Typing of sequential bacterial isolates by pulsed-field gel electrophoresis

We typed 39 sets of multiple bacterial isolates of the same species from patients by pulsed-field gel electrophoresis of genomic DNA (PFGE). Isolates were cultured from different sites or over a 2-week or longer interval. Staphylococcus aureus, Enterococcus faecalis, Pseudomonas aeruginosa, Escherichia coli, Klebsiella pneumoniae, and Enterobacter cloacae were tested. Excluding E. cloacae, 28 of 32 sets of isolates (87%) demonstrated only identical or highly related PFGE types. Four of the seven sets of E. cloacae showed different types. For species other than E. cloacae, our results suggest that patients are usually colonized and infected with a single strain of these bacterial pathogens. Unlike all of the other tested species, E. cloacae PFGE typing differences suggested the presence of multiple strains causing colonization and infection.