Characterization of Plasmids from Antibiotic-resistant Shigella Isolates by Agarose Gel Electrophoresis

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Abstract
Gel electrophoresis of DNA from 95 clinical isolates of S. sonnei and S. flexneri resistant to antibiotics revealed a heterogeneous plasmid population. Most plasmids were smaller than 6 megadaltons (Mdal). Six S. sonnei isolates with the most common antibiotic resistance pattern were characterized. They had 2 plasmids in common: 1 was a self-transmissible Fi+ plasmid of 46 Mdal encoding tetracycline resistance, while the other was a 5.5 Mdal non-conjugative plasmid encoding resistance to streptomycin and sulfafurazole. Several cryptic plasmids ranging in size from 1.0-24.5 Mdal were present. Mobilization of the 5.5 Mdal SuSm plasmid and a 1.0 Mdal cryptic plasmid was demonstrated with all 6 S. sonnei isolates during conjugation. This mobilization was mediated by the 46 Mdal self-transmissible Fi+ R plasmid and also by a 24.5 Mdal Fi- plasmid carrying no known drug resistance determinants.