Broth culture: the modern 'guinea-pig' for isolation of mycobacteria

Three cases of mycobacterial infection were confirmed in our laboratory by exclusive isolation of the pathogen in Middlebrook 7H9 broth medium. The mycobacteria failed to grow in all three cases on three different solid media. In one patient Mycobacterium tuberculosis was isolated from a pleural biopsy; a lymph node biopsy from another patient also yielded M. tuberculosis and M. avium-intracellulare was isolated from the spinal fluid of a patient with acquired immunodeficiency syndrome (AIDS). Only the broth culture was positive in all three cases. We suggest these findings emphasise the need for laboratories to use a liquid medium in addition to egg-based or agar media, especially for isolation of mycobacteria from specimens with small numbers of bacilli.