biofilm metabolism*

Bacteria are colorful, both literally and figuratively. And some change color depending on what is present in their environment. Take the case of Pseudomonas aeruginosa, whose cultures toggle between blue-green and clear. The name “aeruginosa” derives from the Latin word for copper rust, which is of the same blue-green hue. While microbiologists and clinicians have long used color to identify the organism, why it is colored in the first place—and why its color changes with aeration—is a question that not many have considered. We now know that phenazines, a class of redox-active pigments, are responsible not only for the blue-green color of P. aeruginosa in the presence of oxygen, but also for different colors displayed by other Pseudomonas species. In the early 20^th century, Ernst Friedheim and colleagues postulated that phenazines are “accessory respiratory pigments” that sustain bacterial “respiration” based on their ability to stimulate oxygen consumption. Their work was carried out before respiratory pathways were fully understood and well before the importance of microbial biofilms in nature and disease was widely recognized. In the interval between Friedheim’s pioneering studies and our recent work, attention shifted to exploring the roles of phenazines as virulence factors. Phenazines came to be known as "secondary metabolites", molecules produced at late stages of microbial growth in laboratory cultures whose function was thought to be to protect Pseudomonas from competitors. While the antibiotic activity of phenazines has been elegantly shown in a variety of contexts, labeling phenazines as “secondary metabolites” suggests that they are not essential to the growth or survival of their producers. In this talk, I will champion Friedheim’s original hypothesis and extend it. Specifically, I will discuss our deepening understanding of how phenazines are vital to the metabolic survival of cells in biofilms that are oxidant-limited.