediated transformation
  3. The streptomycin sensitive cells were transduced to streptomycin resistance by transduction