Use of thiosemicarbazone and para-nitrobenzoic acid in screening tests for anonymous mycobacteria

In screening for anonymous mycobacteria the use of thiosemicarbazone (10mug./ml.) may give rise to false positive results. Although these can usually be overcome by further tests, additional time and effort is required, which seriously limits the value of thiosemicarbazone as a true screening agent. Para-nitrobenzoic acid (500 mug./ml.) has proved more satisfactory in our hands; no false positive results have been obtained and it has the additional advantage that strains of M. kansasii (usually thiosemicarbazone sensitive) are resistant to para-nitrobenzoic acid.