Saturday, October 25, 2014

Contestant for Clostridium difficile- C scindens


    C difficile has represented important part of current clinical microbiology research. Its attachment with antibiotic induced complications has caused sufficient investment of research. The primary current goal is treatment. As discussed previously (Link), antibiotic and vaccine strategies are currently highly explored. Of everything else the most popularity is gained by a method called Fecal transplantation or Fecal microbiota transplantation (FMT). Over the last couple of years a huge amount of research is focused on improvement of the method.

   FMT works something like the following. A fresh stool sample is obtained from a healthy donor, screened for potential enteric pathogens and a serology for antibodies to hepatitis A, B, and C, HIV etc. The stool sample is homogenized and filtered for debris. The extracted material is administered rectally through a colonoscopy or nasopharyngeally through ryles tube. The basic problem with the approach is that the testing takes takes time and the transplant is best administered within 6-8 hrs. Moreover, healthy donors are not always available at disposal. This has been overcome by using synthetic pills containing frozen fecal matter. Study has shown that with administration of 30-60 capsules over a 8 week period or less most of the participants had completely recovered.

Table 1: Organisms implicated in
interference with C difficile.
   However, there are several problems with this approach. The foremost being the fact of consuming fecal sample no matter how well it is treated. The second important problem is in getting the right donors. The screening of the donor is only as good as the laboratory test and limited to number of tests that can be done. This means that though the technique is safe, it is not out of risks, though the risk is significantly low. It is arguably better to figure out what components of the flora are really important in defense. Considering the extremely huge number of colonizing bacteria in gut, it is very difficult to ascertain the right organisms. It is also very much possible that the effect is brought by combination of bacteria's. For example various Lactobacillus and Bifidobacterium strains have been shown to significantly impact C diff invasion. Table 1 is a list of proposed organisms involved in fighting C difficile infections.

      Clostridium difficile is a anaerobic spore-forming Gram-positive bacterium. the organism has been identified in normal gut of normal individuals also. Its number is however, very low. In patients receiving heavy antibiotic dose, the gut flora is wiped out in high levels, providing a chance (C diff is resistant to multiple drugs) to colonize and cause infection. The first step in this process is spore germination. The germination of spores is a specialized process based on sensing of environment. The signals that favor germination are called germinants. The germinants vary for species and can include anything from an ion gradient to a specific chemical such as sugar or amino acid. There are also negative signals which alert the bacteria to the hostility of environment and can restrict the bacteria from germination. In the context of C difficile taurocholate, glycocholate, cholate, and deoxycholate act as germinants; and chenodeoxycholate can competitively inhibit taurocholate-mediated germination.

       So it makes sense as to consider chenodeoxycholate, a modified bile acid or similar compounds to be an important player. In an experiment with rodent model, scientists were able to identify signature of organisms causing resistance to C diff infections. Though the exact composition was not identical between members, there was a single topper- C scindens. This is indeed a breakthrough, and fits really well into the current model. It is known that C scindens produce enzymes which convert primary bile acids into secondary bile acids in the large intestine through 7 alpha-dehydroxylation. Also, important as shown in paper, C scindens when given as a cocktail with other microbiome was more effective than C scindens alone.

     This has important implications. Foremost, is in considering a probiotic C scindens is an important candidate for the mix. Second, it is very likely that there are more organisms involved in this effect and hence a combination of just few microbes may not work effectively enough. However, if we could figure out the right combination of organisms, even partially, it could go way beyond having to administer fecal transplant and finding the right donor.
Youngster I, Russell GH, Pindar C, Ziv-Baran T, Sauk J, & Hohmann EL (2014). Oral, Capsulized, Frozen Fecal Microbiota Transplantation for Relapsing Clostridium difficile Infection. JAMA PMID:25322359

Hickson M (2011). Probiotics in the prevention of antibiotic-associated diarrhoea and Clostridium difficile infection. Therapeutic advances in gastroenterology, 4 (3), 185-97 PMID:21694803

Sorg JA, & Sonenshein AL (2010). Inhibiting the initiation of Clostridium difficile spore germination using analogs of chenodeoxycholic acid, a bile acid. Journal of bacteriology, 192 (19), 4983-90 PMID: 20675492

Buffie CG, Bucci V, Stein RR, McKenney PT, Ling L, Gobourne A, No D, Liu H, Kinnebrew M, Viale A, Littmann E, van den Brink MR, Jenq RR, Taur Y, Sander C, Cross J, Toussaint NC, Xavier JB, & Pamer EG (2014). Precision microbiome reconstitution restores bile acid mediated resistance to Clostridium difficile. Nature PMID: 25337874