Rabbit Fever Bacteria Survives by Building Protective Armor, Study Finds

Posted: June 21, 2010 at 12:10 pm, Last Updated: June 22, 2010 at 4:02 pm

By Marjorie Musick

Mason microbiologist Monique van Hoek. Creative Services photo

How do the delicate bacteria that cause tularemia or “Rabbit Fever” survive in the wilderness? By building a fortress and pulling an army of allies inside their walls.

This is the conclusion of Mason researchers who investigated how the bacterium Francisella tularensis survives and thrives in the environment.

By growing the bacteria and using special stains to examine the growth, Mason microbiologist Monique van Hoek and biosciences postgraduate students Meghan Durham-Colleran and Anne Brooks Verhoeven found that the organisms constructed a biofilm or protective coating resembling a “sticky matrix” to survive in harsh environments.

They also identified the gene that regulates the biofilm formation and some of the different types of sugars that were used to build the film.

Tularemia is a zoonotic disease, meaning that it can be transferred from animals to humans through skin contact. The disease is most commonly seen in hunters, trackers and outdoor enthusiasts who interact with animals outdoors or become infected through tick and deer fly bites or by drinking contaminated water or inhaling contaminated dusts or aerosols.

Approximately 100 cases occurred in the United States in 2008, according to the Centers for Disease Control. Although not usually fatal in humans, tularemia causes high fevers and other uncomfortable symptoms that range from mild to life-threatening. Most infections can be treated successfully with antibiotics.

However, the research has important national defense implications. In its aerosol form, tularemia can be used to infect large groups of people, making the bacteria a substantial bioterror threat.

“Because of the nature of the disease and the organism itself, people in at least two countries have weaponized it for use as a bioweapons agent,” says Durham-Colleran.

Francisella tularensis. CDC/Larry Stauffer, Oregon State Public Health Laboratory (PHIL #1910), 2002.

“It’s a naturally occurring disease, and the reason it makes a good weapon is that, unlike most bacteria, which require thousands or millions of organisms to cause disease, it only takes inhalation of between one to 10 Francisella tularensis bacteria to cause pneumonic tularemia. Even though it might not cause a lot of deaths, it could cause a lot of panic because so many people could get it if it were to be released into the air.”

Although their findings don’t directly relate to a treatment for tularemia, the research does give the scientific community some idea of how the disease is harbored in the wild and how it is transmitted to humans.

“We want to understand how Francisella survives in the environment and how animals become infected with it,” says van Hoek, who is an assistant professor of molecular and microbiology at Mason’s National Center for Biodefense and Infectious Diseases. “If it’s persisting in water or waterways within the biofilm, that could be one way that animals get infected — by drinking from that water. And since human outbreaks usually occur after animal outbreaks, there’s a link there.

“The ultimate goal of this research is to create a vaccine and better therapeutics that will make the population less vulnerable to this bioterror threat.”

Van Hoek notes that biofilms are common in nature. For example, plaque on a person’s teeth can have as many as 500 bacterial species sheltered within it.

“A biofilm is a community of organisms that use these sugar shells to protect themselves. By working together, they thrive,” says van Hoek. “More research needs to be done to understand how [biofilms are] made and how we can get rid of them in order to destroy bacteria. We hope to use this information to better protect humans against disease.”

The results of the study, “Francisella novicida Forms In Vitro Biofilms Mediated by an Orphan Response Regulator,” were published in the journal Microbial Ecology in April 2010.

The study was supported by the Virginia Academy of Science, Mason’s National Center for Biodefense and Infectious Diseases, the National Science Foundation and the Joint Science and Technology Office for Chemical and Biological Defense/Defense Threat Reduction Agency.

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