Antimicrobial peptides are an important component of the innate immune defense. Mycobacterium avium subsp hominissuis (M. avium) is an organism that establishes contact with the respiratory and gastrointestinal mucosa as a necessary step for infection. M. avium is resistant to high concentrations of polymyxin B, a surrogate for antimicrobial peptides. To determine gene-encoding proteins that are associated with this resistance, we screened a transposon library of M. avium strain 104 for susceptibility to polymyxin B. Ten susceptible mutants were identified and the inactivated genes sequenced. The greatest majority of the genes were related to cell wall synthesis and permeability. The mutants were then examined for their ability to enter macrophages and to survive macrophage killing. Three clones among the mutants had impaired uptake by macrophages compared to the wild-type strain, and all ten clones were attenuated in macrophages. The mutants were shown also to be susceptible to cathelicidin (LL-37), in contrast to the wild-type bacterium. All but one of the mutants were significantly attenuated in mice. In conclusion, this study indicated that the M. avium envelope is the primary defense against host antimicrobial peptides
