CHARACTERIZATION OF HSD PLASMID IN SALMON ELLA-TYPHI

An Hsd (host specificity for DNA) plasmid designated pSTd4 was found i n Salmonella Typhi D4, one of the standard phage typing strains. The p lasmid was introduced into E. coli K-12, and the restriction map of th is plasmid was constructed. The efficiency of plating of lambda . 0 ph age was drastically reduced with pSTd4 as well as other small Hsd plas mids of Enterobacteriaceae origin. The results clearly indicate that p STd4 is a type-determining plasmid in S. Typhi D 4. We propose that ra re phage types such as D4 carrying type-determining plasmids or lysoge nic phages should be eliminated from the collection of standard phage typing strains of S. Typhi.