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Ferric Dicitrate Transport System (Fec) of Shigella flexneri 2a YSH6000 Is Encoded on a Novel Pathogenicity Island Carrying Multiple Antibiotic Resistance Genes

ABSTRACT Iron uptake systems which are critical for bacterial survival and which may play important roles in bacterial virulence are often carried on mobile elements, such as plasmids and pathogenicity islands (PAIs). In the present study, we identified and characterized a ferric dicitrate uptake system (Fec) in Shigella flexneriserotype 2a that is encoded by a novel PAI termed theShigella resistance locus (SRL) PAI. Thefec genes are transcribed in S. flexneri, and complementation of a fec deletion inEscherichia coli demonstrated that they are functional. However, insertional inactivation of fecI, leading to a loss in fec gene expression, did not impair the growth of the parent strain of S. flexneri in iron-limited culture media, suggesting that S. flexneri carries additional iron uptake systems capable of compensating for the loss of Fec-mediated iron uptake. DNA sequence analysis showed that thefec genes are linked to a cluster of multiple antibiotic resistance determinants, designated the SRL, on the chromosome ofS. flexneri 2a. Both the SRL and fec loci are carried on the 66,257-bp SRL PAI, which has integrated into theserX tRNA gene and which carries at least 22 prophage-related open reading frames, including one for a P4-like integrase. This is the first example of a PAI that carries genes encoding antibiotic resistance and the first report of a ferric dicitrate uptake system in Shigella.