Sometime last year, on his now-defunct tumblr feed, Recursive Muffin, Ethan Mitchell asked a question I thought the folks on This Week In Microbiology (which I’ve written about here and here) might be able to answer. I emailed them and they did so, at the end of episode 73. I thought the question and answer were interesting enough to transcribe here:
Question of the day: A strain of Flavobacterium (KI72) evolved the capacity to digest nylon, obviously in recent history. Fine and well. How long will it be until one of the cariogenic bacteria species evolves the ability to digest dental resin? After all, we are putting a lot of it on their dinner table.
Answered primarily by Michael Schmidt, who teaches microbiology to dental students at MUSC, with help by Michele Swanson and Vincent Racaniello. Keep in mind that this is a transcribed conversation, so informal, and that I don’t know how to spell some of these words:
Michael: This has already happened to some extent. In the United States we currently spend 5 billion dollars a year replacing resin-based composite fillings due to failure. The average lifespan of a resin-based filling that a dentist will put in today is around 6 years. And the recurrent decay usually compromises the restoration earlier in its lifespan, and that’s when the bacteria are effectively going after the “glue.” And they’re going—because it’s a polymerization and the microbes—if the restoration isn’t properly fixed and properly cured there’s enough carbon in there that they can get at before the polymerization is completely done, that they can actually get after it. And then afterwards, matrix metalloproteinases and cathepsins places the longevity ceiling at that 6 years, even at healthy and bacterial free restorations. And so these matrix metalloproteinases and cathepsins which are expressed specifically in dentin, they come in and they cause the restoration to fail.
Michele: But those are bacterial, Michael, or those are…
Michael: No, those are eukaryotic. The bacterial failure… places the longevity cap at around 6 years, so there’s currently a resin-based product in the market that’s from a Japanese company that puts chlorhexidine into some of these resin-based products in order to prevent microbial attack and to take out the… bacterial attack. So we’ve been looking at copper nano-particles to effectively prevent some of this decay, but what he is hypothesizing has already been happening in the US and it happens throughout the developed world, anyplace people are using resin-based fillings. The old silver fillings typically last 25-30 years without incident. Most people were concerned about the mercury issue but the amount of mercury in an amalgam based filling is insignificant in terms of health consequences if you look at the evidence-based literature. It really has no issue associated with the health of the individual.
Michele: And I don’t suppose there’s any data saying the half-life of the resin is decreasing? Which would be consistent with this idea that we’ve selected for bacteria, we’re enriching for bacteria, that can break it down more readily?
Michael: No, because the resins have gotten better. The polymerization agents and the curing times, so we don’t have a clean experiment to do it. The folks haven’t actually looked to see if resin-based dissolving bacteria… but that’s a question that I can ask my friends at the Forsyth Institute to see if they’ve hunted to see if there are any resin-eating bacteria out there. But it’s all about the polymerization because the polymer needs to be perfectly cured and any of you who’ve had a recent composite filling you remember the dentist putting on the dark glasses and giving you a pair of dark glasses and they put the magic light into your mouth to cure the filling. And typically they only put the UV lamp in there for 20-30 seconds and that’s what starts the curing process. And we’ve all made polyacrylamide gels and it’s a variation of polyacrylamide gels except we use… Temid and whats the inorganic… the inorganic salt. I haven’t made a gel in…
Vincent: I can’t remember either.
Michael: Because you just pull MP… not MPS…
Vincent: APS
Michael: APS. Ammonium persulfate and that’s what goes bad. The binary catalyst.
Nathen's Miraculous Escape 
March 2, 2015 at 7:43 pm
Wow, that’s awesome. I got a version of this story from my own dentist, who suggested that the resin people are basically on the third generation of bacterial evolution. That sounds more or less consistent with what Schmidt is saying.
I hadn’t thought of the question in these terms, but it actually seems like a good salvo in debates about evolution. One of the odd features of that debate is how irrelevant it is to most people’s lived experience. But I would bet that most American adults have a filling or two.
March 3, 2015 at 7:38 pm
That’s one of the things that really blew my mind when I started listening to that podcast. For microbiologists, evolution is a reality they can watch, understand and influence. They can create selection pressures and watch the adaptation happen, or create specific mutations and watch the results. Bacteria may be something of a special case because of how they can share genes (though it might be more accurate to say that it’s the non-bacteria that are the special case) but it would be difficult to follow microbiology and maintain that evolution doesn’t happen.
March 5, 2015 at 2:06 pm
I have spoken to people (like Rosemary Grant) who study evolution in much slower systems than bacteria, and while they can “see” it in their own research lifetimes, it seems clear to me that they can only see it because they are looking for it. It’s an interesting epistemological issue. If you are working in a field where the signal is amplified (in this case microbiology) you are apt notice it even if you don’t have a theoretical framework for it in advance. But in other situations, you can only detect the signal if you are looking for it, and that blurs into confirmation bias.
March 7, 2015 at 1:38 pm
Interesting! What kind of stuff have they “seen”? Is it like changes in genomes that don’t amount to anything visible, or have they found changes in chemistry or structure?