Methylglyoxal is a small, electrophilic ketoaldehyde produced as a byproduct of carbohydrate metabolism. In many organisms it arises when the flow of sugar phosphates through central metabolism becomes imbalanced, particularly when dihydroxyacetone phosphate accumulates. The enzyme methylglyoxal synthase, encoded by the mgsA gene, channels this intermediate into methylglyoxal rather than allowing it to continue down the glycolytic route. Once formed, methylglyoxal attacks nucleophilic side chains on amino acids, most notably lysine, arginine, and cysteine residues, forming covalent adducts known as advanced glycation end products. These modifications can distort protein structure, interfere with enzyme active sites, and disrupt protein-protein interactions. Because of this destructive chemistry, the accumulation of methylglyoxal has been described as carbonyl stress, a condition generally assumed to be harmful or outright lethal to microbial cells.

The research team set out to determine whether this pathway even operates in spirochetes, a group of spiral-shaped bacteria with distinctive genetics and metabolism. B. burgdorferi has an unusually reduced genome and depends almost entirely on its tick and mammalian hosts for nutrients, so the question of how it manages carbohydrate and phosphate balance is biologically important. The investigators identified several environmental signals that trigger carbonyl stress in the Lyme disease spirochete, including glycerol and peptides. Glycerol is particularly interesting because B. burgdorferi can use it as a carbon source during its tick phase, when blood is unavailable and the bacterium must survive on alternative substrates. Peptides, meanwhile, reflect the amino acid-rich environment the spirochete encounters in mammalian tissues and blood. The finding that both classes of nutrients feed into the carbonyl stress pathway suggests that methylglyoxal production is tied to the specific metabolic states the bacterium passes through during its enzootic cycle between tick and host.

Central to the study was the demonstration that methylglyoxal production and the resulting glycation of proteins depend on the product of the mgsA gene, designated bb0364 in the B. burgdorferi genome. When this gene’s function was removed, the spirochete no longer produced detectable methylglyoxal, and the characteristic protein adducts of advanced glycation disappeared. This genetic dependency establishes that the pathway is not an incidental chemical side reaction but a controlled enzymatic process. The researchers could therefore treat mgsA as a molecular switch, turning carbonyl stress on or off and observing the consequences for the bacterium’s physiology and its ability to infect animals.

One of the most consequential discoveries was phylogenetic. The carbonyl stress pathway proved to be active in Lyme disease spirochetes but absent in function from relapsing fever Borrelia species and from B. mayonii, another human pathogen in the Lyme disease group. Relapsing fever spirochetes are famous for achieving enormous densities in the bloodstream, cycling through antigenic variation to evade antibodies and producing recurrent febrile episodes. The fact that these blood-dwelling relatives lack an active methylglyoxal pathway, while the tissue-adapted Lyme disease spirochete retains one, hints at an evolutionary trade-off. The authors suggest that carbonyl stress may be selected against in spirochetes that reach high numbers in the blood, where the metabolic consequences of glycation could become intolerable at large population sizes.

To test whether methylglyoxal production matters during actual infection, the team turned to the tick-mouse model of Lyme disease, the standard experimental system for studying B. burgdorferi in its natural transmission context. Ticks infected with spirochetes were allowed to feed on mice, and the course of infection was followed through the mammalian host. The results provided evidence that the production of methylglyoxal and the glycation of proteins are not necessarily lethal to the spirochete, at least in this organism, contradicting the long-standing assumption that carbonyl stress is uniformly toxic. Bacteria carrying an intact pathway persisted through the infection cycle, indicating that B. burgdorferi can tolerate a level of self-inflicted protein damage that would cripple many other microbes. This tolerance raises the possibility that the pathway confers some benefit that offsets its biochemical costs.

The clearest hint of that benefit came from an elegant cross-species experiment. The researchers introduced the B. burgdorferi mgsA gene and its associated carbonyl stress pathway into B. hermsii, a relapsing fever spirochete that naturally lacks the system. When the engineered bacteria were used to infect mice, the peak level of spirochetemia during the relapse phase of infection dropped significantly compared with infections caused by unmodified B. hermsii. In other words, forcing a blood-dwelling spirochete to run the Lyme disease carbonyl stress pathway curtailed its ability to flood the bloodstream. This result supports the idea that methylglyoxal production imposes a cost on explosive growth in blood, and it explains why relapsing fever species, which depend on high-density blood infections for transmission, would have lost or never maintained this pathway.

Taken together, these findings define a previously unrecognized post-translational modification system in B. burgdorferi. Rather than relying solely on the phosphorylation, acetylation, and proteolytic processing that bacteria typically use to tune protein function, the Lyme disease spirochete appears to exploit direct chemical glycation by methylglyoxal as a regulatory layer. Because the pathway is triggered by glycerol and peptides, nutrients encountered at particular stages of the tick-mouse cycle, methylglyoxal-mediated modification could help the bacterium reprogram its proteome as it transitions between metabolically distinct environments. Proteins damaged by glycation might be preferentially degraded, redirected, or inactivated, allowing the spirochete to adjust its physiology in ways that standard transcriptional regulation alone cannot achieve.

The study also reframes how scientists think about carbonyl stress in microbial pathogenesis. For decades, methylglyoxal has been studied mainly as a threat that bacteria must detoxify through glyoxalase systems and related enzymes. The new work shows that at least one pathogen has integrated this reactive metabolite into its normal biology, tolerating the damage it causes and possibly using it to govern its lifestyle. The contrast between Lyme disease and relapsing fever spirochetes offers a natural experiment in evolutionary strategy: one pathogen persists at low densities in collagen-rich tissues for months or years, while the other races through the blood in massive waves. The presence of an active carbonyl stress pathway in the former and its apparent incompatibility with the latter suggests that methylglyoxal biology is entwined with the fundamental ecological strategies of these bacteria.

Future work will need to identify which B. burgdorferi proteins are glycated in vivo, how those modifications alter protein function, and whether the pathway influences the bacterium’s ability to establish persistence in joints, heart, and nervous system tissue. The findings also raise questions about whether carbonyl stress contributes to the chronic inflammatory environment characteristic of Lyme disease, since advanced glycation end products are known to interact with host immune receptors in other contexts. For now, the study stands as a vivid example of how a molecule once dismissed as metabolic waste can turn out to be a purposeful instrument of pathogen regulation, and it underscores how much remains to be learned from comparing closely related spirochetes that have evolved to occupy radically different niches within their hosts.

Subject of Research: Methylglyoxal-mediated carbonyl stress and protein glycation regulation in Lyme disease and relapsing fever spirochetes

Article Title: Metabolic regulation of the carbonyl stress pathway and methylglyoxal in Lyme disease and relapsing fever spirochetes

Article References: Drecktrah, D., Hall, L. S., Cheff, B., Richards, C., Raffel, S. J., Wulf, M., Bailey, I. T., Gherardini, F., & Samuels, D. S. (2026). Metabolic regulation of the carbonyl stress pathway and methylglyoxal in Lyme disease and relapsing fever spirochetes. PLOS Pathogens, 22(9), e1014657. https://doi.org/10.1371/journal.ppat.1014657

Image Credits: AI Generated

DOI: 10.1371/journal.ppat.1014657

Keywords: Lyme disease, Borrelia burgdorferi, methylglyoxal, carbonyl stress, advanced glycation end products, mgsA, spirochetes, relapsing fever, Borrelia hermsii, protein glycation, tick-mouse model, metabolic regulation