If I were to define myself professionally, I’d say I am equal parts microbiologist, molecular biologist, and biochemist. I obtained my doctorate in biochemistry in 1986, working on the genetic instability and antibiotic resistance of bacteria. Since then, I’ve studied bacterial pathogenesis, vaccine biotechnology, and more recently, the biochemistry of iron uptake by Escherichia coli and Listeria monocytogenes, two potentially lethal bacterial contaminants of food. As a result of these experiences, I have strong opinions about the use of antibiotics in food production.
Watching the news this week, I saw a pig farmer from my home state (Oklahoma) defending the practice of feeding pigs antibiotics to improve their health and weight. I’d love nothing better than to invite that man over for a cup of coffee and a conversation, so I could ask him what exactly he knows about the use of antibiotics. For example, has he ever heard of the term “plasmid?” What about you? If you care to read about it, then follow me to the next page….
Bacteria possess efficient and sophisticated responses that block antibiotic action, which existed long before we “discovered” them (in 1929), before we learned to produce them (in 1943), and before we created new, more potent derivatives, sometimes with a broader spectrum of microbial targets. Just like any living organism, bacteria organize their DNA in a chromosome, usually a single, large molecule that contains all the genes for survival and replication (in E.coli, that’s almost 4,500 genes). But, bacteria often possess additional DNA, much smaller and not part of the chromosome: this DNA is in the form of plasmids.
Keep in mind that the cartoon is not to scale: the chromosome is huge, and very tightly packed. Plasmids are a few thousand times smaller, but they may be present in high numbers, as much as hundreds of copies per cell. Plasmids usually contain genes that are not needed in “normal” situations, but are useful in adverse situations. Indeed, in the 1960’s scientists found that the genes that confer antibiotic resistance are often located on plasmids. This fact gives the microbes an added bonus, because plasmids may be transferred from one bacterium to another. A conjugative plasmid sends a copy of itself to other bacteria, in a very cool mating process. Yes, even these tiny unicellular creatures are into sexual activities, if the conditions are just right for it… ;-)
Since the first antibiotic – penicillin – was discovered (in a very fortuitous way by Fleming), we’ve witnessed incessant evolution in the microbiological world: every new weapon designed by humans to eliminate bacteria is soon followed by the development of drug resistance, and as new resistance genes arise, they often associate together to form larger and larger plasmids, containing multiple drug resistance genes. You can read about a carefully studied example by clicking here.
At first, scientists were a bit puzzled by resistance genes ending up all neatly organized in plasmids, but later it became clear that these genes efficiently move among bacterial populations, literally “jumping around” from one place to another. They are classified as “transposons“, which gives an intuitive idea of what they do: they transpose themselves to new locations, while leaving the original copy of the antibiotic resistance gene in place.
Back to food. When farmers treat animals (like pigs) with antibiotics to produce more meat and “healthier” animals, they create strong selective pressure on the bacteria in the pig stomachs and on their skins, so that rare, antibiotic-resistant variants will arise, survive and take over the population. It’s only a matter of time. Many of these bacteria are potentially pathogenic in humans, and they will have plasmids that can be transmitted to other bacteria, including bacteria in the human digestive tract. So, even if the bacteria are themselves non-pathogenic, they may pass their antibiotic resistance to more harmful microbes by conjugation or other processes. Besides that, the antibiotics are eliminated in the animals waste, and end up permeating the soil, ultimately draining into rivers, lakes and reservoirs used for drinking water and irrigation of food crops in the surrounding regions. So the selective pressure for antibiotic resistance becomes ubiquitous, leading to the development of more and more dangerous bacterial strains.
Here’s some good news: the selective pressure works both ways. Carrying the extra DNA associated with plasmids is a heavy metabolic load for the bacterial cell. If you stop adding antibiotics to animal food, plasmid-free bacteria will have an advantage in growth and replication, and soon take over the population. The upshot of these explanations is clear: the less antibiotics that we use, in food production and in medicine, the better it is for the future of mankind.
Antibiotics as a preemptive measure in food production are unnecessary and driven by obsession with profit. This practice, that has undeniably caused infections in humans with microorganisms that are completely resistant to multiple drugs (click here for a very good article) was already banned in many European countries. Here in the US we witness a huge opposition to responsible action in the right direction. Why? In part because drug companies do not want to lose their profits from drug sales to the food industry. And in part because large-scale farming walks hand in hand with the pharmaceutical industry, hoping to raise their own profits by making meat artificially cheap, so that we eat lots and lots of it, even knowing that the practice is unhealthy. Ironically, if farmers stopped excessive antibiotic use, the price of pork would only increase by a meager 20 cents per pound. Twenty – cents – per – pound. Does that 20 cents a pound justify the threat of infections by antibiotic resistant bacteria?
Michael Pollan wrote two masterpieces on the subject of the American food industry. I highly recommend that if you haven’t read these books, do so as soon as possible. The bottom line is that by producing our food as cheaply as we possibly can, we are affecting the balance of our planet. We are raising cattle, pigs and chickens in horribly crowded conditions so that we can buy cheap meat, chickens for $3 per pound and eggs less than $3 per dozen. I do not advocate turning into vegetarians: my motto has always been “everything in moderation“. But, at some point, we need to stand up for what is right.
The practice of using antibiotics on healthy animals is irresponsible. Don’t let anyone fool you.