Virulent to avirulent conversion of Legionnaires' disease bacterium (Legionella pneumophila)--its effect on isolation techniques.

Suspensions of the Legionnaries' disease bacterium (Legionella pneumophila; LDB) were prepared from the yolk sacs of infected egg embryos, the spleens of infected guinea pigs, and cultures of the organism propagated on enriched Mueller-Hinton agar. Each suspension was titrated to determine the number of bacterial colonies (cfu), yolk sac 50% lethal doses (YSLD50), guinea pig 50% infectious doses (GPID50), and guinea pig 50% lethal doses (GPLD50) produced by 1 ml of inoculum. The numbers of cfu/YSLD50, GPID50, and GPLD50 were then calculated for each suspension. The suspension from yolk sacs had 1 cfu/YSLD50 and 10 cfu/GPID50. The suspension from spleens of guinea pigs also had 1 cfu/YSLD50. Organisms propagated on Mueller-Hinton agar, however, had greater than 10(7) cfu/YSLD50 and 10(5) cfu/GPID50. Thus, the LDB lost virulence when it was cultivated on agar. Guinea pigs vaccinated either subcutaneously or intraperitoneally with LDB grown on Mueller-Hinton agar resisted challenge with virulent LDB.