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Background   Salmonella enterica  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.  In vitro , static-broth culture favors the growth of  S.  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  fimZ ,  fimY ,  fimW,  and  fimU  within the  fim  gene cluster. Genome sequencing revealed a novel gene,  stm0551 , located between  fimY  and  fimW  that encodes an 11.4-kDa putative phosphodiesterase specific for the bacterial second messenger cyclic-diguanylate monophosphate (c-di-GMP). The role of  stm0551  in the regulation of type 1 fimbriae in  S . Typhimurium remains unclear.    Results  A  stm0551 -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,  fimA,  and one of the regulatory genes,  fimZ , were comparably increased in the  stm0551 -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  stm0551 , expression of  fimA  and  fimZ  decreased in the  stm0551  mutant strain in both culture conditions, whereas transformation with the control vector pACYC184 relieved this repression. A purified STM0551 protein exhibited a phosphodiesterase activity  in vitro  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.    Conclusions  The finding that the  stm0551  gene plays a negative regulatory role in the regulation of type 1 fimbriae in  S.  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.

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