Rekindling old controversy on elusive lair of latent tuberculosis

Understanding how Mycobacterium tuberculosis lies quiescent for years if not decades, how the immune system fails to detect and eradicate it, and the nature of the stimuli for its reactivation remains nearly as uncertain today as in Robert Koch's time. An even simpler issue, but no less elusive, has been the question of where tubercle bacilli reside during latency. At issue is whether latent bacilli inhabit old granulomatous lesions in the lung and lymphatics or whether they reside remotely in non-granulomatous tissue. In today's Lancet R Hernandez-Pando and colleagues rekindle this smouldering controversy with molecular evidence that normal-appearing human lung, rather than granulomatous lesions, may harbour latent M tuberculosis. Persistence of DNA from Mycobacterium tuberculosis in superficially normal lung tissue during latent infectionM tuberculosis can persist intracellularly in lung tissue without histological evidence of tuberculous lesions. M tuberculosis DNA is situated not only in macrophages but also in other non-professional phagocytic cells. These findings contradict the dominant view that latent organisms exist in old classic tuberculous lesions, and have important implications for strategies aimed at the elimination of latent and persistent bacilli.