In the copy number of the -specific insertion element IS was found to vary from four to six. All strains tested contained at least one common insertion site which was serovar specific, and most strains contained three common sites. Concurrent analysis of plasmids indicated that all insertion sequence copies were chromosomally located, and also supported the equivalence of an IS fingerprint and clonality. Seven intra-serovar clonal lines were thereby identified. One of these was associated with human infections, including septicaemias. Another was associated with chicken as a host: all these strains also carried a unique plasmid of 23 MDa, which was typed as a member of the IncX group. The chromosomal fingerprint of a third clone showed it to be a descendant of the chicken line marked by a single IS transposition. One or two representatives of four other clonal lines were identified. These lines of could be related by divergent evolution, and the most recent relatives conformed to a continuous branching process model of IS transposition. This insertion sequence provided a highly discriminatory molecular marker of the chromosome, and two of the seven clonal lines so identified were associated with distinct clinical/epidemiological contexts.


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