Non-tuberculosis mycobacteria in Africa. 3. Formamidase activity--its evaluation and practical application.

The third study in a series on the prevalence of non-tuberculosis mycobacteria in Africa is devoted to the investigation of the formamidase activity of 288 cultures of mycobacteria, already typed by a battery of standard tests as pathogenic or atypical (184 strains) and saprophytic (104 strains). Of the latter, 96 (92.3%) were formamidase-positive, as compared with only 6 (3.3%) of the former. A close correlation was observed between the speed of growth on Löwenstein-Jensen medium and formamidase activity, 98 (96.1%) of the positive strains showing visible growth within 1-3 days. The relation between formamidase activity and growth on nutrient media was less clear-cut, however, and it was concluded that for the routine differentiation of saprophytic from other mycobacteria the formamidase test should be combined with simple tests such as speed of growth on L-J medium and ability to grow on nutrient media. Russel's method and Nessler's reagent for the detection of ammonia in the formamidase test were compared; the authors consider the former to be preferable, since the reaction is easier to read.