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Epidemiological markers of Shigella sonnei infections: R-plasmid fingerprinting, phage-typing and biotyping
Abstract:
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.
Insights
An epidemic of Shigella sonnei in Sicily was linked to a specific phage type and antibiotic resistance pattern. Plasmid analysis confirmed a common source, highlighting phage-typing and R-plasmid characterization for epidemiological studies.
Area of Science:
- Microbiology
- Epidemiology
- Molecular Biology
Background:
- A significant increase in Shigella sonnei isolates was observed in Sicily in 1980.
- The majority of isolates (80%) belonged to phage-type 3 and shared identical antibiotic resistance profiles.
Purpose of the Study:
- To investigate the cause of the apparent epidemic of Shigella sonnei in Sicily.
- To determine the role of plasmids in the observed antibiotic resistance and spread of the bacteria.
Main Methods:
- Phage-typing of Shigella sonnei isolates.
- Antibiotic resistance profiling.
- Plasmid DNA extraction and agarose gel electrophoresis.
- Plasmid transfer experiments (conjugation) to Escherichia coli K12.
- R-plasmid characterization including incompatibility grouping and endonuclease cleavage pattern analysis.
Main Results:
- Phage-type 3 isolates exhibited consistent plasmid profiles, including 80 Mdal and 55 Mdal plasmids, with some variations in smaller plasmids.
- The 80 Mdal plasmid was self-transmissible and conferred the donor's antibiotic resistance pattern to recipient Escherichia coli K12.
- All transmissible R plasmids belonged to incompatibility group I1 and showed similar endonuclease cleavage patterns.
- A non-phage-type 3 isolate displayed a distinct plasmid profile.
Conclusions:
- Phage-typing and R-plasmid characterization are highly reliable methods for the epidemiological study of Shigella sonnei outbreaks.
- The consistent plasmid profiles and transferability of resistance suggest a common origin and clonal spread of the dominant phage-type 3 strain during the Sicilian epidemic.
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