I like dabbling in bio, so I keep abreast of recent developments by reading Nature and Science. One article in particular caught my eye the other day–George Church’s “Bacteria Subsisting on Antibiotics” in Science (April 4, 2008, Vol 320, p. 100).

The common wisdom is that “superbugs” — antibiotic resistant bacteria — are being bread inside humans who don’t finish their full course of antibiotics. The theory is that when you don’t finish your full course of antibiotics, you only weaken them, killing off the ones most susceptible to antibiotics: the remaining few were the ones most resistant to antibiotics. If these remaining bacteria cause you to relapse, the new infection will have a greater resistance to antibiotics. Repeat this process a few times, and you are the culture dish for evolving antibiotic resistant bacteria. Clearly, the solution to this problem is to just make sure we all take our antibiotics to the end of its course. Or is it?

The interesting part about Church’s report is that the bacteria commonly found all around us in the soil has a high chance of being resistant to every known antibiotic; and not only do they resist them, they can use these antibiotics as a food source! They are “ultimate superbugs”. The obvious question is, why haven’t these just taken over and killed every human? [Note: the rest is all my speculation, and not part of Church’s report…] The answer probably lies along several reasons. Typically, soil-based bacteria doesn’t grow well in human hosts; however, it was noted in the article that several strains of resistant bacteria are close relatives to human pathogens. So maybe that’s not the reason. My thought is that antibiotic resistance requires the bacteria to spend extra energy and resources, so when left in a nutrient-rich environment — like the mucous lining of your sinus — they are out-reproduced by the more nimble, but less robust human pathogens. Since bacterial reproduction happens on an exponential curve, even tiny extra metabolic costs add up to a huge disadvantage in the end. Anyone who has financed a mortgage is aware of how a change of a few fractions of a percentage compound interest per year can add up to a lot over many years!

So, I guess that’s good — the superbugs aren’t winning yet. However, the remaining threat is that bacteria are very promiscuous. They will acquire or exchange DNA under a large number of conditions, including changes in heat, pH, and electric current, as well as viral vectors. My thought is that human pathogens could “acquire” genomes from their resistant soil-based kin when they mix together, and that the slow-growing but long-lived soil based bacteria are acting like a genome archive where useful but expensive bacterial genes are stored. The problem with this theory, of course, is that when the human pathogen acquires the resistance genes, they reproduce slower than those that don’t, so they eventually go extinct, probably before they can infect a human host.

But there’s one other factor that’s missing. A lot of antibiotics used on humans and animals are excreted through urine, feces, and sweat. These antibiotics are concentrated in sewage and released into the environment — into the soil. The presence of these antibiotics, even in small quantities, combined with the genetic archive stored in soil bacteria, could be enough to bias natural selection to favor the bacteria that have acquired the antibiotic resistance genes, thus providing a natural environmental reservoir for the breeding and storage of superbugs.

Think about it: the mere prescription of an antibiotic may ultimately lead to environmental bacteria acquiring a resistance to them, and no amount of care or attention on the part of you and me in finishing our antibiotic courses can prevent this.

That being said, it’s all just speculation on the part of someone who’s really an electronics hacker and not a biologist, so I wouldn’t go sounding any alarms. But it is interesting to think about the role of environmental DNA and the evolution of species; it may be one of those rule-changing disruptive concepts. I’ve been reading about how sea water contains lots of DNA that codes for all kinds of interesting genes, and how our DNA contains lots of “junk” DNA introduced by viruses, etc. Maybe there is more to evolution and genetics than just simple random mutation and how genes are selected from a pool defined by only those found in the parents. With the incorporation of environmental DNA, totally random, unexpected whole genes can be introduced by the environmental library, absent of any parent. Furthermore, genes that fall out of favor (become “extinct”) due to external changes can be archived in this environmental library and brought back into service at a later time, so evolution, at least for simple organisms like bacteria, might not be a simple linear progression.

Also, in the same issue of Science, there is a snazzy article titled “Single-Molecule DNA Sequencing of a Viral Genome”. Really, really clever stuff going on in there that probably has application beyond just DNA sequencing; if you have an interest in nanotechnology or single-molecule data storage/manipulation/retrieval it’s worth the read.

This entry was posted on Sunday, April 13th, 2008 at 5:16 am and is filed under Biology, Ponderings. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.