Bistable Expression of CsgD in Biofilm Development ofSalmonella entericaSerovar Typhimurium

Abstract

Bacterial persistence in the environment and in the infected host is often aided by the formation of exopolymer-enclosed communities known as biofilms. Heterogeneous gene expression takes place in microcompartments formed within the complex biofilm structure. This study describes cell differentiation within an isogenic bacterial cell population based on the example of biofilm formation by Salmonella enterica serovar Typhimurium. We analyzed the expression of the major biofilm regulator CsgD at the single-cell level with a chromosomal CsgD-green fluorescent protein (GFP) translational fusion. In individual cells, CsgD-GFP expression is mostly found in the cytoplasm. Quantitative expression analysis and results from three different models of S. Typhimurium biofilms demonstrated that CsgD is expressed in a bistable manner during biofilm development. CsgD expression is, however, monomodal when CsgD is expressed in larger amounts due to a promoter mutation or elevated levels of the secondary signaling molecule c-di-GMP. High levels of CsgD-GFP are associated with cellular aggregation in all three biofilm models. Furthermore, the subpopulation of cells expressing large amounts of CsgD is engaged in cellulose production during red, dry, and rough (rdar) morphotype development and in microcolony formation under conditions of continuous flow. Consequently, bistability at the level of CsgD expression leads to a corresponding pattern of task distribution in S. Typhimurium biofilms.

Iron induces bimodal population development by Escherichia coli

“…2 and 3). Studies in S. typhimurium have previously demonstrated that levels of CsgD and curli can vary from cell to cell in bacterial communities (17,49). Here we have demonstrated a distinct bimodal restructuring in which matrix-producing bacteria line the air-biofilm interface, and non-matrix-producing bacteria fill the wrinkle interiors.…”

Bistable Expression of CsgD in Salmonella enterica Serovar Typhimurium Connects Virulence to Persistence

“…The bacterial secondary messenger c-di-GMP is known to play a role in regulating csgD transcription and CsgD activity (30,31). High levels of c-di-GMP are associated with biofilm formation, whereas low levels are associated with a single-cell lifestyle (44).…”

“…In addition to the cells in the fluid phase, there were cells that formed an adherent biofilm at the air-liquid interface of the culture flask. These cells differed in terms of oxygen availability (30), and the amount of biofilm formed was highly variable (data not shown); therefore, they were excluded from further analysis.…”

“…Although they are informative, a potential problem with strain-to-strain comparisons is that they could miss fluctuations that exist within individual populations of cells. It was recently demonstrated that when S. Typhimurium is grown under biofilm-inducing conditions in liquid medium, the population of cells in the fluid phase differentiates into large, multicellular aggregates and single (planktonic) cells (25,30). This bifurcation is caused by the bistable synthesis of CsgD, the central transcriptional regulator of the rdar morphotype (31).…”

“…Aggregated cells are in a CsgD-on state, whereas the nonaggregative planktonic cells are primarily in a CsgD-off state. The multicellular aggregates can be thought of as noncanonical biofilms, as the cells strongly express extracellular matrix components (25,30) but are not in direct contact with a surface. While it is assumed that the aggregates retain the resistance properties of classic adherent biofilms, the planktonic cells that result from CsgD bistability have never been analyzed in detail, and it is unclear what role they may have in S. Typhimurium physiology.…”