One Step Closer to Curbing Columnaris

It’s insidious, it’s unpredictable, and it’s one of the biggest threats faced by any freshwater aquaculture operation. Flavobacterium columnare, the bacteria behind the dreaded columnaris disease, can move with frightening quickness, leaving tanks of dead trout, perch and catfish in its wake.

But here’s the good news: Mark McBride and David Hunnicutt, a pair of Wisconsin professors, are discovering some of the bacterium’s weapons, and, thanks to research funded in part by Wisconsin Sea Grant, they’re several steps closer to developing a vaccine that could neutralize them.

McBride, a professor of microbiology and molecular biology at the University of Wisconsin-Milwaukee, spent years studying flavobacterium, honing in on the basic biology that allows it to wreak havoc in aquaculture systems and in the wild.

Years of research revealed that Flavobacterium cells have a novel secretion system that transports proteins to the cell surface and beyond. Some of the secreted proteins are needed for the bacterium to attach to and move over surfaces. McBride and Hunnicutt suspected that these secreted proteins might allow the bacteria to cause disease in fish. This gave them a clear strategy to attack the bacterium.

“When we disrupted the secretion system, we expected that the cells would lose motility and attachment,” said McBride. “But more importantly, we were also looking to see if they would lose their virulence,”

Early tests on zebrafish and perch suggest that that their hunch was correct—the mutant bacteria failed to cause disease. This suggests that proteins secreted by this system are needed to cause disease.

The researchers set out to find those proteins. They bathed fish in a solution containing proteins secreted from the wild bacteria and the fish died rapidly, even with no bacteria present.

“What we found is that the proteins may be toxic all by themselves, without the bacteria present.” This helps the researchers to think of how the bacterium causes disease, and strategies to stop it. One approach is to use their mutant bacteria that don’t cause disease as vaccine strains.”

A few weeks ago, Hunnicutt, a professor at St. Norbert College in De Pere, Wisc., gave students in his immunology class a chance to test-drive the latest version of a nascent columnaris vaccine. Students inoculated more than 100 zebrafish with the harmless strain, and have been monitoring them for signs of resistance to infection with the deadly strain.

Hunnicutt said he didn’t expect protective results from the first batch of fish, but a second inoculation, given in April, will tell whether the vaccine strategy is working. The incorporation of research with a real-world application into the classroom setting has been exciting for his students.

“When you can bring in ongoing research, it gets the students motivated,” Hunnicutt said. “We’ve been using it as a way to talk about comparative immunology between fish and mammals.”

Here in Wisconsin, McBride and Hunnicutt have focused primarily on perch, because, obviously, it’s the staple of the legendary Wisconsin fish fry.  Wild perch populations have plummeted, but commercial aquaculture operations in the upper Midwest have risen to meet the demand for this popular fish Several of these facilities have been plagued by—you guessed it—columnaris disease. However the significance of the research conducted by McBride, Hunnicutt, and their students extends beyond the Midwest. For example they are collaborating with a team at the USDA labs in Auburn, Alabama to develop vaccines that might protect catfish, a common aquaculture fish in the southern US, and also to protect rainbow trout.

It is still an open question whether the phase one vaccine will provide strong resistance to wild-type F. columnare in diverse aquaculture fish. “The genetic tools that we developed allow us to determine which proteins are important in the disease process” said McBride. “Even if our first vaccine strains are not optimal, we have a rationale path to follow to generate new and improved strains that are likely to function as better vaccines.”

McBride said he gives special credit to visiting scientist Nan Li from China, where columnaris disease also devastates the aquaculture industry. Li played a large part in developing the genetic tools to unravel the bacterium’s basic biology.

If the eventual vaccine proves effective, others will likely want to try it with other aquaculture species, such as arctic char, a species that’s being raised in places like the UW-Stevens Point Northern Aquaculture Demonstration Facility in Bayfield, Wis.

“If we can accomplish this, we may be able to limit the problems caused by Flavobacterium columnare in many aquaculture systems,” said Hunnicutt. “It’s always gratifying for those of us in the lab when something is useful in the field.”