Response of the chick embryo to live and heat-killed Campylobacter jejuni injected into the yolk sac

Abstract Graded doses of live and heat-killed cells of Campylobacter jejuni were injected into the yolk-sac of 5-day-old chick embryos, and the 50% lethal dose (LD50) was determined 7 days later. A strain dependent virulence was seen. In the diluted series of cultures the LD50 values for live campylobacter ranged from 10(6) c.f.u. beyond the last dilution showing growth, that is to less than one organism per embryo. When the 22 strains were tested as heat-killed cells, the chick embryo LD50 values retained the same relative order of toxicity obtained with viable cells, but the LD50 values were increased by +1 to +4 log units. Heat-killed cells from strains known to be invasive, but non-toxigenic, were still lethal for the embryos, suggesting that viability was not solely necessary for virulence. Semi-pure lipopolysaccharide from a non-virulent strain of C. jejuni was not toxic to the embryos, but semi-pure and ultracentrifuge-purified lipopolysaccharide from the most lethal campylobacter strains gave LD50 values in the order of 3.0 micrograms lipopolysaccharide per ml (0.6 microgram per embryo) in the yolk-sac assay. No relationship between serotype and lethality was seen. Injection into the yolk-sac appears to be an easy, rapid and reproducible in vivo assay of the virulence of C. jejuni.