Taking apart infectious disease
By studying cholera, Dr. Stefan Pukatzki has uncovered a new method that bacteria use to cause disease.
Story by Tara Narwani/Illustration by Douglas Fraser
Bacteria are sophisticated organisms that have evolved complex mechanisms to cause disease. Dr. Stefan Pukatzki is hunting for new ways in which bacteria are harmful to humans. While studying Vibrio cholerae, the bacterium that causes cholera in humans, Dr. Pukatzki found a new disease-causing mechanism called the type VI secretion system that this pathogen and many others carry. "For us, the discovery was a real jackpot," says Dr. Pukatzki.
Prior to his arrival at the University of Alberta in 2007, Dr. Pukatzki began this research in the Harvard laboratory of world-renowned microbiologist John J. Mekalanos. To understand how Vibrio infects organisms, Dr. Pukatzki decided to investigate how Vibrio cholerae interacts with its natural competitors. Examining how these bacteria behave in their natural environment where the disease-causing mechanisms evolved could help us understand how they infect humans.
Vibrio cholerae is typically found in marine environments where it encounters amoebae, fungi, and other bacteria. To mimic this, Dr. Pukatzki grows the cholera bacterium on Petri plates together with an amoeba called Dictyostelium.
The amoeba ingests most strains of Vibrio and uses them as food. But Dr. Pukatzki found that a unique strain called V52, which caused a deadly outbreak of cholera in Sudan in 1968, killed the amoeba instead.
By turning off certain genes in V52, Dr. Pukatzki stopped this bacterial strain from killing the amoeba and, in the process, identified some essential elements of the type VI secretion system.
With the evidence gathered to date, Dr. Pukatzki has developed a basic model of how this mechanism might work. "We think of the secretion apparatus as a little syringe that is sitting on top of the bacterium. And because it's a syringe-like structure, it can puncture a host membrane, secrete proteins directly into the host cell, and interfere with whatever these cells do," he explains.
So how does this strain of Vibrio cause disease in humans? Although it's too early to draw any conclusions, Dr. Pukatzki speculates that Vibrio uses this secretion system to kill healthy bacteria in our gut, allowing it to colonize our digestive system. Another possibility is that the secretion system allows Vibrio to immobilize key cells in our immune system that are responsible for destroying foreign invaders.
Once Dr. Pukatzki discovered the type VI secretion system in the cholera bacterium, he soon found it in other bacteria, such as Pseudomonas aeruginosa, which causes lung infections in immune-compromised patients, as well as a strain of E. coli that causes illness after consumption of contaminated beef.
The prevalence of this disease-causing mechanism makes it even more compelling to understand how it's regulated, what it secretes, and how it affects host cells. These are the questions Dr. Pukatzki is now pursuing in his lab at the University of Alberta. For one of his current projects, Dr. Pukatzki and his team are changing the activity of a gene product that turns on important components of the type VI secretion system in Vibrio. As expected, when they activate this gene in strains of Vibrio that don't produce this secretion system under laboratory conditions, it triggers the bacterium to secrete a set of proteins that are part of the system.
In the long term, Dr. Pukatzki is confident that this research will have broad implications for the treatment of infectious diseases. "I'm very hopeful that, at some point, we'll find a component of the system to target, and if we can knock it out with a drug, then we can prevent infections by pathogens that rely on this mechanism."