Growth of Mycobacterium paratuberculosis in radiometric, Middlebrook and egg-based media

The ability of BACTEC radiometric 7H12 broth, Middlebrook 7H10 Tween broth, Middlebrook 7H10 agar, and Herrold's egg-yolk medium to provide early detection of Mycobacterium paratuberculosis was evaluated. The minimum detection times in days for the various media were: 7H12, 9; 7H10 agar, 23 (plate), 28 (slant); 7H10 Tween broth; 27; and Herrold's egg-yolk medium, 43 (plate), 49 (slant). The radiometric broths provided the earliest detection of M. paratuberculosis, and 3625 organisms ml-1 were required to produce a positive, radiometric growth-index reading. Of the non-radiometric plate and slant media evaluated, microscope examination of the translucent 7H10 agar plate resulted in the earliest detection and highest mean colony counts (387) as compared with Herrold's egg-yolk agar plate (208). Similar results were noted for 7H10 and Herrold's egg-yolk agar slants; however, accurate colony counts could not be determined because of confluent growth. All media were supplemented with 2 micrograms ml-1 of mycobactin J and excess amounts of this supplement inhibited the growth of M. paratuberculosis in radiometric 7H12 media.