Effect of Increased Inoculum Size on Drug Susceptibility of Mycobacterium tuberculosis: Challenges in Reliable Drug Resistance Detection.

Inoculum preparation of Mycobacterium tuberculosis isolates is a critical step for the reproducibility of drug susceptibility tests (DST) and the prevention of false results. In this study, we investigated the effect of increasing inoculum sizes of M. tuberculosis isolates on DST. For this purpose, primary drug susceptibilities of five ATCC strains and 24 M. tuberculosis isolates were determined by the proportion method on 7H10 agar using six different inoculum sizes (10-2 dilution of McFarland no 1 as reference inoculum and McFarland no 0.5-1-2-3-4). Among the tested isolates (including ATCC strain), 22 of them had DST results at increasing inoculum sizes that were 100% consistent with DST results in the reference inoculum and MGIT-960. For alone streptomycin (STR), in seven isolates tested, DST results at increasing inoculum sizes were 100% consistent with MGIT-960 results but were inconsistent with results determined with the reference inoculum. These isolates were found to be susceptible to STR with reference inoculum, but resistant to STR in MGIT-960 and increasing inoculum sizes. Our study reveals that inoculum size does not affect DST results of M. tuberculosis; on the contrary, increasing the inoculum may have positive results for some antibiotics, such as STR.