CONTRIBUTION OF HORIZONTAL GENE-TRANSFER AND DELETION EVENTS TO DEVELOPMENT OF DISTINCTIVE PATTERNS OF FIMBRIAL OPERONS DURING EVOLUTION OFSALMONELLA SEROTYPES

Only certain serotypes of Salmonella represent 99% of all human clinic al isolates. We determined whether the phylogenetic distribution of fi mbrial operons would account for the host adaptations observed for Sal monella serotypes. We found that three fimbrial operons, fim, lpf, and agf, were present in a lineage ancestral to Salmonella. While the fim and agf fimbrial operons were highly conserved among all Salmonella s erotypes, sequence analysis suggested that the lpf operon was lost fro m many distantly related lineages. As a consequence, the distribution of the lpf operon cannot be explained easily and may be a consequence of positive and negative selection in different hosts for the presence of these genes. Two other fimbrial operons, sef and pef, each entered two distantly related Salmonella lineages and each is present only in a small number of serotypes. These results show that horizontal gene transfer and deletion events have created unique combinations of fimbr ial operons among Salmonella serotypes. The presence of sef and pef co rrelated with serotypes frequently isolated from common domesticated a nimals.