Camplobacter fetus subsp. fetus is a zoonotic pathogen and cause opportunistic systemic infection in human who either are pregnant or have an underlying illness. Crystalline surface layer (S-layer), an external structure on the outer membrane is the prime virulence factor of this organism. S-layer is responsible for resistance to both leukocyte phagocytosis and the bactericidal effects of human serum. Antigenic variation of C. fetus surface antigen during infection also help the organism to escape from host immune-system and it is due to variation in the expression of the S-layer proteins (SLPs). C. fetus has multiple SLP-encoding genes and control the expression with inversion of a DNA segment containing the SLP promoter and/or homologous recombination.