Virulence of Escherichia coli strains for chicken embryos.

The virulence of 66 Escherichia coli strains was evaluated in 12-day-old chicken embryos inoculated by the allantoic route. The index of virulence used was the proportion of dead embryos within 3 days of inoculation. The strains were classified into "highly virulent," "moderately virulent," and "avirulent" groups. Although both virulent and avirulent strains grew equally well in vivo in the allantoic and yolk sacs, virulent E. coli invaded the whole embryo but avirulent ones failed to do so.