Changes in the virulence of Mycobacterium avium after passage through embryonated hens' eggs

Eight-day-old embryonated hen's eggs were used as a model to study Mycobact erium avium virulence. Strains isolated from human patients caused 20-90% m ortality when eggs were infected by injection of bacterial suspensions into the amniotic sac, Virulence of examined strains subsequently decreased wit h passage through eggs to between 0 and 40% mortality in four passages. Vir ulence of the egg-attenuated strains could be restored by passage through h uman peripheral blood mononuclear cells. The site of infection in the egg w as usually the mesodermal layer of the chorioallantoic membrane. A few smal l granulomas containing acid-fast bacteria were seen in the liver, but not in other organs. Death of chicken embryos may have resulted from destructio n of the mesodermal layer of the chorioallantoic membrane with consequent r espiratory failure. PBMCs infected with less virulent egg-passaged strains of M. avium produced higher levels of tumor necrosis factor-alpha than did peripheral blood mononuclear cells infected with more virulent nonpassaged strains. (C) 2000 Federation of European Microbiological Societies. Publish ed by Elsevier Science B.V. All rights reserved.