Fig. 4.
From: Competition for nutritional resources masks the true frequency of bacterial mutants
a A single PCM (blue bacterium) produces glycerol molecules that diffuse out and are captured by the surrounding wild-type cells. The concentration of glycerol in the PCM vicinity is insufficient to enable the growth of the mutant or the growth of the neighboring wild-type cells, leading to the collapse of the population. 
, glycerol molecules produced by the PCM. b A microcluster of two PCMs mutually feeding each other. Under these conditions, the concentration of glycerol available to the PCMs increases. Glycerol freely moves between PCM cells, being thus sufficient to foment the growth of a colony