CHARACTERIZATION OF HSD PLASMID IN SALMONELLA-TYPHI

An Hsd (host specificity for DNA) plasmid designated pSTd4 was found in Salmonella Typhi D4, one of the standard phage typing strains. The plasmid was introduced into E. coli K-12, and the restriction map of this plasmid was constructed. The efficiency of plating of lambda . 0 phage was drastically reduced with pSTd4 as well as other small Hsd plasmids of Enterobacteriaceae origin. The results clearly indicate that pSTd4 is a type-determining plasmid in S. Typhi D 4. We propose that rare phage types such as D4 carrying type-determining plasmids or lysogenic phages should be eliminated from the collection of standard phage typing strains of S. Typhi.