What Are Phenazines and What Do They Do?

Phenazines are small, colorful, nitrogen-containing molecules made primarily by bacteria, and they do a remarkable number of things. Over 150 natural phenazines have been identified, and their ability to shuttle electrons between different chemical partners underpins everything from helping bacteria breathe in oxygen-starved biofilms to killing competing fungi in soil to worsening lung infections in humans.¹ Despite being discovered in the mid-nineteenth century, researchers have only recently pieced together the molecular machinery behind their production and the full scope of their ecological influence.

The Basic Chemistry Behind Their Versatility

At their core, phenazines share a flat, three-ringed aromatic scaffold that contains two nitrogen atoms. That structure lets them accept electrons from one source, hold onto them briefly, and then pass them to something else. This electron-shuttling ability is the single trait that explains almost everything phenazines do. In reduced form, a phenazine can hand electrons to iron minerals, to oxygen, or to other molecules; in oxidized form, it can pull electrons away from cellular machinery. Different side groups attached to the core scaffold shift the molecule’s properties in meaningful ways, changing how eagerly it picks up or releases electrons, how quickly it reacts with oxygen versus iron, and which environments it favors.

A good example of how these chemical differences matter: researchers tested four natural phenazines against two forms of iron oxide at varying pH and found that each one behaved differently. Phenazines with a lower reduction potential generally dissolved iron faster, and the reaction slowed as pH rose. But pyocyanin, the best-known phenazine, broke the pattern. It was sluggish with iron at neutral pH yet reacted with oxygen faster than any of the others.² The implication is that different phenazines are tuned for different jobs, and bacteria that make multiple phenazines have a toolkit rather than a single tool.

Who Makes Them

The best-studied phenazine producers belong to the genus Pseudomonas, especially Pseudomonas aeruginosa (the species responsible for serious lung infections) and Pseudomonas chlororaphis (a soil bacterium used in agricultural biocontrol). But phenazine production extends far beyond one genus. A large genomic survey assembled 181 phenazine-producer genomes spanning 15 different bacterial and archaeal orders, with roughly equal representation from proteobacteria and actinobacteria.³ The phylogenetic tree of phenazine-making genes does not match the family tree of the organisms themselves, meaning phenazine production has been passed sideways between unrelated species through horizontal gene transfer multiple times over evolutionary history.

This scattered distribution hints at something important: the ability to make phenazines is valuable enough that bacteria in very different ecological niches have acquired or retained it. The producers include free-living soil bacteria, plant root colonizers, marine vibrios, and even some archaea.

How Bacteria Build Them

All known phenazine biosynthetic pathways funnel through the same starting compound: phenazine-1-carboxylic acid, or PCA. A core set of biosynthetic genes (typically labeled phzA through phzG) converts a precursor from the shikimate pathway into PCA. In P. aeruginosa, each of two copies of this gene cluster is sufficient on its own to produce PCA.⁴

From PCA, additional “decorating” enzymes tack on various chemical groups to create the full diversity of natural phenazines. In P. aeruginosa, a methyltransferase called PhzM and an oxygenase called PhzS work together to convert PCA into pyocyanin, the blue-green pigment that gives infected wound dressings their characteristic color. A separate enzyme, PhzH, converts PCA into phenazine-1-carboxamide instead.⁴ Other species apply hydroxyl groups, carboxyl groups, or other modifications, each tuning the molecule for particular redox behavior or environmental conditions. The result is a family of more than 150 structurally related but functionally distinct compounds.

Keeping the Lights On Inside Oxygen-Starved Cells

One of the most surprising discoveries about phenazines is that they are not just weapons. They are part of the basic metabolic plumbing of the bacteria that produce them. When a bacterium like P. aeruginosa runs low on oxygen or other terminal electron acceptors, its internal pool of NADH (the molecule that carries electrons through metabolism) starts to pile up with nowhere to dump its electrons. This metabolic backup can stall growth. Phenazines provide a relief valve. A mutant strain of P. aeruginosa that cannot make phenazines accumulates more NADH in stationary phase than the wild type; adding pyocyanin back to the mutant brings NADH levels back down.⁵ In other words, phenazines allow cells to keep their electron-producing metabolism running smoothly even when oxygen is scarce.

This is not a minor side function. Phenazines have been described as respiratory pigments that extend into primary metabolism, not merely secondary metabolites produced for defense.¹ They act as electron shuttles to alternative terminal acceptors, modify the cell’s internal redox balance, regulate gene expression, and enhance bacterial survival.⁶

Breathing at a Distance in Biofilms

Bacteria living in thick biofilms face a fundamental problem: cells deep inside the community are too far from the surface to access oxygen directly. Phenazines solve this with a trick called extracellular electron transfer. A cell deep in the biofilm reduces a phenazine molecule, passing its excess electrons to it. That reduced phenazine then diffuses outward through the biofilm matrix until it encounters oxygen or another oxidant at the surface, where it dumps its electrons and gets re-oxidized. The oxidized phenazine then diffuses back inward to pick up more electrons. The molecule effectively acts as a tiny, self-propelled wire.

This process has real consequences for biofilm shape and behavior. In P. aeruginosa, phenazines interact with extracellular DNA in the biofilm matrix, and this interaction supports a rapid and efficient redox cycle.⁷ When researchers used an electrochemical setup to oxidize phenazines at a distal electrode, mimicking what happens when phenazines reach an oxidant at the biofilm’s edge, it suppressed the characteristic wrinkle formation in P. aeruginosa biofilms.⁸ The wrinkles appear to be a physical response to metabolic stress: when cells deep in the biofilm cannot offload electrons, they push upward to get closer to oxygen. Phenazine-mediated electron transfer reduces that stress, so the biofilm stays flatter.

More broadly, phenazine production influences swarming motility and the overall surface-to-volume ratio of mature biofilms, though the details vary between species.⁹ In P. chlororaphis, phenazines are actually required for normal biofilm formation on surfaces, including plant roots and seeds. Mutants unable to make phenazines are significantly impaired in establishing surface-attached communities, and neither simply restoring the chemical signaling molecules nor growing near wild-type bacteria can fully compensate unless phenazines themselves are present.¹⁰

Weapons Against Competitors

The antibiotic activity of phenazines was noticed long before anyone understood their metabolic roles. Because phenazines cycle between oxidized and reduced states inside a target cell, they generate reactive oxygen species as a byproduct. This oxidative barrage can damage DNA, deplete protective antioxidants, and kill or suppress competing microbes. Against the fungal pathogen Candida albicans, a phenazine called 5-methylphenazinium-1-carboxylate penetrates the cells, gets reduced by NADH, and then spontaneously re-oxidizes in the presence of oxygen, generating reactive oxygen species that precede fungal death.¹¹

A survey of over 100 bacterial strains isolated from a dryland wheat field found that sensitivity to PCA varies dramatically. Gram-positive bacteria tend to be more susceptible than Gram-negative ones, and there was significant variability even within the same phylum. Resistance was more common among bacteria isolated from the wheat rhizosphere, where PCA-producing bacteria were also abundant, suggesting that coexistence with producers selects for resistance over time. Toxicity was pH-dependent for most susceptible strains and broadly correlated with how rapidly the organisms reduced PCA, pointing to redox cycling as the key mechanism of harm.¹²

The Dark Side in Human Infections

When the producer is P. aeruginosa and the environment is a human lung, the same redox cycling that kills fungi becomes a virulence mechanism against host tissue. Pyocyanin has been recognized as an important virulence factor with effects across the respiratory, cardiovascular, urological, and central nervous systems, and a large share of the damage comes from the reactive oxygen species it generates.¹³

The consequences are especially serious in people with cystic fibrosis. In human airway cells, exposure to pyocyanin tripled the rate of hydrogen peroxide release, oxidized the cytoplasm, and inhibited a critical chloride-secretion channel (CFTR) by about 86%. The decline in chloride transport was directly proportional to how oxidized the cell interior became. Pyocyanin also depleted total cellular glutathione to about 62% and ATP to about 46% within 24 hours. Airway cells carrying the most common cystic fibrosis mutation (ΔF508) lost chloride secretion entirely when exposed to pyocyanin, confirming that the oxidant specifically targets the already-compromised chloride channel.¹⁴ This helps explain why chronic P. aeruginosa infections are so destructive in cystic fibrosis lungs: the pathogen’s phenazines actively undermine the airway’s already-fragile defense mechanisms.

Protecting Crops as Biological Control Agents

The same antifungal punch that makes phenazines dangerous in clinical settings makes them useful in agriculture. Pseudomonas chlororaphis 30-84, a well-studied biocontrol strain, produces three phenazines: PCA, 2-hydroxy-PCA, and a small amount of 2-hydroxyphenazine. These compounds are required both for inhibiting fungal pathogens and for competing successfully on wheat roots. When researchers engineered the strain to produce different combinations of phenazines, the spectrum of fungi it could inhibit changed, and so did the degree of disease suppression against take-all, a devastating wheat root disease.¹⁵

A broader review of phenazine-producing biocontrol strains found that isolated natural phenazines sometimes matched or even outperformed commercial fungicides in laboratory assays. In practice, though, most phenazines except PCA were secreted below their minimum inhibitory concentrations, and in-field biocontrol performance was generally modest. The best results came from PCA-producing strains of P. chlororaphis and P. fluorescens.¹⁶ The gap between lab and field performance is a familiar challenge in biocontrol research, and it underscores that simply killing a pathogen in a petri dish does not guarantee a strain will thrive, persist, and protect a root system in actual soil.

Reshaping Soil Chemistry Around Plant Roots

Beyond killing pathogens, phenazines alter the geochemistry of the soil directly. Because reduced phenazines can donate electrons to iron and manganese minerals, phenazine-producing bacteria effectively dissolve these minerals, converting them from solid, largely inert forms into more reactive or mobile ones. In laboratory cultures, P. chlororaphis reductively dissolved poorly crystalline iron and manganese oxides, but a mutant unable to make phenazines could not; adding purified phenazine back to the mutant restored the ability. The amount of iron reduced far exceeded the amount of phenazine produced, confirming that each phenazine molecule was recycled many times as an electron shuttle rather than consumed in a one-off reaction.¹⁷

In field-relevant conditions, inoculating wheat roots with a PCA-producing strain increased the concentration of poorly crystalline iron in the rhizosphere by roughly 30 to 40% compared to a PCA-deficient mutant. Interestingly, total iron uptake into the plant did not change significantly, but the PCA-producing strain altered how iron was distributed within the plant, shifting translocation into shoots.¹⁸ The takeaway is that phenazines reshape the mineral landscape around roots in ways that could matter for plant nutrition and soil health, even when the effects do not show up as simple increases in total nutrient uptake.

How Phenazines Get Broken Down

Phenazines are not permanent fixtures in soil or biofilms. Other microorganisms actively degrade them, and some even use them as a food source. Researchers identified several mycobacterial species that can break down PCA, phenazine-1-carboxamide, and pyocyanin. PCA served as the sole carbon source for these organisms, fueling their growth. Deleting certain genes in Mycobacterium fortuitum abolished the ability to degrade phenazines, and expressing just two of those genes in a different species was enough to confer degradation of both phenazine-1-carboxamide and pyocyanin. The degradation also protected otherwise sensitive bacteria from pyocyanin toxicity, suggesting that phenazine-degrading organisms can provide a form of community-level detoxification.¹² An earlier study had independently identified a Sphingomonas strain capable of completely degrading PCA in about 40 hours, using it as its sole carbon and nitrogen source.¹⁹

The existence of dedicated phenazine degraders matters for understanding phenazine ecology. It means the impact of these molecules in soil or in an infection depends not just on how much is produced but on how fast nearby organisms are breaking it down. In the wheat rhizosphere, the genomic survey mentioned earlier found that phenazine biodegradation genes were also broadly distributed, pointing to an ongoing arms race between phenazine producers and consumers.³

Technology Applications

The electron-shuttling ability that makes phenazines biologically versatile has attracted engineers, too. In microbial fuel cells, where electricity is generated by bacteria oxidizing organic matter and transferring electrons to an electrode, the rate of electron transfer to the anode is a major performance bottleneck. Researchers found that adding phenazine-producing Pseudomonas strains to a mixed-culture fuel cell could double the electric current output. Purified PCA and phenazine-1-carboxamide had similar effects, confirming that the phenazines themselves, not some other product of the bacteria, were responsible. Using an engineered strain that overproduced phenazine-1-carboxamide sustained the improvement over longer periods and increased total charge production by about 50%.²⁰

Because each phenazine has a distinct electrochemical signature, phenazines can also serve as a diagnostic fingerprint for detecting P. aeruginosa in clinical or environmental samples. Transparent carbon ultramicroelectrode arrays have been used to detect pyocyanin and 5-methylphenazine-1-carboxylic acid in real time from growing cultures, tracking not just how much of each phenazine is present but how their production changes over time.²¹ A separate approach using cellulose-based laser-induced graphene electrodes tracked phenazine release from both planktonic and surface-attached bacterial cultures, revealing marked differences in production dynamics depending on how the bacteria were growing.²² These kinds of rapid, real-time detection systems could eventually speed up clinical identification of P. aeruginosa infections, where early diagnosis improves outcomes.

Synthetic Phenazines as Drug Candidates

The broad antimicrobial activity of natural phenazines has spurred chemists to design synthetic versions optimized for medical use. A series of phenazine sulfonamides built around the natural scaffold showed promising activity against methicillin-resistant Staphylococcus aureus (MRSA), S. epidermidis, and Neisseria gonorrhoeae. One lead compound, designated 7i, was active in the low micromolar range against MRSA and S. epidermidis, boosted the effectiveness of existing antibiotics like vancomycin and linezolid, and powerfully inhibited biofilm formation by suppressing key virulence genes. Crucially, it showed no toxicity to mammalian cells, no hemolysis, and no mutagenic activity.²³

An earlier medicinal chemistry effort had also identified synthetic phenazine derivatives with meaningful activity against both MRSA and E. coli, including one compound that demonstrated significant biofilm dispersion activity against S. aureus.²⁴ The interest in phenazine-based drug design reflects a broader trend: as resistance to conventional antibiotics grows, researchers are increasingly returning to naturally evolved antimicrobial scaffolds, hoping that the structural features bacteria have used against each other for millions of years can be repurposed against human pathogens. These efforts are still in early-stage development, with no phenazine-based antibiotic yet in clinical trials, but the safety profiles and mechanism of action look encouraging enough to keep the chemistry moving forward.

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