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

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.