Dr. Keith Poole

Professor, Department of Biomedical and Molecular Sciences Queen’s University, Kingston, ON
Researcher of the month: 
Aug 2016

Accidents of Antibiotic Resistance

What secrets lie in a test tube of Pseudomonas aeruginosa? Perhaps the reasons why these and other bacteria develop antibiotic resistance, says Dr. Keith Poole, Professor in the Department of Biomedical and Molecular Sciences at Queen’s University, Kingston, ON. 

Chronic lung infections are the perfect breeding ground for this opportunistic pathogen in people who suffer from cystic fibrosis. The trouble is, knowing which antibiotics to use to fight Pseudomonas infections is becoming more problematic, as antibiotic resistance grows.

We need to know more about how P. aeruginosa works to answer this complex question. Basic research opens new doors to our understanding of how Pseudomonas and other bacteria work inside the human body. For example, when a bacterium infects a human host, it steals nutrients from the host, which in turn, tries to withhold those nutrients in a bid to keep the invader from multiplying. The bacteria evolve ways to evade such tricks, and the body responds by developing new ones.

“It’s a tug-of-war,” says Poole, who began his journey to discovery by studying iron transport mechanisms in Pseudomonas to find ways to fight infection. Iron is one of those nutrients that’s essential for bacteria growth and survival. For the first five years of his career, Poole made great inroads in studying iron transport in Pseudomonas. Then, with one serendipitous finding, his work completely changed course.

“We cloned a gene that I was convinced was involved in iron transport,” he says. “I tried to prove it, but as it turned out, I was trying to fit a round peg in a square hole. Then, just as serendipitously, doing a variety of experiments, we found that this gene had an impact on antibiotic resistance.”

He had stumbled across the first multidrug efflux system to be identified and cloned in P. aeruginosa. Since that discovery, more than 20 years ago, his research has focused on the role of bacterial efflux pumps on antibiotic resistance. This field of study has blossomed, attracting wide interest and spawning research worldwide.

Antibiotics must reach a certain concentration within bacterial cells to have a lethal impact. However, as soon as antibiotic levels begin to rise, multidrug efflux systems pump these drugs out of the bacterium. Not enough antibiotic accumulates within the cell to kill it.

Poole studies a particular family of multidrug efflux pumps called Mex. “Any given pump in this family has the ability to recognize and pump out a variety of structurally different antibiotics. Once activated in a bacterial cell, one pump can make that cell resistant to multiple antibiotics.”

His challenge is to discover how multidrug-resistant efflux pumps recognize antibiotics with widely different chemical structures. Usually, cellular proteins are keyed to fit only one lock, i.e., bind to one chemical structure, so their ability to recognize and pump out different antibiotics poses a mystery.

Poole’s work has unravelled one piece of this puzzle, and his research may profoundly influence our understanding of antibiotic resistance. Testing at his laboratory has revealed that some efflux pumps in this family only appear in P. aeruginosa cell membranes in response to environmental stress. The bacteria produce the efflux pump as a survival mechanism – a way to adapt to stressors that threaten its survival in the host environment. As the organism works to eliminate the stressor, it concomitantly pumps antibiotics out of the cell.

“Antibiotic resistance is an unintended consequence,” he explains.

One of the Mex family of drug pumps, MexXY, is frequently found in aminoglycoside-resistant P. aeruginosa. These bacteria cause chronic lung infections in cystic fibrosis (CF) patients, and aminoglycoside antibiotics are important for their treatment. The infections cause chronic lung inflammation, which typically produces reactive oxygen leading to what is called oxidative stress.

Poole and his research team have found that oxidative stress triggers the production of MexXY efflux pumps as an environmental defence mechanism in P. aeruginosa. As the pumps work to alleviate damage to the bacterium from oxidative stress, they inadvertently pump out the aminoglycosides, conveying resistance to these antibiotics.

In other words, Poole says, antibiotic resistance may be accidental. That means, stopping the use of aminoglycosides to renew antibiotic sensitivity won’t curb aminoglycoside resistance in these organisms.

“It’s somewhat concerning that resistance mechanisms can be driven by something other than antibiotics,” says Poole. There may be no way to combat such resistance, and physicians may have to prescribe different antibiotics – ones that aren’t inadvertently pumped out of bacterial cells.

And, it’s unlikely that antibiotic sensitivity tests in the laboratory will detect this type of resistance, he states, because the test tube does not contain the same environmental stressors as a working lung.

“We have no clue what kind of lung conditions may impact (bacterial) cell physiology and recruit genes or proteins that may influence antibiotic resistance in unpredictable ways,” he adds. Poole’s research now focuses on bacterial stress responses.

“We’re starting to find other environmental conditions that effect antibiotic resistance, although we don’t know why. We’re trying to figure out what’s actually happening at the infection site, so that we can recreate that environment in a test tube to get more accurate measures of antibiotic susceptibility.”

These challenges keep his work “fresh”, he claims. “When you don’t quite know what’s going to happen next – that’s what I find exciting about science. I like to be surprised on a regular basis.”

When Poole was a third-year student in a pre-med program at the University of British Columbia (UBC), his plans to enter medical school were thrown into chaos when he took a microbiology course. Realizing “this is what I have to do”, in his fourth year, he took a course load-and-a-half in order to graduate with a BSc in microbiology.

He then worked as a technician in Bob Hancock’s microbiology lab at UBC for a year before pursuing his Masters and PhD. He did a post-doc in molecular biology in Germany for two years before serendipitously landing a tenure-track position at Queen’s University in 1988 as an assistant professor.

Now a full professor, Poole was recently elected as a fellow of the Royal Society of Canada. The Canadian Society of Microbiologists’ honoured him with the Murray Award for lifetime achievement in 2014. He is a fellow of the American Academy of Microbiology and a past recipient of the Queen’s University Prize for Excellence in Research. Cystic Fibrosis Canada and the Canadian Institutes of Health Research fund his work.