Several inbred strains of mice were inoculated with Serpula (Treponema) hyodysenteriae B204 to determine susceptibility to infection. Challenge doses of 107 or 108 spirochetes induced cecal lesions in C3H/HeJ mice and other C3H strains of mice. However, more than a 100-fold difference existed between the dose required to induce lesions in 50% of the infected C3H/HeJ mice (8.3 x 107) and that required to induce them in 50% of the infected C3H/HeN mice (5 x 105). C3H/HeJ mice lack a splenocyte mitogenic response to Escherichia coli lipopolysaccharide but exhibited a mitogenic response comparable to those of other C3H strains of mice when stimulated with S. hyodysenteriae endotoxin (butanol-water extract). Different inbred strains exhibited different susceptibilities to infection, with the strain C3H/HeN being the most susceptible on the basis of colonization and development of macroscopic cecal lesions. The ity gene had no apparent effect on susceptibility of mice challenged with S. hyodysenteriae. The involvement of the H-2 haplotype with susceptibility is unclear, but the mice bearing H-2k were more susceptible than mice with the H-2b, H-2d, or H-2q haplotype. These data support the hypothesis that the host's responsiveness to lipopolysaccharide influences the susceptibility to infection with S. hyodysenteriae. However, differences in susceptibility between inbred mice exist independent of the Ips locus, suggesting that there are other inherent differences between mouse strains that affect susceptibility to infection by S. hyodysenteriae.

Lipopolysaccharide (LPS) is a classic inducer of inflammatory cytokines and is a key virulence factor for most gram-negative pathogens. The effect of phenol-water (LPS) and butanol-water (endotoxin) extracts from Serpulina hyodysenteriae on inflammatory cytokine mRNA expression from porcine alveolar macrophages was investigated. The LPS and endotoxin extracts from S. hyodysenteriae induced a dose-dependent expression of interleukin 1␤ (IL-1␤) and IL-8 which was weak compared with the responses induced by Escherichia coli LPS. In addition, the spirochetal extracts induced no detectable upregulation of mRNA expression for either IL-6 or tumor necrosis factor alpha.

A defined diet was used to increase the susceptibility of mice to Serpulina hyodysenteriae. BALB/cByJ, C3H/HeN, and C3H/HeJ mice, when fed the defined diet 7 to 14 days prior to and throughout the challenge period, consistently showed higher incidences of disease than mice maintained on normal rodent chow. The use of this defined diet will increase the consistency of in vivo studies following infection with S. hyodysenteriae in the mouse model.

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