Host modification of chlamydiae: presence of an egg antigen on the surface of chlamydiae grown in the chick embryo.

Egg-grown chlamydiae (EGO) have a yolk sac antigen assoicated with their surface which is absent from cell monolayer-grown organisms (CGO). EGO infectivity was specifically neutralized by rabbit antiserum to normal yolk sac; CGO infectivity, before or after incubation with normal yolk sac material, was not neutralized. Treatment of EGO with Clostridium welchii culture filtrate, containing phospholipase C, abolished spontaneous infectivity for monolayers and neutralization by anti-yolk sac antiserum but did not affect centrifuge-assisted infectivity. The possible significance of host antigen on the chlamydial surface is considered.