RAPID RADIOMETRIC SUSCEPTIBILITY TESTING OF MYCOBACTERIUM-TUBERCULOSIS

Abstract

A 48 h radiometric test for determining the drug susceptibility of M. tuberculosis was developed. The test is based on the measurement of 14CO2 produced by the oxidation of formate labeled with 14C. The test system uses 5 .times. 107 organisms in 1 ml of Middlebrook 7H9 medium plus albumin-dextrose-catalase enrichment and 1 .mu.Ci of [14C]formate. The 14CO2 produced is measured in an ionization chamber at 24-, 48-, and 72-h intervals, with and without the addition of antituberculous drugs. Isoniazid, streptomycin, rifampin and ethambutol were each tested at 3 concentrations by the radiometric method and the reference (agar dilution) method. Six standard strains and 21 patients (human) isolates were compared by both methods. Production of 14CO2 was quantitatively decreased in the presence of drugs that inhibit the organism. The radiometric method requires 2 days; the agar dilution, 14-21 days.