The identification of genes expressed within host cells would contribute greatly to the development of new strategies to combat tuberculosis. By combining the natural fluorescence of the green fluorescent protein (GFP) with the counterselectable property of the SacB protein, promoters displaying enhanced activity have been isolated. Macrophages were infected with recombinant bacille Calmette–Guérin containing a library of promoters controlling and expression, and fluorescent bacteria recovered by fluorescence-activated cell sorting. The expression of was used to eliminate clones with strong promoter activity outside the macrophage, resulting in the isolation of seven clones containing promoters with greater activity intracellularly. The gene products identified displayed similarity to proteins from other organisms whose functions include nutrient utilization, protection from oxidative stress and defence against xenobiotics. These proposed functions are consistent with conditions encountered within the host cell and thus suggest that the augmented activity of the isolated promoters/genes may represent strategies employed by to enhance intracellular survival and promote infection.


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