High‐frequency S‐layer protein variation in Campylobacter fetus revealed by sapA mutagenesis

Campylobacter fetus utilizes paracrystalline surface (S‐) layer proteins that confer complement resistance and that undergo antigenic variation to facilitate persistent mucosal colonization in ungulates. C. fetus possesses multiple homologues of sapA, each of which encode full‐length S‐layer proteins. Disruption of sapA by a gene targeting method (insertion of kanamycin (km) resistance) caused the loss of C. fetus cells bearing full‐length S‐layer proteins and their replacement by cells bearing a 50 kDa truncated protein that was not exported to the cell surface. After incubation of the mutants with serum, the survival rate was approximately 2 × 10^‐2. Immunoblots of survivors showed that phenotypic reversion involving high‐level production of full‐length (98, 127 or 149 kDa) S‐layer proteins had occurred. Revertants were serum resistant but caused approximately 10‐fold less bacteraemia in orally challenged mice than did the wild‐type strain. Southern hybridizations of the revertants showed rearrangement of sapA homologues and retention of the km marker. These results indicate that there exists high‐frequency generation of C. fetus sapA antigenic variants, and that intracellular mechanisms acting at the level of DNA reciprocal recombination play key roles in this phenomenon.