A previously uncharacterized gene <it>stm0551</it> plays a repressive role in the regulation of type 1 fimbriae in <it>Salmonella enterica</it> serotype Typhimurium

<p>Abstract</p> <p>Background</p> <p><it>Salmonella enterica</it> serotype Typhimurium produces surface-associated fimbriae that facilitate adherence of the bacteria to a variety of cells and tissues. Type 1 fimbriae with binding specificity to mannose residues are the most commonly found fimbrial type. <it>In vitro</it>, static-broth culture favors the growth of <it>S.</it> Typhimurium with type 1 fimbriae, whereas non-type 1 fimbriate bacteria are obtained by culture on solid-agar media. Previous studies demonstrated that the phenotypic expression of type 1 fimbriae is the result of the interaction and cooperation of the regulatory genes <it>fimZ</it>, <it>fimY</it>, <it>fimW,</it> and <it>fimU</it> within the <it>fim</it> gene cluster. Genome sequencing revealed a novel gene, <it>stm0551</it>, located between <it>fimY</it> and <it>fimW</it> that encodes an 11.4-kDa putative phosphodiesterase specific for the bacterial second messenger cyclic-diguanylate monophosphate (c-di-GMP). The role of <it>stm0551</it> in the regulation of type 1 fimbriae in <it>S</it>. Typhimurium remains unclear.</p> <p>Results</p> <p>A <it>stm0551</it>-deleted stain constructed by allelic exchange constitutively produced type 1 fimbriae in both static-broth and solid-agar medium conditions. Quantative RT-PCR revealed that expression of the fimbrial major subunit gene, <it>fimA,</it> and one of the regulatory genes, <it>fimZ</it>, were comparably increased in the <it>stm0551</it>-deleted strain compared with those of the parental strain when grown on the solid-agar medium, a condition that normally inhibits expression of type 1 fimbriae. Following transformation with a plasmid possessing the coding sequence of <it>stm0551</it>, expression of <it>fimA</it> and <it>fimZ</it> decreased in the <it>stm0551</it> mutant strain in both culture conditions, whereas transformation with the control vector pACYC184 relieved this repression. A purified STM0551 protein exhibited a phosphodiesterase activity <it>in vitro</it> while a point mutation in the putative EAL domain, substituting glutamic acid (E) with alanine (A), of STM0551 or a FimY protein abolished this activity.</p> <p>Conclusions</p> <p>The finding that the <it>stm0551</it> gene plays a negative regulatory role in the regulation of type 1 fimbriae in <it>S.</it> Typhimurium has not been reported previously. The possibility that degradation of c-di-GMP is a key step in the regulation of type 1 fimbriae warrants further investigation.</p>