Comparison of plasmid profile analysis, phage typing, and antimicrobial susceptibility testing in characterizing Salmonella typhimurium isolates from outbreaks.

We compared the phage types, antimicrobial resistance patterns, and plasmid profiles of 20 groups of isolates received at the Centers for Disease Control from Salmonella typhimurium outbreaks between 1975 and 1982 to determine the most useful laboratory method for identifying epidemiologically related isolates of S. typhimurium. In 18 (90%) of the 20 outbreaks, epidemiologically related isolates were identified as being the same by each of the three methods. In a subgroup of nine outbreaks in which isolates unrelated to the outbreak were submitted for comparison, outbreak isolates were differentiated from such control isolates six times (67%) by phage typing alone, four times (44%) by antimicrobial susceptibility testing alone, and eight times (89%) by plasmid profile analysis alone. Epidemic isolates were multiply susceptible, nontypable, or without plasmids in 14 (70%), 1 (5%), and 3 (15%), respectively, of the 20 outbreaks. Plasmid analysis appeared to be at least as specific as phage typing in identifying epidemiologically related isolates of S. typhimurium as being the same or in differentiating them from control specimens; both techniques appeared to be superior to antimicrobial susceptibility testing.