Transposition of the heat-stable toxin astA gene into a gifsy-2-related prophage of Salmonella enterica serovar Abortusovis

The horizontal transfer and acquisition of virulence genes via mobile genetic elements have been a major
driving force in the evolution of Salmonella pathogenicity. Serovars of Salmonella enterica carry variable
assortments of phage-encoded virulence genes, suggesting that temperate phages play a pivotal role in this
process. Epidemic isolates of S. enterica serovar Typhimurium are consistently lysogenic for two lambdoid
phages, Gifsy-1 and Gifsy-2, carrying known virulence genes. Other serovars of S. enterica, including serovars
Dublin, Gallinarum, Enteritidis, and Hadar, carry distinct prophages with similarity to the Gifsy phages. In
this study, we analyzed Gifsy-related loci from S. enterica serovar Abortusovis, a pathogen associated exclusively
with ovine infection. A cryptic prophage, closely related to serovar Typhimurium phage Gifsy-2, was
identified. This element, named Gifsy-2AO, was shown to contribute to serovar Abortusovis systemic infection
in lambs. Sequence analysis of the prophage b region showed a large deletion which covers genes encoding
phage tail fiber proteins and putative virulence factors, including type III secreted effector protein SseI (GtgB,
SrfH). This deletion was identified in most of the serovar Abortusovis isolates tested and might be dependent
on the replicative transposition of an adjacent insertion sequence, IS1414, previously identified in pathogenic
Escherichia coli strains. IS1414 encodes heat-stable toxin EAST1 (astA) and showed multiple genomic copies in
isolates of serovar Abortusovis. To our knowledge, this is the first evidence of intergeneric transfer of virulence
genes via insertion sequence elements in Salmonella. The acquisition of IS1414 (EAST1) and its frequent
transposition within the chromosome might improve the fitness of serovar Abortusovis within its narrow
ecological niche.