Metabolite toxicity as a driver of bacterial metabolite externalization

ABSTRACT Some microbes externalize costly biosynthetic precursors in sufficient quantities to sustain a recipient population through cross-feeding. However, it is unclear whether metabolites are externalized purely for a reciprocal benefit or if metabolite externalization also plays a physiological role for the producer. Here we focus on adenine, a metabolite externalized by some strains of the phototrophic bacterium Rhodopseudomonas palustris at sufficient levels to support Escherichia coli growth. In 10 long-term monocultures and 22 cocultures pairing R. palustris with E. coli , extracellular adenine externalized by all 140 isolates screened was 1.7 – 3.4-fold higher than that by the ancestor, suggesting that there was selective pressure for adenine externalization. We hypothesized that adenine is toxic to R. palustris . The CGA0092 growth rate decreased by half in the presence of about 0.3 mM external adenine. This inhibitory effect increased by an order of magnitude when we over-expressed adenine phosphoribosyltransferase to overcome a bottleneck in the purine salvage pathway, suggesting that toxicity stems from a metabolite derived from adenine. To assess whether adenine tolerance is connected to adenine externalization, we surveyed 12 evolved isolates and 49 environmental strains that externalized different levels of adenine, revealing a significant positive correlation. Our data suggests a physiological role for externalization of costly-metabolites like adenine at the origin of cross-feeding. In addition to cross-feeding, resulting metabolic interactions could be negative, considering that even a biosynthetic precursor like adenine can be inhibitory.