BACKGROUND:Biofilm formation enhances the capacity of pathogenic Salmonella
bacteria to survive stresses that are commonly encountered within
food processing and during host infection. The persistence of Salmonella
within the food chain has become a major health concern, as biofilms
can serve as a reservoir for the contamination of food products.
While the molecular mechanisms required for the survival of bacteria
on surfaces are not fully understood, transcriptional studies of
other bacteria have demonstrated that biofilm growth triggers the
expression of specific sets of genes, compared with planktonic cells.
Until now, most gene expression studies of Salmonella have focused
on the effect of infection-relevant stressors on virulence or the
comparison of mutant and wild-type bacteria. However little is known
about the physiological responses taking place inside a Salmonella
biofilm.RESULTS:We have determined the transcriptomic and proteomic
profiles of biofilms of Salmonella enterica serovar Typhimurium.
We discovered that 124 detectable proteins were differentially expressed
in the biofilm compared with planktonic cells, and that 10% of the
S. Typhimurium genome (433 genes) showed a 2-fold or more change
in the biofilm compared with planktonic cells. The genes that were
significantly up-regulated implicated certain cellular processes
in biofilm development including amino acid metabolism, cell motility,
global regulation and tolerance to stress. We found that the most
highly down-regulated genes in the biofilm were located on Salmonella
Pathogenicity Island 2 (SPI2), and that a functional SPI2 secretion
system regulator (ssrA) was required for S. Typhimurium biofilm formation.
We identified STM0341 as a gene of unknown function that was needed
for biofilm growth. Genes involved in tryptophan (trp) biosynthesis
and transport were up-regulated in the biofilm. Deletion of trpE
led to decreased bacterial attachment and this biofilm defect was
restored by exogenous tryptophan or indole.CONCLUSIONS:Biofilm growth
of S. Typhimurium causes distinct changes in gene and protein expression.
Our results show that aromatic amino acids make an important contribution
to biofilm formation and reveal a link between SPI2 expression and
surface-associated growth in S. Typhimurium.
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