Escherichia coli R-factors unstable in Salmonella typhi are deleted before being segregated in this host

Although resistant Salmonella typhi strains are found in the environment, many epidemiological data indicate that most isolates are multisensitive. Plasmids are not common in S. typhi, contrasting with other enterobacteria. Since S. typhi is able to receive R plasmids from other enterobacteria, such plasmidless condition may be due to destabilization of the plasmids acquired. The segregation process of three plasmids known to behave unstably in S. typhi strains was analyzed, and evidence was obtained that the three of them lost an antibiotic-resistance determinant due to small deletions before plasmid loss occurred. Once deleted, the plasmids entered a segregation phase with particular kinetic features in each case. Selective pressure prevented plasmid deletion and segregation. Like their parental plasmids, the deleted plasmids were unstable in S. typhi but stable in Escherichia coli, suggesting that deletions imply loss of genetic information relevant for a plasmid stability mechanism operative in S. typhi, but not in E. coli.