"We've discovered, through genetics, a factor that is important in the normal biology of the organism out in the environment and it is also one of the very initial factors for cholera colonization in the intestine," said Dr. Ronald Taylor, professor of microbiology and immunology at DMS who led the research. "Now that we know what the bacterium attaches to in the intestine, we can find ways to block that initial contact."
Cholera and the bacterium that causes it is found in contaminated drinking water and food, often in underdeveloped countries and refugee camps. Once the disease takes hold, it causes diarrhea, vomiting, severe dehydration and can result in death if treatment is not promptly given. In 2001 alone, 28 countries reported over 40 outbreaks of cholera to the World Health Organization, resulting in the deaths of thousands.
Large outbreaks are often traced back to contaminated water supplies that are commonly associated with algal or zooplankton blooms. For the V. cholerae bacterium to infect someone with cholera, the bacterium often binds to plankton in the aquatic environment before it arrives at the human intestine via contaminated food and water sources. V. cholerae attaches to the outer surface of plankton, made up of a carbonate substance called chitin. Once attached to the plankton's chitin, the bacterium thrives on the carbon and multiplies. Humans do not have chitin in the surface of intestinal cells, where the bacterium takes hold, and researchers have been searching for another substance that could be responsible for playing a role in attachment.
In the study, Taylor and colleagues screened cultured intestinal cells and found mutant bacteria that had trouble binding to the intestinal cells. One mutant strain of V. cholerae lacks a gene that enables it to properly bind with a sugar called GlcNAc. When they compared it with normal, wild-type V. cholerae bacteria, the researchers found that the protein encoded by this gene provided normal bacteria the ability to attach to the GlcNAc on cells. The team verified that the GlcNAc in the intestine initiates the attachment and colonization of the bacteria by testing the mutant strain on zooplankton and cultured intestinal cells in vitro as well as in an in vivo cholera model.
"We set out to find factors that would reduce the bacteria's ability to bind to the epithelial lining of the intestine," said Taylor. "What's interesting is that we've identified a factor that works both in the environment and in the human body. This type of link hasn't been discussed before and it has a strong potential for vaccine and therapeutic development."
These findings could lead to a new form of therapy to treat people with cholera. "Now that we know that the bacteria are binding this particular sugar, we could essentially trick the infecting bacteria to bind to the sugar included in a derivative of oral rehydration therapy solution instead of the intestine," said study co-author Brooke Jude, a fourth-year PhD student at Dartmouth Medical School.
A vaccine for cholera already exists, but only works 50% of the time and people who take it are only immune for 12 months, according to Taylor. Taylor believes that a more effective vaccine could be developed by inducing the production of antibodies directed against the protein his research team has discovered, thereby blocking its function. This would inhibit an early step in the intestinal colonization process, and the bacteria would pass harmlessly through the body. The authors acknowledge that in addition to GlcNAc, there may be other points of attachment that could still be responsible for allowing the bacteria to bind to the intestine, and they are currently focusing their research to identify any other areas of attachment..
"There may be more of these factors and as we find them and knock them out, we'll decrease the ability for cholera bacteria to colonize even further," said Taylor.
This study, "A colonization factor links Vibrio cholerae environmental survival and human infection," was funded by the NIH and a Rosalind Borison memorial fellowship.