Division of Labor during Biofilm Matrix Production

Abstract

Organisms as simple as bacteria can engage in complex collective actions, such as group motility and fruiting body formation. Some of these actions involve a division of labor, where phenotypically specialized clonal subpopulations or genetically distinct lineages cooperate with each other by performing complementary tasks. Here, we combine experimental and computational approaches to investigate potential benefits arising from division of labor during biofilm matrix production. We show that both phenotypic and genetic strategies for a division of labor can promote collective biofilm formation in the soil bacterium Bacillus subtilis. In this species, biofilm matrix consists of two major components, exopolysaccharides (EPSs) and TasA. We observed that clonal groups of B. subtilis phenotypically segregate into three subpopulations composed of matrix non-producers, EPS producers, and generalists, which produce both EPSs and TasA. This incomplete phenotypic specialization was outperformed by a genetic division of labor, where two mutants, engineered as specialists, complemented each other by exchanging EPSs and TasA. The relative fitness of the two mutants displayed a negative frequency dependence both in vitro and on plant roots, with strain frequency reaching a stable equilibrium at 30% TasA producers, corresponding exactly to the population composition where group productivity is maximized. Using individual-based modeling, we show that asymmetries in strain ratio can arise due to differences in the relative benefits that matrix compounds generate for the collective and that genetic division of labor can be favored when it breaks metabolic constraints associated with the simultaneous production of two matrix components.

Dual functionality of the TasA amyloid protein inBacillusphysiology and fitness on the phylloplane

“…To analyze the presence of TasA in the different strains, biofilms were fractionated into cells and ECM as described elsewhere 96 , and both fractions were analyzed separately. Briefly, 72-hour-old colonies grown under biofilm-inducing conditions on MSgg-agar plates were carefully lifted from the plates and resuspended in 10 ml of MS medium (MSgg broth without glycerol and glutamate, which were replaced by water) with a 25 5 /8 G needle. Next, the samples were subjected to mild sonication in a Branson 450 digital sonifier (4-5 5 seconds pulses at 20% amplitude) to ensure bacterial resuspension.…”

“…All of the assays were performed in duplicate. The collected cells were resuspended and homogenized via passage through a 25 5 /8 G needle in BirnBoim A 101 buffer (20% sucrose, 10 mM Tris-HCl pH 8, 10 mM EDTA and 50 mM NaCl). Lysozyme (10 mg/ml) was added, and the mixture was incubated for 30 minutes at 37 °C.…”

“…Cells were grown on MSgg agar at 30 °C. At different time points, colonies were recovered in 500 μL of PBS and resuspended with a 25 5 /8 G needle. For the promoter expression assays, colonies were gently sonicated as described above to ensure complete resuspension, and the cells were fixed in 4% paraformaldehyde in PBS and washed three times in PBS.…”

“…In response to a wide range of environmental factors 1, 2 , some bacterial species establish complex communities called biofilms 3 . To do so, planktonic cells initiate a transition into a sedentary lifestyle and trigger a cell differentiation program that leads to two distinctive features: (1) a division of labor, in which different subpopulations of cells are dedicated to covering different processes needed to maintain the viability of the community 4, 5 , and (2) the secretion of a battery of molecules that assemble the extracellular matrix (ECM) 3, 6 . Like in eukaryotic tissues, the bacterial ECM is a dynamic structure that supports cellular adhesion and regulates the flux of signals to ensure cell differentiation, both of which are key ECM functions in biofilms 7, 8 .…”

Differential equation based minimal model describing metabolic oscillations inBacillus subtilisbiofilms

“…Biofilms are thus complex communities of bacteria and as such, many types of social dynamics come into play. One of these is the division of labour 1,2 . The core of the biofilm growing on a solid surface shows a different metabolic state than the periphery.…”