Evaluation of a rapid differentiation test for the Mycobacterium tuberculosis complex by selective inhibition with rho-nitrobenzoic acid and thiophene-2-carboxylic acid hydrazide.

ABSTRACT

SETTING: Mycobacterial growth in media to which inhibitory substances are added has been used in species identification. Growth of the Mycobacterium tuberculosis complex (MTC) is inhibited by rho-nitrobenzoic acid (PNB), whereas non-tuberculous mycobacteria (NTM) are resistant. Thiophene-2-carboxylic acid hydrazide (TCH) is useful in the differentiation of MTC when performed together with other tests. OBJECTIVE: To develop a test using PNB or TCH added to culture medium, and to evaluate its usefulness in the screening of mycobacteria isolates. DESIGN: In 2001, PNB testing was performed in 109 M. tuberculosis strains identified by Instituto Adolfo Lutz (IAL) and 52 NTM strains from the institute's culture collection. The drugs were added to Löwenstein-Jensen (LJ) medium and to BBL-MGIT. RESULTS: Species differentiation of MTC with the MGIT/TCH method was similar to that observed using the conventional LJ/TCH method. The accuracy of the MGIT/PNB method to differentiate NTM and MTC strains was 99.4%. The BBL-MGIT system allowed presumptive identification in 3-11 days, compared to > or =12 days with LJ medium. CONCLUSION: A simple, low-cost test using growth inhibitors may be incorporated into a modern, safe and quick methodology enabling differentiation of MTC and NTM.