High Genetic Stability of Integrons in Clinical Isolates of Shigella spp. of Worldwide Origin

Over a 12-year period, 68 Shigella strains (31 S. sonnei, 30 S. flexneri, 4 S. dysenteriae, and 3 S. boydii strains) were collected in a French University Hospital from the stools of patients who generally had a recent history of travel to various parts of the world (91%), particularly Africa (67%). These strains were often resistant (streptomycin, spectinomycin, trimethoprim, tetracycline, and sulfonamides, 66 to 84%; ampicillin and chloramphenicol, 34 to 38%; nalidixic acid, 4%) and even multiresistant (87%), and they generally carried integrons (81%) of class 1 (21%), class 2 (47%), or both (13%). Class 1 integrons were associated with ampicillin resistance due to the production of an OXA-30 beta-lactamase in S. flexneri and S. dysenteriae. Class 2 integrons were associated with trimethoprim resistance in S. sonnei. Class 1 and class 2 integrons were inserted within transposons Tn21 and Tn7, respectively, themselves located on the bacterial chromosome, except in one strain. Class 1 integrons showed an atypical organization consisting of the insertion sequence IS1 at the 3' end instead of the typical 3' conserved segment and two blaOXA-30 and aadA1 gene cassettes, despite the absence of epidemiological relationships between the strains, and an apparently functional integrase. Class 2 integrons showed the same albeit classical organization with the three dfrA1, sat, and aadA1 gene cassettes. Occasionally, the 3' end was deleted and the aadA1 gene cassette was unexpressed. Thus, integrons contributed only in part to the multidrug resistance of the Shigella strains. The highly conserved organization of integrons might be related to their location within mobile genetic superstructures.