Role of genomic rearrangements in producing new ribotypes of Salmonella typhi

Salmonella typhi is the only species of Salmonella which grows exclusively in humans, in whom it causes enteric typhoid fever. Strains of S. typhi sho w very little variation in electrophoretic types, restriction fragment leng th polymorphisms, cell envelope proteins, and intervening sequences, but th e same strains are very heterogeneous for ribotypes which are detected with the restriction endonuclease PstI. In addition, the genome of S. typhi has been proven to undergo genomic rearrangement due to homologous recombinati on between the seven copies of rrn genes. The relationship between ribotype heterogeneity and genomic rearrangement was investigated. Strains of S. ty phi which belong to 23 different genome types were analyzed by ribotyping. A limited number of ribotypes were found within the same genome type group; e.g., most strains of genome type 3 belonged to only two different ribotyp es, which result from recombination between rrnH and rrnC operons. Differen t genome type groups normally have different ribotypes. The size and identi ty of the PstI fragment containing each of the seven different rm operons f rom S. typhi Ty2 were determined, and from these data, one can infer how ge nomic rearrangement forms new ribotypes. It is postulated that genomic rear rangement, rather than mutation, is largely responsible for producing the r ibotype heterogeneity in S. typhi.