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Antimicrob Agents Chemother. 1977 March; 11(3): 553-558
Copyright © 1977 American Society for Microbiology. All Rights Reserved.

Characterization of an R-Plasmid Associated with Ampicillin Resistance in Shigella dysenteriae Type 1 Isolated from Epidemics

^* Department of Microbiology, School of Medicine, University of Washington, Seattle, Washington 98195
Laboratorio de Bacteriologia Intestinal, Hospital Infantil de México, Mexico City, Mexico; and Instituto de Investigaciones en Salud, University of Costa Rica, San Jose, Costa Rica

ABSTRACT

Ampicillin-resistant strains of Shigella dysenteriae type 1 isolated in epidemics in Mexico, Central America, and Bangla Desh were examined for the presence of plasmid deoxyribonucleic acid (DNA) by gel electrophoresis. All strains contained a heterogeneous population of plasmids. Transfer experiments to Escherichia coli K-12 indicated that the ampicillin resistance determinant (Ap^r) was located on a 5.5-megadalton (Mdal) plasmid identical in all Shiga strains examined, as judged by DNA hybridization and by its molecular properties. This 5.5-Mdal plasmid contained the ampicillin transposon (TnA) sequences. There was not a high degree of homology between the Shiga Ap^r plasmid DNA and DNA obtained from Ap^rSalmonella typhi strains isolated from typhoid epidemics in Mexico, previous to the dysentery outbreaks. Although low, the degree of reassociation observed indicated that probably part of the TnA sequence was present in S. typhi DNA. The DNA hybridization experiments showed, in addition, that there was a high degree of homology among Ap^r plasmids isolated from different enterobacteria, and this identity was confirmed by restriction endonuclease activity. These results together with their similarities in molecular and replicative properties indicate that the Ap^r plasmids, as was suggested for the Sm^r Su^r plasmids, possibly evolved once and then epidemiologically spread in the Enterobacteriaceae.