Protein structure of C. diff toxin B
Clostridia difficile infections are often the subject of this blog, but we rarely ever discuss how the infection actually occurs. Scientists know that C. diff spores, which are not very uncommon in nature, can enter the gut from a variety of sources. Once the spores reach the gut they normally just pass through unnoticed. However, given the right conditions, these spores can take hold, germinate, and grow. At some point during infection, the C. diff produces toxins which can compromise gut permeability (i.e. cause ‘leaky gut’) which leads to inflammation and all the nasty effects associated with the disease. The exact gut conditions that trigger C. diff spore germination are not known, but scientists are convinced that the microbiome is involved because taking antibiotics, which wipe out the normal gut flora, make people susceptible to C. diff infection. Some research has suggested that certain bugs in the microbiome outcompete C. diff for resources. Other research shows that secondary bile acids, which are produced when the microbiome breaks down bile acids, inhibit C. diff germination. Scientists are still working hard to understand the mechanisms of this infection, and just this week research out of the University of Michigan, published in Infection and Immunity, has shed new light on the process.
The scientists first gave a group of mice antibiotics to make them susceptible to infection, and then fed the mice C. diff spores. After, they euthanized mice every 6 hours to measure the progression of C. diff infection. They learned that within 6 hours the spores had already germinated and entered the vegetative state in the feces and large intestine of the mice. Over time, the C. diff progressed their way up the distal end of the large intestine all the way to the stomach, until the entire gastrointestinal (GI) tract was infected. After 30 hours, sporulation of C. diff occurred, and interestingly this coincided with the production of C. diff toxins. These toxins were found throughout the GI tract, however, inflammation only occurred in the large intestine, and not in the small intestine. After 36 hours the infection had become severe enough that all animals were euthanized.
The scientists also measured the bacterial population and bile acid content of the gut during the infection. After antibiotic treatment the microbiome was drastically altered and Lactobacillaceae flourished. Once infection took hold the Lactobacillaceae were supplanted by C. diff in the large intestine, although the Lactobacillaceae still dominated the small intestine population, which, notably, did not become inflamed. Secondary bile acids, which are produced by the microbiome and linked to C. diff germination, were abundant prior to antibiotics. After antibiotic treatment, the large intestine had fewer secondary bile acids, and in the most infected regions had no detectable secondary bile acids.
This research is the first to develop a timeline for C. diff infection in mice, and strikingly it occurs very rapidly, with symptoms showing within 2 days. This study also supports the notion that an altered microbiome is critical to C. diff infection, and that secondary bile acids may in fact play a crucial role in keeping C. diff from vegetating. Interestingly, this study fits in well with a previous study we wrote about that showed the benefits of secondary bile acids in preventing C. diff infection.