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Minimal colonization inocula for two broiler strains of Campylobacter jejuni were determined in broiler chicks aged 2-3 days and 2 weeks. Individually housed chicks were exposed to a single oral or cloacal challenge. Diarrhoeal symptoms were absent in all 380 chicks included in the study. Chick susceptibility to the two C. jejuni strains varied. Colonization was effected by less than 10(2)-10(4) colony forming units (c.f.u.) via cloacal challenge and 10(4)-10(6) c.f.u. via the oral route. Colonization inocula for 2- to 3-day and 2-week-old chicks were similar. Treatment of 1-day-old chicks with fresh adult caecal flora or an anaerobic broth culture of adult caecal flora did not inhibit colonization after challenge with low-dose C. jejuni. Susceptible chicks were colonized rapidly. C. jejuni was detected in 167 of 189 (88%) colonized chicks within 3 days of challenge and persisted during the 2-week monitoring period. Our data suggest that colonization of broiler chicks with C. jejuni is effected more easily by the cloacal than the oral route and is independent of age.

The biochemical and biological properties of the flagella of Campylobacter jejuni have been investigated using two variants selected from a flagellate, motile clinical isolate (strain 81116): a flagellate, non-motile variant (SF-1) and an aflagellate variant (SF-2). Phenotypic and biochemical analysis of the strains and amino acid analysis of the isolated flagella suggest that the variants differed from the wild-type strain only in the absence of flagella and/or motility. The aflagellate variant poorly colonized the gastrointestinal tract of infant mice but the flagellate, non-motile variant colonized the mice as successfully as the wild-type strain. 35S-labelled organisms were used to investigate the attachment of the variants to human epithelial cell monolayers in vitro. The flagellate, non-motile strain attached more efficiently to the cells than the wild-type strain or the aflagellate strain. Differences in attachment suggest that an adhesin is intimately associated with flagella of C. jejuni and that active flagella mediate only a tenuous association with host cells. This adhesin attached most efficiently to cells of intestinal epithelial origin and was not specifically inhibited by various sugars.

The virulence of Campylobacter jejuni and C. coli isolated from various water sources was compared with that of clinical strains by in vitro assays of adhesion, invasion and cytotoxicity to HeLa cells. Variation in degree of attachment was observed, but this did not appear to be related to strain source, However, water strains were less invasive and less cytotoxic to HeLa cells than clinical strains as shown by immunofluorescence and electron microscopy. These differences were particularly evident between clinical and water isolates of the same serotype and biotype implicated in an outbreak of campylobacter enteritis in a school. The enhanced virulence of the clinical isolates, possibly induced by passage, was confirmed by colonization tests on infant mice.

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.