Antibiotic efficacy can be antagonized by bioactive metabolites and other drugs present at infection sites. Pseudomonas aeruginosa, a common cause of biofilm-based infections, releases metabolites called phenazines that accept electrons to support cellular redox balancing. Here, we find that phenazines promote tolerance to clinically relevant antibiotics, such as ciprofloxacin, in P. aeruginosa biofilms and that this effect depends on the carbon source provided for growth. We couple stable isotope labeling with stimulated Raman scattering microscopy to visualize biofilm metabolic activity in situ. This approach shows that phenazines promote metabolism in microaerobic biofilm regions and influence metabolic responses to ciprofloxacin treatment. Consistent with roles of specific respiratory complexes in supporting phenazine utilization in biofilms, phenazine-dependent survival on ciprofloxacin is diminished in mutants lacking these enzymes. Our work introduces a technique for the chemical imaging of biosynthetic activity in biofilms and highlights complex interactions between bacterial products, their effects on biofilm metabolism, and the antibiotics we use to treat infections.
Bacteria typically rely on secreted metabolites, potentially shareable at the community level, to scavenge resources from the environment. The evolution of diffusible, shareable metabolites is, however, difficult to explain because molecules can get lost, or be exploited by cheating mutants. A key question is whether natural selection can act on molecule structure to control loss and shareability. We tested this possibility by collating information on diffusivity properties of 189 secreted iron-scavenging siderophores and the natural habitats occupied by the siderophore-producing species. In line with evolutionary theory, we found that highly diffusible siderophores have preferentially evolved in species living in structured habitats, such as soil and hosts, because structuring can keep producers and their shareable goods together. Poorly diffusible siderophores, meanwhile, have preferentially evolved in species living in unstructured habitats, such as seawater, indicating that these metabolites are less shareable and more likely provide direct benefits to the producers.
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