Fig. 3.

Cross-protection of susceptible cells by resistant cells is supported by the degree of cell filamentation. Confocal images of the colony edge after 2 days show carbenicillin causes susceptible cells to filament, when grown alone on increasing concentrations of antibiotic (a, moving horizontally across the first row, both red and yellow cells are susceptible). The presence of the resistant strain causes susceptible cells to filament less (b, moving across second row, susceptible cells in yellow, resistant cells in red) than when they are grown alone at the same concentration of antibiotic (a, top row). At intermediate concentrations of carbenicillin there is enrichment at the edge for the susceptible strain when in competition with the resistant strain (b, centre panels, susceptible strain in yellow, resistant strain in red). When the resistant cells are grown alone on the antibiotic (c, both colours are resistant) they do not filament at these concentrations of carbenicillin. The MIC (minimum inhibitory concentration) is the lowest concentration of antibiotic that completely inhibits growth of susceptible cells (here measured in a colony (Colour figure online)) (Supplementary Table S1)