Three independent assay methods were used to investigate the activities of
antimicrobial peptides (human and rabbit
defensins and protegrin from porcine leukocytes) against Mycobacterium tuberculosis in vitro. M.
tuberculosis H37Ra was cultured in the presence of
human neutrophil peptide 1, synthetic
rabbit neutrophil peptide 1, or porcine protegrin 1 at 37 degrees C for 6 to 48 h, and antimycobacterial activity was measured by CFU assay. These
peptides at a concentration of 50 microg/ml showed significant antibacterial effects on M.
tuberculosis after 24 and 48 h of incubation (85.9 to 97.5% at 24 h and 91.6 to 99.4% at 48 h). A radiometric method and a radial diffusion assay confirmed these observations. Antibacterial activity against M.
tuberculosis was independent of
calcium (1.0 mM) or
magnesium (1.0 mM) and not inhibited by
sodium chloride (100 mM). The optimal pH for antibacterial activity against M.
tuberculosis was greater than 4.0. Three clinical isolates of M.
tuberculosis were also studied, and these
peptides showed 86.3 to 99.0% reduction in CFU of these organisms. Morphological studies using scanning electron microscopy showed that
defensins caused lesions on the surface of H37Ra. These observations suggest that
antimicrobial peptides such as
defensins and protegrins may represent an important component of the host defense mechanism against M.
tuberculosis and offer a potential new approach to
therapy.