Cyanobacteria are a diverse group of photosynthetic bacteria that exhibit phototaxis, or motion in response to light. Cyanobacteria such as Synechocystis sp. secrete a mixture of complex polysaccharides that facilitate cell motion, while their type 4 pili allow them to physically attach to each other. Even though cells can respond individually to light, colonies are observed to move collectively towards the light source in dense finger-like projections. We present an agent-based model for cyanobacterial phototaxis that accounts for slime deposition as well as for direct physical links between bacteria, mediated through their type 4 pili. We reproduce the experimentally observed aggregation of cells at the colony boundary as a precursor to finger formation. Our model also describes the changes in colony morphology that occur when the location of the light source is abruptly changed. We find that the overall motion of cells toward light remains relatively unimpaired even if a fraction of them do not sense light, allowing heterogeneous populations to continue to mount a robust collective response to stimuli. Our work suggests that in addition to bio-chemical signalling via diffusible molecules in the context of bacterial quorum-sensing, short-ranged physical interactions may also contribute to collective effects in bacterial motility.

Cells in microbial colonies integrate information across multiple spatial and temporal scales while sensing environmental cues. A number of photosynthetic cyanobacteria respond in a directional manner to incident light, resulting in the phototaxis of individual cells. Colonies of such bacteria exhibit large-scale changes in morphology, arising from cell-cell interactions, during phototaxis. These interactions occur through type IV pili-mediated physical contacts between cells, as well as through the secretion of complex polysaccharides ('slime') that facilitates cell motion. Here, we describe a computational model for such collective behaviour in colonies of the cyanobacterium Synechocystis. The model is designed to replicate observations from recent experiments on the emergent response of the colonies to varied light regimes. It predicts the complex colony morphologies that arise as a result. We ask if changes in colony morphology during phototaxis can be used to infer if cells integrate information from multiple light sources simultaneously, or respond to these light sources separately at each instant of time. We find that these two scenarios cannot be distinguished from the shapes of colonies alone. However, we show that tracking the trajectories of individual cyanobacteria provides a way of determining their mode of response. Our model allows us to address the emergent nature of this class of collective bacterial motion, linking individual cell response to the dynamics of colony shape.

Cyanobacteria rely on photosynthesis and, thus, have evolved complex responses to light. These include phototaxis, the ability of cells to detect and respond to the direction, wavelength, and intensity of light.

13Cyanobacteria are a widely distributed, diverse group of photosynthetic bacteria that exhibit 14 phototaxis, or motion in response to light. Cyanobacteria such as Synechocystis sp. secrete 15 a mixture of complex polysaccharides that facilitate cell motion, while their type 4 pili allow 16 them to physically attach to each other. Even though cells can respond individually to light, 17 colonies of such bacteria are observed to move collectively towards the light source in dense 18 finger-like projections. Agent-based models are especially useful in connecting individual cell 19 behaviour with the emergent collective phenomena that arise out of their interactions. We 20 present an agent-based model for cyanobacterial phototaxis that accounts for slime deposition 21 as well as for direct physical links between bacteria, mediated through their type 4 pili. 22 We reproduce the experimentally observed aggregation of cells at the colony boundary as a 23 precursor to finger formation. Our model also describes the changes in colony morphology 24 that occur when the location of the light source is abruptly changed. We find that the overall 25 motion of cells toward light remains relatively unimpaired even if a fraction of them do 26 not sense light, allowing heterogeneous populations to continue to mount a robust collective 27 response to stimuli. Our work suggests that in addition to bio-chemical signalling via diffusible 28 molecules in the context of bacterial quorum-sensing, short-ranged physical interactions may 29 also contribute to collective effects in bacterial motility.

No abstract