New combination drug therapy offers hope for treating chronic wound infections

University of Oregon researchers have tested a new combination drug therapy that could dismantle the difficult-to-treat bacteria inhabiting chronic wound infections. 

Their findings, published Sept. 29 in the journal Applied and Environmental Microbiology, illuminate ways to develop more effective antimicrobial treatments that promote healing in chronic wounds. Such treatments also could help reduce the risk of severe infections that sometimes lead to amputations, such as diabetic foot ulcers. 

Funded by the National Institutes of Health, the approach pairs long-known substances that do little on their own against hard-to-treat pathogens festering in chronic wounds, namely the bacterium Pseudomonas aeruginosa. But by adding small doses of a simple molecule called chlorate to standard antibiotics, the combination proved 10,000 times more effective at killing bacterial cells in the lab than single-drug antibiotics. That kind of potency reduced the dose of medication required to kill P. aeruginosa. 

If the findings can be translated to humans, they could help shorten the time patients need to be on antibiotics and lower the risks of toxicity, said Melanie Spero, an assistant professor of biology in the UO’s College of Arts and Sciences and senior author of the study. 

Though investigated here in the context of chronic wound infections, the strategy may hold promise for addressing antibiotic resistance more broadly. 

I think that drug combinations will be a critical approach that helps us fight against the rise of antibiotic resistance. Finding examples of synergy among antimicrobials that are already on the market is going to be really valuable. And we’ll need to dig further into the mechanisms behind why they work well together.” 

 Melanie Spero, assistant professor of biology in the UO’s College of Arts and Sciences 

Challenges in treating chronic wound infections 

A chronic wound is injured tissue that hasn’t started to heal within normal time frames of four to 12 weeks. The most common type is a diabetic foot ulcer, Spero said, which is an open sore on the foot’s underside that forms from poor circulation, prolonged pressure and a lack of sensation. 

According to research published by the American Diabetes Association, about 1 in 4 people with Type 2 diabetes develop a foot ulcer, and more than half of those cases become infected. 

“An active infection is the most common complication that prevents the wound from healing and closing,” Spero said, adding that when severe, 1 in 5 diabetic foot ulcers require an amputation. “It’s very debilitating, but there’s not a lot of microbiology research being done in this field. So it’s an opportunity to make a big difference.” 

Shifts in blood flow, the high oxygen demand of inflammatory cells, and the presence of bacteria in the chronic wound site all limit how much oxygen reaches the tissue, preventing healing. Those low-oxygen conditions also are the very problem that makes bacterial infections hard to fight: It unmasks antibiotic resistance and tolerance. 

When a wound site becomes oxygen-limited, bacteria switch to breathing nitrate for energy, known as nitrate respiration. Their growth slows without oxygen, but they still survive and continue to spread. 

The resulting slow growth of the bacteria, particularly P. aeruginosa, makes them notoriously tolerant to conventional antibiotics. That’s because many medications are rated based on how well they kill fast-growing bacteria, Spero said. But if the bacteria are growing slowly, those antibiotics, which also are often tested only in oxygen-rich conditions, end up being ineffective, she said. 

At least when administered on their own, Spero has found. 

Getting more mileage out of current antibiotics 

When the antibiotics are combined with a small molecule called chlorate, it “stresses the bacterial cell in a way that makes it super susceptible to antibiotics,” Spero said. 

The research builds on studies Spero first conducted as a postdoctoral scholar at the California Institute of Technology. She previously found that chlorate, a simple compound that’s harmless to mammals and humans in the low doses used in her studies, turns antibiotics from lukewarm performers into potent bacteria killers in cell cultures and diabetic mouse models. 

Thanks to a $1.84 million grant over five years from the National Institutes of Health, Spero has been able to continue the work in her new lab at the UO. Her latest study shows that chlorate works to make all kinds of antibiotics more effective at killing P. aeruginosa and can lower the antibiotic dose needed to fight the pathogen. With a small amount of chlorate in the mix, her team could use 1 percent of the standard dose of the broad-spectrum antibiotic ceftazidime, the research found. 

“In the case of chronic infections, people are often on antibiotics for long periods of time, and that can wreak havoc on the body,” Spero said. “Drugs with high toxicities can disrupt gut microbes and have severe side effects. Anything we can do to shorten the amount of time that a person is going to be on antibiotics and lower the dosage, the better.” 

The results come from controlled lab tests on bacterial cell cultures, so translation to the clinic is still far down the line. Especially since chronic infections usually don’t involve a single bacterium, Spero said, as they host whole microbial neighborhoods living and interacting together. So uncovering how drug combinations affect those complex communities in model organisms is an obvious next step, she added. 

The exact mechanism for how chlorate boosts antibiotics also is still a mystery. Spero explained that chlorate has been known by scientists to hijack nitrate respiration, so in the complete absence of oxygen, microbes are wiped out. But in microenvironments of low – or high – levels of oxygen, the bacteria can somehow repair that damage and tolerate the chemical. So in traditional single-drug screenings, which are usually performed in high-oxygen conditions, chlorate has been overlooked, Spero said. 

“I think that’s what we don’t fully appreciate: the types of stresses these compounds impose on the cell that are invisible to us,” she said. “If our only metric is viability – did the bacteria live or die? – that’s all we’ll look for. We need to be asking what processes are being pushed on or stressed out in the cell that can lead to its collapse in the presence of antibiotics.” 

Spero hopes that looking “under the hood” of a cell during chlorate-antibiotic exposure will show scientists the biological machinery of how bacteria become susceptible to a range of antibiotics. 

“This will have important implications not only for treating chronic wound infections but also broadly for the infectious disease field and our fight against antibiotic resistance and treatment failure,” Spero said. “Once we understand the mechanisms of drug synergy, we can start to find other molecules that elicit these synergistic behaviors, and it won’t feel like a guessing game where we test every possible drug combination. We can start doing rational drug design, using molecules that have already been approved.”