An image-based screening system for the study of Mycobacterium avium persistence and drug discovery

Mycobacterium avium infects human macrophages causing opportunistic infections. A steady increase of these infections over the past four decades and resistance to common anti-mycobacterial drugs, create an urgent need for new treatments; however, drug discovery is held back by a lack of knowledge about how M. avium replicates or persists in host cells.

We implemented an image-based assay using a fluorescence dilution (FD) system to measure M. avium replication and persistence. M. avium strain 104 carrying a plasmid encoding GFP and TurboFP635 under constitutive and inducible promoters, respectively, is induced prior to infection of THP1 macrophages and the fluorescent signals are tracked over time in the absence of the inducer. Loss of the TurboFP635 signal while GFP signal is maintained, identifies replicating and retention of both signals non-replicating bacteria.

In the absence of inducer, the M. avium 104 FD strain replicated in the macrophages, leading to increasing numbers of GFP-expressing intracellular bacteria and concomitant loss of TurboFP635 signal in >90% of the infected cells after 24 hours. Upon re-induction, these bacteria expressed TurboFP635, suggesting they are metabolically active and alive. We observed the presence of a small non replicating population that persisted over 96 hours pi. We applied our assay to compare the effect of a panel of anti-mycobacterial drugs, revealing different effects on killing, intracellular replication and induction of persisting, non-replicating bacteria, illustrating the power of this system to facilitate the dissection of the biology of persistence and anti-mycobacterial drug discovery in the future.