Abstract
genetically compared with the R factors carried by the Salmonella typhi responsible for the 1972 outbreak of typhoid fever in Mexico. R factors from both genera confer the same pattern of resistance to chloramphenicol (Cm), streptomycin (S), sulfonamide (Su), and tetracycline (Tc) on their hosts. When introduced into an Escherichia coli recipient, the levels of resistance to antibiotics and heavy metals were identical except that the Shigella R factor provided slightly greater resistance to Cm. Both R factors are of the fi- phenotype and do not restrict or modify bacteriophage. However, E. coli harboring the Shigella R factors were found to have approximately 1,000-fold higher frequencies of transfer into drug-sensitive E. coli and S. typhi. Despite the differences mentioned above, the two epidemic R factors share many similarities, and thus a common ancestry is suspected.
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