Description of an unusual class 2 integron in Shigella sonnei isolates in Senegal (sub-Saharan Africa)

This is the authors' abstract. We don't add key points for this paper.

Sir, Integrons are genetic elements that acquire gene cassettes by integrase-catalysed site-specific recombination.1 Integrons contain various combinations of gene cassettes encoding antibiotic resistance determinants and are widespread among Gram-negative bacteria. Five classes of integrons have been described to date, based on the integrase coding sequence. Classes 1 and 2 are the most frequent. Class 2 was found in transposon Tn7 and its derivatives (Tn1825, Tn1826 and Tn4132), and its 3′ segment contains five tns genes involved in transposon movements. The intI2 integrase gene of class 2 integrons contains a premature stop codon at position 179, rendering the integrase non-functional.2 Therefore, class 2 integrons usually contain the same array of four gene cassettes, three antibiotic resistance gene cassettes (dfrA1, sat and aadA1, conferring resistance to trimethoprim, streptothricin and spectinomycin/streptomycin, respectively) and an orfX of unknown function. However, variations of the cassette contents have been described, resulting from Intl1 integrase-catalysed co-integrate formation between a class 1 and a class 2 integron, or from an RecA-dependent homologous recombination between two copies of the same cassette found in both classes of integron. Indeed, the dfrA1 and aadA1 cassettes, initially described in class 2 integrons, have also been found in class 1 integrons. Class 1 and 2 integrons have frequently been found in Shigella strains, and class 2 integrons predominate in Shigella sonnei and Shigella flexneri.3–5