Differences in antimicrobial susceptibility of pigmented and unpigmented colonial variants of Mycobacterium avium

Abstract

Unpigmented colonial variants were isolated from pigmented Mycobacterium avium isolates recovered from patients with acquired immunodeficiency syndrome and the environment. The variants were interconvertible: the rate of transition from unpigmented to pigmented type was 4.0 x 10-5 variants per cell per generation, and that from pigmented to unpigmented type was 1.5 x 10-5 per cell per generation. The unpigmented variants were more tolerant to antibiotics, especially β-lactams, and Cd2+ and Cu2+ salts than were their pigmented parents. Both pigmented and unpigmented variants of the strains produced β-lactamase, although β-lactamase did not appear to be a determinant of β-lactam susceptibility. Pigmented variants grew more rapidly in a number of commonly used mycobacterial media, were more hydrophobic, and had higher carotenoid contents than their unpigmented segregants.