Ferric dicitrate transport system (Fec) of Shigella flexneri 2a YSH6000 isencoded on a novel pathogenicity island carrying multiple antibiotic resistance genes

Iron uptake systems which are critical for bacterial survival and which may play important roles in bacterial virulence are often carried on mobile el ements, such as plasmids and pathogenicity islands (PAIs). In the present s tudy, we identified and characterized a ferric dicitrate uptake system (Fee ) in Shigella flexneri serotype 2a that is encoded by a novel PAI termed th e Shigella resistance locus (SRL) PAL The fec genes are transcribed in S. f lexneri, and complementation of a fee deletion in Escherichia coli demonstr ated that they are functional. However, insertional inactivation of fecI, l eading to a loss in fec gene expression, did not impair the growth of the p arent 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 the fec genes are linked to a cluster of multiple antibiotic resistance de terminants, designated the SRL, on the chromosome of S. flexneri 2a. Both t he SRL and fee loci are carried on the 66,257-bp SRL PAI, which has integra ted into the serX tRNA gene and which carries at least 22 prophage-related open reading frames, including one for a P4-like integrase. This is the fir st example of a PAI that carries genes encoding antibiotic resistance and t he first report of a ferric dicitrate uptake system in Shigella.