Inhibition of Mycobacterium tuberculosis secretory serine protease blocks bacterial multiplication both in axenic culture and in human macrophages

Abstract

To study the possible importance of mycobacterial ES-31 serine protease for bacterial cell growth, the effect of serine and metalloprotease inhibitors, anti-tubercular drugs such as isoniazid and anti-ES-31 antibody, was evaluated on mycobacterial ES-31 serine protease in vitro and on bacilli in axenic and macrophage cultures. Serine protease inhibitors such as pefabloc, 3,4 dichloroisocoumarin, phenyl methyl sulfonyl fluoride (PMSF) and metalloprotease inhibitors such as ethylene diamine tetracetic acid (EDTA) and 1,10 phenanthroline inhibited 65–92% serine protease activity in vitro. Isoniazid showed 95% inhibition on mycobacterial ES-31 serine protease. These inhibitors also showed decreased bacterial growth in axenic culture and inhibition was further confirmed by a decreased amount of ES-31 serine protease in culture filtrate. In human macrophage culture, highly inhibitory pefabloc, 1,10 phenanthroline and isoniazid inhibited infectivity of virulent as well as avirulent M. tuberculosis bacilli to macrophages. It was observed that addition of mycobacterial ES-31 serine protease to macrophage culture enhanced the entry of bacilli and their multiplication in human macrophages. However, the addition of anti-ES-31 serine protease antibody strongly inhibited the mycobacterial growth as observed by decreased CFU count, showing the importance of mycobacterial ES-31 serine protease for entry of bacilli and their multiplication.