No abstract

In Saccharomyces cerevisiae, the FLO1 gene encodes flocculins that lead to formation of multicellular flocs, that offer protection to the constituent cells. Flo1p was found to preferentially bind to fellow cooperators compared to defectors lacking FLO1 expression, resulting in enrichment of cooperators within the flocs. Given this dual function in cooperation and kin recognition, FLO1 has been termed a 'green beard gene'. Because of the heterophilic nature of Flo1p binding however, we hypothesize that kin recognition is permissive and depends on the relative stability of FLO1 + /flo1 − versus FLO1 + /FLO1 + bonds, which itself can be dependent on environmental conditions and intrinsic cell properties. We combine single cell measurements of adhesion strengths, individual cell-based simulations of cluster formation and evolution, and in vitro flocculation experiments to study the impact of relative bond stability on defector exclusion as well as benefit and stability of cooperation. We hereto vary the relative bond stability by changing the shear flow rate and the inherent bond strength. We identify a marked trade-off between both aspects of the green beard mechanism, with reduced relative bond stability leading to increased kin recognition, but at the expense of decreased cluster sizes and benefit of cooperation. Most notably, we show that the selection of FLO1 cooperators is negative-frequency dependent, which we directly attribute to the permissive character of the Flo1p bond. Taking into account the costs associated to FLO1 expression, this asymmetric selection results in a broad range of ecological conditions where coexistence between cooperators and defectors is stable. Although the kin recognition aspect of the FLO1 'green beard gene' is thus limited and condition dependent, the negative-frequency dependency of selection can conserve the diversity of flocculent and non-flocculent phenotypes ensuring flexibility towards variable selective pressures.studying ST 9-11 and CT 12-14 group formation, although S. cerevisiae does not have any known obligate multicellular descendants 4 . A key gene family involved in group formation in yeast comprises the FLO genes, which encode for flocculins, proteins involved in cell adhesion 4,12,[15][16][17][18][19] . These flocculins possess an N-terminal domain protruding from the cell surface, a central domain of tandem repeated sequences, and a C-terminal glycosylphosphatidylinositol (GPI) domain anchored in the cell wall 15,20 . Based on the N-terminal domain, two types of flocculins can be distinguished. Flo11p harbors a fibronectin type III-like domain that confers homophilic protein-protein interaction with neighbouring cells 11,21 . Flo11p-mediated adhesion partakes in multiple ST group phenotypes such as biofilm 11,22 and pseudohyphae formation 23 . In contrast, the FLO1 gene encodes for a PA14-like N-terminal domain that binds to mannose residues on the cell wall of neighbouring cells, a mechanism that is heterophilic in nature 24,25 . Flo1p controls the CT flocculatio...

In Saccharomyces cerevisiae, the FLO1 gene encodes flocculins that lead to formation of multicellular flocs, that offer protection to the constituent cells. Flo1p was found to preferentially bind to fellow cooperators compared to defectors lacking FLO1 expression, resulting in enrichment of cooperators within the flocs. Given this dual function in cooperation and kin recognition, FLO1 has been termed a ‘green beard gene’. Because of the heterophilic nature of Flo1p binding however, we hypothesize that kin recognition is permissive and depends on the relative stability of FLO1+/flo1− versus FLO1+/FLO1+ bonds, which itself can be dependent on environmental conditions and intrinsic cell properties. We combine single cell measurements of adhesion strengths, individual cell-based simulations of cluster formation and evolution, and in vitro flocculation experiments to study the impact of relative bond stability on defector exclusion as well as benefit and stability of cooperation. We hereto vary the relative bond stability by changing the shear flow rate and the inherent bond strength. We identify a marked trade-off between both aspects of the green beard mechanism, with reduced relative bond stability leading to increased kin recognition, but at the expense of decreased cluster sizes and benefit of cooperation. Most notably, we show that the selection of FLO1 cooperators is negative-frequency dependent, which we directly attribute to the permissive character of the Flo1p bond. Taking into account the costs associated to FLO1 expression, this asymmetric selection results in a broad range of ecological conditions where coexistence between cooperators and defectors is stable. Although the kin recognition aspect of the FLO1 ‘green beard gene’ is thus limited and condition dependent, the negative-frequency dependency of selection can conserve the diversity of flocculent and non-flocculent phenotypes ensuring flexibility towards variable selective pressures.

No abstract

Extracellular polymeric substances (EPS) in bacterial biofilms complicate treatment by inactivating drugs and slowing down diffusion. Through enhanced penetration and resistance to degradation in bacterial biofilms, nanoparticle (NP) carriers can help improve biofilm treatment. However, the way in which biofilm architecture influences the diffusive properties and penetration of NPs in biofilms is still poorly understood. In this work, we combined single particle tracking (SPT) and confocal laser scanning microscopy (CLSM) in Salmonella biofilms with simulations of a Brownian dynamics model to quantify how macro- (spatial organization of the bacteria) and micro- (EPS dependent) structure of the biofilm affects NP penetration. In CLSM images we observed immobilization of NPs in the EPS, which allows shielding of bacteria from the NPs, an effect that was more pronounced in dispersed biofilms, grown in nutrient-rich conditions, than in compacted biofilms, grown in nutrient-poor conditions. SPT experiments revealed anomalous diffusion, with an increased probability for small displacements near clusters of bacteria. Simulations of a Brownian dynamics model revealed that EPS reinforces shielding by affecting the pore structure of the biofilm. Finally, in virtual biofilms with varying spatial distribution of bacteria, we found that even for the same number of bacteria, dispersed biofilm structures provide more shielding than biofilms organized in dense, compacted clusters, even when accounting for decreased NP diffusivity.