Starting a bacterial colony in a new environment often leads to a lag time before growth starts. This duration of this lag phase decreases with the size of the initial cell population, an effect known in microbiology as the “inoculum effect”, which has been reported over 100 years ago but is still poorly understood. Possible explanations range from statistical sampling to collective population effects.
We used a millifluidic droplet device which enables to monitor the growth dynamics of hundreds of populations founded by controlled numbers of P fluorescens cells. The decrease of average lag time and its variance across droplets, as well as lag time distribution and its scaling properties, follow the predictions of extreme value theory. The timescales and population sizes in the experiment indicate that exit from lag phase depends on interactions among cells, consistent with a leader cell signalling and triggering the end of lag phase for the entire population.