The idea would be to have two types of E. coli :
One type would express GFP (they look green)
The other type would express RFP (they look red)
The starting material is an unorganized mixture of red and green bacteria
The GFP expressing bacteria would start, under induction, to express Antigen 43 (Ag43)which is an aggregation factor working on stable media culture.
The "green" bacteria would start to gather making green dots on the plates.
The "red" bacteria express, with a certain delay(long enough to let the green ones to cluster first), the aggregation factor (Ag43).
This would produce a sorted phenotype (green bacteria surrounded by red bacteria)