The present invention pertains to a method to obtain McCoy cells persistently infected with Lawsonia intracellularis bacteria, comprising infecting McCoy cells with Lawsonia intracellularis bacteria, growing the infected McCoy cells in a suitable medium at an oxygen concentration less than 18% to arrive at a culture of McCoy cells infected with Lawsonia intracellularis bacteria, passing at least a part of the said culture to fresh medium, and without adding uninfected McCoy cells to the medium, growing the infected McCoy cells contained in the said at least part in the fresh medium at an oxygen concentration less than 18%, to obtain the persistently infected McCoy cells. The invention also pertains to the use of such persistently infected cells to grow and obtain these bacteria in purified form.