Mutual sensing of R and E. coli colonies. a At time 0, both partners were planted simultaneously 5 or 15 mm apart. Negative value: R planted to E. coli colony one day old. Positive value: E. coli planted to R colony 1 and 2 days old. Day 10 after planting E. coli. Micro-photographs taken from areas indicated. b Interaction on MMA, planting distance 3 mm, dashed line delineates the contours of both colonies. (Day 7).