Bioengineered Yeast Combat Clostridium difficile

Clostridium difficile (C. diff for short) is an environmental bacterium that has become increasingly common as an opportunistic pathogen. Like its relatives, Clostridium botulinum (the cause of botulism) and Clostridium tetani (the cause of tetanus), C. diff is a spore-forming organism. Spores are dormant forms of bacteria that are very stable and resistant structures. Their resistant nature allows the spores to persist in the environment for years and survive through heat, cold, wet, or dry conditions. C. diff spores are found in soil, water, air, feces, and even some food products, especially meats and vegetables. For the spores to germinate back into typical, active bacteria, they must be in an environment without oxygen (an anaerobic environment) as oxygen is poisonous to this family of microbes. We all likely ingest these C. diff spores on occasion, and even though our intestinal tract is anaerobic, generally we are not affected. Our normal intestinal bacteria (our gut microbiome) protect us from C. diff by outcompeting the geminating C. diff organisms and preventing them from colonizing our intestines. Even when C. diff does manage to establish itself permanently in some individuals (1-3% of healthy adults) it is only at low levels that do not cause disease.

The clinical problem with C. diff arises primarily in older individuals, especially those in hospitals or long-term care facilities, after antibiotic therapy for the treatment of other infections. While antibiotics will kill the target pathogen causing the infection, they also can indiscriminately kill many of our indigenous bacteria in our gut microbiome. These off-target effects of antibiotics can disrupt the intestinal flora and reduce the levels of healthy, protective bacteria in our gut microbiome. Without a robust and vigorous normal flora in the intestines, C. diff can proliferate to high levels and establish itself as a permanent infection. Most strains of C. diff produce and release two toxins called TcdA and TcdB that can kill human cells. As the number of C. diff organisms grows in the intestine the amount of these toxins accumulates to dangerous levels. Symptoms range from persistent diarrhea to intense intestinal inflammation and sometimes fatal sepsis. The annual number of C. diff cases in the United States is approaching 500,000 annually with roughly 30,000 deaths. Infections are even becoming more common in younger people and individuals without antibiotic usage, indicating that C. diff cases are unlikely to wane in the coming years.

Effective treatment for C. diff infections has been problematic. The standard approach to bacterial infections is antibiotic treatment, but this is a double-edged sword for C. diff infections. While certain antibiotics are effective against C. diff, at the same time these antibiotics further disrupt the normal flora and prevent the beneficial action of the flora against C. diff organisms. Add to that the emergence of antibiotic-resistant strains of C. diff and there is an urgent need for alternative therapeutic strategies. Transplant of fecal microorganisms to restore the normal intestinal flora has had some success, but this therapy is difficult to standardize and carries some risk of transmitting other infections to the recipient. Another innovative method with positive results uses monoclonal antibodies that neutralize the TcdA and TcdB toxins. Unlike antibiotics, these antibodies don’t affect the indigenous bacteria so there is no further prevention or delay in the regeneration of a healthy gut. However, these antibodies have to be administered intravenously which makes it complicated for home use.

A recent publication in the journal Science Translational Medicine expands on the antibody approach by using the yeast,  Saccharomyces boulardii, to express an anti-toxin antibody directly within the gut. This type of yeast was chosen because it is widely regarded as a safe organism with beneficial properties for promoting intestinal health and is already used in some over-the-counter probiotics. The authors of the study bioengineered an antibody that neutralized both the TcdA and TcdB toxins of C. diff and created an S. boulardii strain that constitutively secretes this antibody. When taken orally this yeast colonizes the intestines and continually produces the neutralizing antibody, thus eliminating the need for intravenous administration. Additionally, since yeast aren’t affected by anti-bacterial antibiotics, the therapeutic yeast strain could be given in conjunction with antibiotics targeted at C. diff to provide a dual attack against the pathogen. To date, this has only been tested in a mouse model, but the results are promising. After infecting the mice with C. diff, 70% of the untreated mice died by day 4. In contrast, only about 30% of the yeast-treated mice succumbed to the infection, a significant clinical improvement. Importantly, an even more encouraging result was obtained when the mice were yeast treated in advance of the C. diff infection. Mice that were given the yeast daily for 3 days prior to C. diff infection all survived, while 60% of the untreated mice died from C. diff infection within 4 days. This suggests that the yeast treatment could be used as a prophylactic to help prevent C. diff in high-risk patients who are going to start an antibiotic treatment or are admitted into the hospital for an extended stay. Much more work is needed to refine this approach and eventually move it into human trials, but it would a great boon if C. diff therapy was a simple as taking a probiotic.

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