Bacterial contact inhibition: single cell to microcolony level analysis using experimental and in silico approaches.

Bacteria have evolved a diverse range of strategies for survival. To colonise a niche and source nutrients, cooperation and antagonism can both prove advantageous. Contact dependent toxin insertion is one way that bacteria are able to antagonistically manipulate growth of other bacteria in close proximity, but the effect of different contact inhibition systems on the spatial structure of bacterial populations has not been explored. The type VI secretion system (T6SS) and contact-dependent inhibition (CDI) are two contact inhibition systems with differing potency; T6SS is highly toxic whereas CDI has a subtler effect on susceptible target cells. Here I show, with an interdisciplinary blend of microscopy and computational modelling, that the potency of the system determines structure in binary competitions. Tracking single cells during competition with phase contrast microscopy shows that both the rate of inhibition and the toxicity of the system (measured by the extent of growth rate reduction of target cells) defines the system’s potency. Simulations exploring the range of these two inhibition potency parameters show continuous change in the structure of microcolonies, from intermixing at low potency, to reduced interaction border and enforcement of clustering at high potency. A T6SS effector mutant with reduced inhibition rate was used to validate simulation predictions, using the fractal dimension as a statistical measure of microcolony structure. Combining empirical and in silico data with spatial statistics has allowed identification of fine scale structural changes imposed by incremental changes in inhibition potency of contact inhibition systems. The methods developed in this analysis can be used to further investigate the diverse range of toxins from these two contact inhibition systems and how other interbacterial interactions affect bacterial population structure. A better understanding of factors affecting population structure is important to understand how natural multispecies populations establish and maintain themselves