Epidemiological markers of Shigella sonnei infections: R-plasmid fingerprinting, phage-typing and biotyping.

In 1980, the number of Shigella sonnei strains isolated in Sicily increased markedly. Approximately 80% of the isolates belonged to phage-type 3 and showed the same antibiotic resistance pattern, suggesting that an epidemic had been going on for several months. Plasmid analysis of strains isolated at various times in different places supported this view. Agarose gel electrophoresis of plasmid preparations from seven selected phage-type 3 isolates showed the presence of two plasmids, of 80 megadaltons (Mdal) and 55 Mdal, respectively. In addition, all but one harboured a 2.8-Mdal plasmid, while a 30-Mdal and a 47-Mdal plasmid were, respectively, present in two other isolates. The 80-Mdal plasmid was self-transmissible to Escherichia coli K12, which acquired "en bloc" the resistance patterns of the donor strains. All the self-transmissible R plasmids fell into the incompatibility group I1 and showed a similar endonuclease cleavage pattern. An S. sonnei strain which was isolated during the same period, but did not belong to phage-type 3, exhibited a totally different plasmid pattern. We can conclude that phage-typing and R-plasmid characterization (i.e. incompatibility group, molecular weight and endonuclease cleavage pattern) represent the most reliable methods for epidemiologic study of S. sonnei isolates.