Abstract
BackgroundOld, 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.MethodsEstimates 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.ResultsPlate 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 log10 cfu /organ from spleens and 5.523 log10 cfu/organ from lungs, while plate counts were 3.858 log10 cfu/organ from spleens and 3.662 log10 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.ConclusionThe proportion growing on plates might be a measure of the "dormancy" of the bacilli equally applicable to cultural and animal models.
Highlights
Old, stationary cultures of Mycobacterium tuberculosis contain a majority of bacteria that can grow in broth cultures but cannot grow on solid medium plates
Cultures that have been grown undisturbed in the depths of liquid medium for 100 days contain a majority population which grows in liquid medium but is not able to form colonies on solid medium [1,2]
Culture media and bacteria The media used were 7H9 liquid medium with 10% albumin, dextrose, catalase supplement and 0.05% Tween 80, and 7H11 agar medium with 10% oleic acid, albumin, dextrose, catalase supplement (Becton Dickinson, Oxford, UK)
Summary
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 nonreplicating, dormant growth phase. The organisms in a log phase, actively multiplying culture of Mycobacterium tuberculosis all grow well on plates and are estimated as colony forming units (cfu). Cultures that have been grown undisturbed in the depths of liquid medium for 100 days contain a majority population which grows in liquid medium but is not able to form colonies on solid medium [1,2]. We estimated populations present in the (page number not for citation purposes)
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