Seminar by Gabriele Micali

Bacteria in nature often live in complex environments where the availability of resources fluctuates unpredictably. When resource availability changes, bacteria may face a `catch-22' situation. They will need to express de novo a set of genes to cope with the new resource. However, initiaion of gene expression may be limited by the supply of gene products of the aforementioned genes. Gene expression noise, where certain isogenic individuals can exhbit differential expression of genes before an environmenal fluctuation can solve this paradox. In addition, bacteria often live in communities where functions are distributed in different genotypes and hence, to cope with environmental fluctuations they may relay not only with the expression of their own genes but also with sharing resources with other partners. The `catch-22' problem may be even more severe in these communities. 

Here, we focus on amino acid production to examine a version of this 'catch-22' problem. We quantify single-cell growth dynamics of E. coli cells as they transition from high to no amino acid media using microfluidic devices. We compare the responses of an isogenic population which can synthesize all amino acids with the responses of a consortium of two coexisting genotypes that in the absence of amino acids need to share resources due to amino acid auxotrophy. 

We found that the auxotrophic consortium takes longer to resume growth after the switch when compared to the community able to synthesize amino acids. However, close spatial proximity of members of the consortia to the other genotype reduces the lag. We conclude that local spatial organization is the main driver of growth resumption in our spatially structured system.