What is sequential passaging?
I am normally reluctant to cite Wikipedia articles, however, the one for Sequential or Serial Passaging explains it quite nicely.

In short:


  In microbiology, sequential or serial passaging refers to the process
  of growing bacteria or viruses in iterations. In essence this means
  that a virus or a strain of bacteria will be isolated and allowed to
  grow for a period of time. After the sample has grown for some time,
  part of it will be transferred to a new environment and allowed to
  grow for the same period of time.This process will be repeated as many
  times as desired. The final product is studied, often in comparison with the original virus or bacterial strain to see whether it has evolved or any other changes have occurred.