This report is based on the study of 735 cultures of tubercle bacilli identified as human type. Sputum specimens were collected from patients attending the Lady Willingdon Tuberculosis Demonstration and Training Centre (LWTDTC), Bangalore, and from the mass Case-finding studies in semi-urban areas. Drug sensitivity tests for streptomycin, isoniazid, PAS and thioacetazone with different drug concentrations, different size of inoculum and for various length of incubation were carried out.

No difference was observed in the duration of growth between sensitive and resitant cultures in their first appearance on primary diagnostic cultures or sub-cultures on drug free slopes when innoculated with standard suspension. The primary cultures took about 3 weeks and sub-cultures 2 weeks to grow on drug free media. Large sensitive bacillary population required higher concentration of thioacetazone to inhibit the growth, suggesting standardization of inoculum size for sensitivity tests. Prolonged incubation period on drug slopes showed profound influence on the level of drug inhibiting concentration of thioacetazone; with the increase in incubation period, fall in growth of sensitive culture was not observed even on high drug concentration. The reproducibility of this observation on duplicate specimens from the same patients after shorter intervals excluded the possibility of experimental error. A reduction in the inhibition of growth of sensitive organisms on drug media with time is presumed to be due to either deterioration of the drugs in the media or due to adaptation by the micro-organisms. Because of the decrease in inhibition of growth, even sensitive organisms may be classified as resistant if reading of culture for drug sensitivity is prolonged beyond 3 weeks of the inoculation period. It is suggested that a standard inoculum size and a maximum limit of 3 weeks incubation period should be adopted for finding out sensitivity to thioacetazone. Cultures classified as sensitive to the three first line drugs or resistant to one or more, showed no difference in the pattern of sensitivity to thioacetazone.