He terms "sterile" and "pure" can be used interchangeably when discussing cultures, because they both refer to a culture that has not been contaminated.

In  microbiology, pure culture is a laboratory  culture  containing a single  species  of organism. it is usually derived from a mixed culture. pure culture transfers a small sample into new, sterile  growth medium  in order  to distribute the individual cells across the medium surface. it separates the individual cells. when the cells multiply, each will form a colony and it then may  be used to inoculate more medium.