MYCOBACTERIAL GROWTH AND SENSITIVITY TO H2O2 KILLING IN HUMAN MONOCYTES IN-VITRO

The intracellular growth and susceptibilities to killing by H2O2 in cu ltured human monocytes of a number of mycobacterial species including laboratory strains and clinical isolates of Mycobacterium tuberculosis , and Mycobacterium bovis bacillus Calmette-Guerin (BCG) and a clinica l isolate of Mycobacterium avium-M. intracellulare were examined, The clinical isolate of M. avium-M. intracellulare did not replicate in fr eshly explanted monocytes (generation time of >400 h); BCG replicated with a generation time of 95 h, and M. tuberculosis strains CDC551, H3 7Rv, and H37Ra replicated with generation times of 24, 35, and 37 h, r espectively, during the 4-day growth assay, When cultured in monocytes for 4 days, the mycobacteria were variably sensitive to H2O2-induced killing, A positive correlation between the generation time and percen t killing of intracellular bacilli was observed, By comparison, mycoba cterial strains were similarly sensitive to H2O2 treatment in cell-fre e culture media and in sonicated cell suspensions, Using a number of i nhibitors of reactive oxygen intermediates we determined that other th an catalase the inhibitors tested did not affect H2O2-induced killing of intracellular mycobacteria, Our studies suggest that the killing of mycobacteria growing in human monocytes in vitro by the addition of e xogenous H2O2 is dependent on the susceptibility to a peroxide-induced killing pathway as well as on the intracellular growth rate of the my cobacteria.