Microbes and ‘Mmunology in the Mountains: Monday addendum

So, it turns out, I accidentally left off the talk by Colin Hill, which is very unfortunate, because I actually really liked his talk. So, here it is now.

Colin Hill’s talk was titled “Controlling Costridium difficile without ‘collateral’ damage to the microbiota”. The main thrust of his talk was the search for novel “pharmabiotics” that we could mine from the microbiota. The idea here is that probiotics so far have not turned out to be as effective as originally advertised, and it may, in the long run, but more effective to use particular compounds made from microbes in the microbiota, but not the microbes themselves.

The focus of his talk was on Clostridium difficile associated diarrhea (CDAD). C. difficile invades the colon after treatment with antibiotics, and treatment of the infection requires further antibiotics which causes a recurrence of the infection. Fecal bacteriotherapy (i.e. transplanting the fecal microbiota from a healthy patient to an infected one) can be very successful; there’s an 89% cure rate after a single transplant, and a 92% cure rate overall. However, this approach is very “messy”. There’s no known mechanism, and patients are essentially just being fed an undetermined community of microbes from someone else’s feces. The use of a Lactobacillus species from a cheap yoghurt was shown to be very effective in a double blind study on treating CDAD, and one month later, doctors in a hospital in Dublin were prescribing yoghurt to treat CDAD (at a much higher price than it can be found in the store, mind you). The problem was, though, the doctors were prescribing a different yoghurt than the one in the study, and it contained a different species of Lactobacillus. Colin compared this to doctors prescribing either an anti-inflammatory pill or Viagra for the same problem, simply because they’re both blue. While the differences may not be that drastic, they could be, and they could even be worse. There is really no guarantee that species of the same genus will have the same effect in the microbiota.

So, Hill and his group decided to look at molecules produced by the microbiota, specifically bacteriocins. These are small peptides that kill other bacteria, and the Hill group wanted to know if they would make good anti-infectives. Lactobacillus salivarius can protect mice against infection with Listeria monocytogenes, but bacteriocin(–) Lactobacillus cannot. A particular bacteriocin, Lacticin 3147 from Lactococcus lactis is active against all Gram (+) bacteria. Subcutaneous lacticin 3147 can control the spread of Staphylococcus aureus as well as protect the teats of cows from infection with Streptococcus dysgalactiae.

One of the drawbacks to using pure bacteriocins is that they take a very long time to isolate. So, they looked at what would happen if they introduced a particular bacterial species that expressed the lacticin (and controlled with a mutant that did not).
They took a fecal slurry from humans, inoculated with C. difficile, then added Lactococcus + and – for lacticin, as well as two different doses of pure lacticin. These communities were then incubated. The pure protein treatments had the best effect on C. difficile, but there was a lot of collateral damage to the rest of the microbiota. The Lactococcus (+) treatment had much less collateral damage on the microbiota, but it was also not significantly better at removing C. difficile than the control. It seemed this lacticin may have been too broad of an approach.

So, they moved on to thuricin CD, a narrow spectrum bacteriocin that is affective against C. difficile. In an overlay experiment (where they first inoculate the plates with the microbiota and C. difficile and then overlay that with the protein) they found it killed of C. difficile, but left the remaining culturable members of the microbiota alive. They repeated the fecal slurry experiment with the thuricin CD and found it to be just as effective as vancomycin and metronidazole, but the bacteriocin had a much lower impact on the microbiota than the antibiotics. An in vivo mouse trial also demonstrated significant results.

One of the questions I asked Colin after his talk was whether or not the evolution of resistance to these bacteriocins might become a problem if we were to start using them widely. He, of course, did not think that that would be much of a problem because we can be so specific with them, and if introduced via a probiotic, the proteins are not administered to the whole community, but rather, each bacteriocin (+) cell can only affect other cells that come into close contact with it. I’m not sure why exactly this prevents the evolution of resistance, if a particular species is almost completely eradicated by this compound, that’s pretty strong selection to be able to resist it. But, overall, I think this is an exciting area of research. Many of the problems we’ve encountered with probiotics are that (a) what exactly they do is poorly defined, (b) they are often not normal members of the microbiota anyway [since they tend to come from fermented cow’s milk products] which means they don’t actually stick around in the community, and (c) the transplant of fecal microbiota from healthy people to CDAD, while effective, is undetermined and potentially dangerous. The isolation of particular bacterial compounds from the normal microbiota provides a way to be much more controlled in how we treat these microbiota-associated conditions in terms of both knowing the mechanism of action for the treatment as well as minimizing the effect on the rest of the community, thus avoiding the cycle of disturbance that repeating administration of antibiotics causes.


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