Question: what are the conditions necessary for a cell to take in a plasmid?
Answer: The bacteria are given a heat shock, which "encourages" them to take up a plasmid. Most bacteria do not take up a plasmid, but some do. Plasmids used in cloning contain an antibiotic resistance gene. Thus, all of the bacteria are placed on an antibiotic plate to select for ones that took up a plasmid.