CHARACTERIZATION OF THE VIRULENCE OF MYCOBACTERIUM-AVIUM COMPLEX (MAC) ISOLATES IN MICE

The virulence of different isolates of MAC was studied in naturally su sceptible BALB/c mice. In preliminary experiments, MAC bacteria formin g smooth transparent colonies on solid media (SmT variants) were found to be virulent for BALB/c mice, causing progressive infection; smooth opaque (SmOp) were generally avirulent, being slowly eliminated from the infected organs; and rough (Rg) variants were either avirulent or as virulent as SmT variants. We chose to compare the virulence of diff erent isolates of MAC of different origins, studying only the SmT morp hotype. Strains of MAC isolated from naturally infected animals were t hose that most consistently caused progressive infections. AIDS patien ts-derived isolates were of intermediate virulence or devoid of virule nce in mice. The environmental strains were eliminated from mice or di d not proliferate. Strains of MAC isolated from individuals who were n ot infected by HIV varied in virulence from completely avirulent to hi ghly virulent. There was no close correlation between virulence and re striction fragment length polymorphism (RFLP) type, although all highl y virulent strains were of the A/I type. There was also no correlation between virulence analysed in vivo and the ability to grow in culture d macrophages.