It has been confirmed that 11- and 13-day-old chick embryos are susceptible to lethal infection with minute inocula of V. cholerae, while 15-day-old embryos are relatively resistant.
Twenty strains of E. coli were found to vary in their capacity to kill chick embryos, without relationship to their human enteropathogenicity. Prior infection of 13-day-old embryonated eggs with E. coli strains selected for low embryo virulence had a marked protective effect against superinfection with V. cholerae.
This effect was duplicated by pretreatment of embryos with killed E. coli cells or endotoxin preparations but not with a suspension of killed cells of a Gram-positive species. Preparatory E. coli infection induces a phase of vibriostasis in the 13-day-old egg which may be sufficient to tide the embryo over into the phase of relative insusceptibility associated with age. Younger embryos exhibit only a lag in death under similar conditions.
The chick embryo is presented as a potentially valuable tool for the study of combined infections and of the mechanisms of natural resistance.