Persister cells can survive antibiotic treatment despite being genetically identical to sensitive cells. Persistence is a particular problem for the treatment of Mycobacterium tuberculosis; however, so far there was no straightforward way to identify and quantify persisters of this bacterium. Jain et al. developed a dual-reporter mycobacteriophage that enables the fluorescent detection of persisters. The authors induced persistence in an in vitro model and identified a set of consistently upregulated genes in these cells. They then constructed a reporter bacteriophage that constitutively expressed GFP to verify infection and a red fluorescent protein, tdTomato, under the control of the dnaK promoter, which was specifically activated in persisters. They showed that a small number of M. tuberculosis cells from the sputum of patients were tdTomato positive and thus primed for persistence and two weeks after the start of antibiotic treatment the number of persister cells increased.