Fig. 8: Acid-induced cross-feeding between soil bacteria growing on GlcNAc.

Monoculture of Citrobacter freundii (Cf) or co-culture of Cf and Pseudomonas fluorescens (Pf) was grown in 10 mM GlcNAc as the sole carbon and nitrogen sources in weak phosphate-based buffer and with shaking; see “Methods”. a–c show data from monoculture growth of Cf with (a) OD and pH, (b) consumption of GlcNAc and excretion of acetate, and (c) excretion of pyruvate. d–g show data from 24-h growth-dilution cycles of Cf+Pf co-culture in the same medium with 100× dilution. The co-culture was started from a 1:1 mixture of exponentially growing cells of each species, each at an OD₆₀₀ of 0.01. d Growth curve of the co-culture over the first cycle is shown as the solid black line. Growth curve of Cf monoculture (dashed gray line, same as the black curve in (a)) is shown for comparison. e The viable counts of Cf and Pf cells at the end of each cycle, with Cf and Pf colonies distinguished by their sizes. The data from six biological replicates are shown as filled black circles for each cycle. f pH dynamics was monitored continuously throughout the first four cycles; see Supplementary Fig. 12. The horizontal dashed black line indicates the pH below which acid-induced excretion was observed for the Cf monoculture (shown as vertical dotted lines in (a)–(c)). g Metabolites measured in the medium at selected times during Cycle 3. Purple triangles indicate the concentration of pyruvate and the red squares indicate the concentration of acetate.