[Comparative evaluation of two different formulae of Middlebrook 7H9 broth in a fully automated mycobacteria culture system, MB/BacT; the effect of Tween 80].

Two different formulae of Middlebrook 7H9 broth, one containing Tween 80 [Tween (+) broth] and the other containing vancomycin but not Tween 80 [Tween (-) broth], were evaluated in parallel for a fully automated mycobacteria culture system, MB/BacT(Organon Teknika, Durham, NC, U.S.A.). A total of 586 clinical sputum specimens were digested and decontaminated by the semi-alkaline protease-N-acetyl-L-cysteine-NaOH (SAP-NALC-NaOH). Each part of sample treated was inoculated into the MB/BacT Process Bottle containing the respective Middlebrook 7H9 broth. Culture bottles were incubated in the MB/BacT at 37 degrees C for up to 56 days. Of 586 samples, 110 isolates of Mycobacterium tuberculosis complex and 77 of nontuberculous mycobacteria (NTM) were isolated. The occurrence of false alarm due to breakthrough contamination was 3.2 in Tween (+) broth and 2.9% in Tween (-) broth. Also, the positivities of mycobacteria by the respective culture media were comparable. However, Tween (-) broth could detect positive cultures for mycobacteria, particularly for M. tuberculosis complex at the earlier incubation cycle when compared to Tween (+) broth. The time to detect 50% positive cultures for M. tuberculosis complex was 20.5 days for Tween (-) broth and 34.3 days for Tween (+) broth, respectively. With the results, it was concluded that; Tween (+) broth produced homogeneous mycobacterial growth in culture media, and thus, it was easy to prepare the inoculum directly adjusted to McFarland turbidity to the susceptibility test. However, the present formula of Middlebrook 7H9 broth supplemented with Tween 80 was not enough suitable for the rapid detection of positive cultures and needs some revisions to improve.