Mutual sensing of F and E. coli colonies. a At time 0, both partners were planted simultaneously at two different distances. Negative values: F planted to E. coli colonies one (−1) or two days old (−2). Positive value: E. coli planted to F colonies 2 and 6 days old (note the different magnification at lower left; arrow shows rudiment of E. coli). 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).