Mycobacterium tuberculosis from chronic murine infections that grows in liquid but not on solid medium

Background: Old, stationary cultures of Mycobacterium tuberculosis contain a majority of bacteria that can grow in broth cultures but cannot grow on solid medium plates. These may be in a non-replicating, dormant growth phase. We hypothesised that a similar population might be present in chronic, murine tuberculosis. Methods: Estimates of the numbers of viable M. tuberculosis, strain H37Rv, in the spleens and lungs of mice in a 7-day acute infection and in a 10-month chronic infection were made by conventional plate counts and, as broth counts, by noting presence or absence of growth in serial replicate dilutions in liquid medium. Results: Plate and broth counts in 6 mice gave similar mean values in the acute infection, 7 days after infection. However, the broth counts were much higher in 36 mice with a chronic infection at 10 months. Broth counts averaged 5.290 log(10) cfu/organ from spleens and 5.523 log(10) cfu/organ from lungs, while plate counts were 3.858 log(10) cfu/organ from spleens and 3.662 log(10) cfu/organ from lungs, indicating that the total bacterial population contained only 3.7% bacilli in spleens and 1.4% bacilli in lungs, capable of growth on plates. Conclusion: The proportion growing on plates might be a measure of the "dormancy" of the bacilli equally applicable to cultural and animal models.