In vitro infection of human cells with Mycobacterium tuberculosis

It has been estimated that approximately 50% of individuals exposed to Mycobacterium tuberculosis never become tuberculin skin test positive, which may indicate that successful human immunological responses are able not only to inhibit mycobacterial growth, but also to kill the bacteria. Nevertheless, it has been extremely difficult to reproduce this effect in vitro and the ability of human phagocytes to eliminate the bacteria is controversial. This is one of the reasons why we do not fully understand either tuberculosis resistance or susceptibility. Nowadays there is a pressing need in tuberculosis vaccine research to find biomarkers of successful responses to tuberculosis and the use of in vitro models may allow the identification of the immunological mechanisms responsible for the mycobacterial killing that could be tested in clinical settings. Besides biosafety concerns, the manipulation of mycobacteria is technically very demanding, and the optimization of in vitro infection protocols have been difficult. As a result, there are a large number of variations in the methodology that make arduous the comparison of results obtained in different research groups. In this review an overview of the mycobacterial and human cellular models most frequently studied are presented, together with a description of several of the modifications tried in infection protocols, from the preparation of the inoculum to the quantification of surviving mycobacteria after infection. A comment about statistical methods that may improve the detection of relevant biological effects is also included.