Abstract
Campylobacter fetus
bacteria, isolated from both mammals and reptiles, may be either subsp.
fetus
or subsp.
venerealis
and either serotype A or serotype B. Surface layer proteins, expressed and secreted by genes in the
sap
locus, play an important role in
C. fetus
virulence. To assess whether the
sap
locus represents a pathogenicity island and to gain further insights into
C. fetus
evolution, we examined several
C. fetus
genes in 18 isolates. All of the isolates had 5 to 9
sapA
or
sapB
homologs. One strain (85-387) possessed both
sapA
and
sapB
homologs, suggesting a recombinational event in the
sap
locus between
sapA
and
sapB
strains. When we amplified and analyzed nucleotide sequences from portions of housekeeping gene
recA
(501 bp) and
sapD
(450 bp), a part of the 6-kb
sap
invertible element, the phylogenies of the genes were highly parallel. Among the 15 isolates from mammals, serotype A and serotype B strains generally had consistent positions. The fact that the serotype A
C. fetus
subsp.
fetus
and subsp.
venerealis
strains were on the same branch suggests that their differentiation occurred after the type A-type B split. Isolates from mammals and reptiles formed two distinct tight phylogenetic clusters that were well separated. Sequence analysis of 16S rRNA showed that the reptile strains form a distinct phylotype between mammalian
C. fetus
and
Campylobacter hyointestinalis
. The phylogenies and sequence results showing that
sapD
and
recA
have similar G + C contents and substitution rates suggest that the
sap
locus is not a pathogenicity island but rather is an ancient constituent of the
C. fetus
genome, integral to its biology.