Comparison of the BBL mycobacteria growth indicator tube, the BACTEC 12B, and solid media for the isolation of Mycobacterium bovis

Bacterial culture is a key diagnostic tool in bovine tuberculosis control programmes and liquid culture systems are the preferred method for the isolation of Mycobacterium bovis. In this study different isolation methods were compared for their ability to isolate M. bovis from tissue samples from animals with lesions resembling bovine tuberculosis. In the first trial M. bovis was isolated from 86 of the 200 tissues which were cultured using the liquid media Bactec 12B and Mycobacteria Growth Indicator Tube (MGIT) and solid, Middlebrook 7H11 supplemented with pyruvate (7H11P). While M. bovis was isolated from 2 samples with MGIT but not Bactec 12B, M. bovis was isolated from 9 samples with Bactec but not MGIT. All these 9 samples came from the North Canterbury /Marlborough region of New Zealand. The proportion of tissues from which M. bovis was isolated with Bactec 12B and MGIT and the mean time for isolation was different for samples from the North Canterbury/Marlborough region but not the rest of New Zealand. In the second trial, M. bovis was isolated from 401 of 1033 tissues which were cultured using MGIT, Middlebrook 7H9 broth and solid 7H11P. The proportion of isolates of M. bovis and the mean time for their isolation with MGIT was different for the North Canterbury/Marlborough and the rest of New Zealand. The reason for the reduced performance of MGIT in isolating M. bovis from Canterbury/Marlborough was not determined but may relate to the set of genotypes present in this region. Typing using variable number tandem repeats (VNTR) of 197 isolates from this study revealed that the 44 isolates from North Canterbury/Marlborough were represented by 2 closely related VNTR types which were not found in 154 isolates from the remainder of New Zealand.