SUMMARY: Semi-solid agar media prepared as previously described (Knox, 1955; Knox, Swait & Woodroffe, 1956) were found especially useful for studying the sensitivity of cultures of to isoniazid, streptomycin and -aminosalicylic acid (PAS). Kirchner, Fisher and Dubos media were compared both as semi-solid and as liquid media. In the semi-solid media when large inocula of H37Rv strain were used, results could be read in 2 days with a hand-lens and were usually easily read with the naked eye in 3–5 days. Results were easier to read and interpret in semi-solid than in the corresponding liquid media both because of the sharpness of the initial end-point defining the minimum inhibitory concentration of the drug (M.I.C.), and because the later growth of numerous or isolated colonies gave a measure, not obtainable with liquid media, of the variability within a culture. Such presumptively resistant colonies appeared most frequently with isoniazid in Kirchner semi-solid medium (in which the drug decayed rapidly), and less frequently with streptomycin and PAS. Colonies which appeared in isoniazid-containing tubes were not always found to consist of drug-resistant organisms on subculture. But when 2–3 days were allowed for microcolonies to appear before the drug was added, then the few large colonies which subsequently developed were found to contain truly resistant organisms. Strains resistant to isoniazid or streptomycin Avere distinguished from sensitive strains more easily than in liquid media. PAS sensitivity tests gave sharp and consistent end-points in Fisher and Kirchner semi-solid agar even when large inocula were used, though the actual level of the m.i.c. varied greatly with inoculum size, and with different strains. This may be partly accounted for by the reversal of PAS by -aminobenzoic acid (PAB). In Dubos medium readable end-points were often obtained only with small inocula. For all three drugs it seemed that Fisher semi-solid agar gave the most satisfactory results.


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