Antibiotics in Meat Do Lead to MRSA in Humans

I was extremely disturbed to see in the NYT's letters a veterinarian's defense of the practice of overuse of antibiotics in animals that suggested transmission of resistant organisms does not occur. Nonsense! It is abundantly clear that antibiotic use in animals results in resistant strains that then colonize humans. They are being recognized as the newest reservoir for strains of MRSA.

Unlike the GMO nonsense, this is a clear public health issue with a plausible (and demonstrated) mechanism of transmitted risk to humans. The author of the letter, Charles Hofacre, says two, wildly misleading things. For one, he suggests the antibiotics they are using are somehow substantively different from those in humans by saying, "About a third of livestock antibiotics used today are not used at all in human medicine." Well, that means 2/3rds are the same and just because we don't use the exact same antibiotics doesn't mean they don't share the exact same mechanism. If he's trying to suggest resistance in livestock antibiotics isn't relevant to human pathogens, he is just wrong, wrong, wrong. Second he says, "There is no proven link to antibiotic treatment failure in humans because of antibiotic use in animals for consumption — a critical point that is often missed. " This is such a misleading statement I can't believe an academic would say such a thing, as it assumes we're just idiots. This suggests that there is not a transmission issue, or at least none of clinical relevance. But this is also wrong. There is extensive documentation of Methicillin-resistant Staph Aureus (MRSA) becoming more common in livestock, being transmitted to humans, and appearing in hospitals. There hasn't been a "treatment failure", because we still have antibiotics that work against MRSA, and MRSA is usually not pathogenic on its own without some failure of the host immune system, broken skin/non-sterile injection, surgery, chemo, etc. That doesn't mean we should go around spreading MRSA! We have to start taking out the big guns to deal with MRSA infections when they do occur (we don't treat colonization), and the more we expose these bacteria towards the better antibiotics, the more we'll train them for resistance to those drugs. But it should be made clear, the transmission of resistant bacteria from farm animals to humans has been documented, just because the patients didn't die doesn't mean that there's no problem here. This is just shameful.

Antibiotic resistance has existed since before we even used antibiotics and will only get worse the more we train the organisms to grow in the presence of antibiotics. These genes for resistance aren't "new", but not all bacteria carry them because there is an energy cost associated with production of proteins, and if it doesn't benefit their survival, those bacterial strains wasting energy will become less common. If we constantly create a selective pressure on bacteria to maintain resistance genes, we are going to increase the proportion of bacteria that carry resistance, and thus the resistant organisms we are exposed to. Then, as we have to use more and more powerful antibiotics to address resistance, we create additional selective pressure on the organisms to carry more and better resistance genes (not all beta-lactamases are created equal), and as they mutate to become more effective, those effective resistance strains will eventually mutate into bacteria for which we have no therapeutic option. These are already starting to emerge as those who followed reports of the MDR-klebs outbreak at NIH know.

In my GMO thread I used the analogy that the beta-lactamase used for genetic modification of organisms by molecular biologists is like a "sharpened stick" it can use against weaker penicillins. This is why those resistance genes aren't a danger for humans. They've been around forever anyway, all the bacteria that are going to carry them already do so we don't even bother using weaker penicillins on those types of infections, and they can't beat our stronger beta-lactam drugs like the anti-staph and extended-spectrum beta lactams. The multiple-resistance and pan-resistance bugs that we are finding in our ICUs are the "multiple nuclear warhead" bugs because they beat multiple classes of drugs as well as our extended-spectrum drugs. We've created these bugs by the steady application of selective pressure with exposure of the organisms to progressively more powerful antibiotics. The continued injudicious use of antibiotics in animals will invariably lead to the same phenomenon, just all over the place in communities and the workplace rather than just in the ICU. We are going to see a higher prevalence of resistant bacteria, those bacteria will mutate their resistance genes to become more and more effective, they're already crossing over to humans and hospitals, and we're going to have to use our big guns more which will speed up the loss of our antibacterials' efficacy.

Some caveats. One, this represents more of a threat for farm workers than consumers, as MRSA is not carried in the meat itself, although it will likely contaminate the meat at higher frequency (this has indeed been shown) as the prevalence increases from slaughterhouse contamination. MRSA usually colonizes the outside of the animal, the nares, etc., not the inside of the animal. Two, standard practices of food handling will also decrease, but not eliminate our risk. Cooking meat and washing hands with soap after meat handling (which should be your standard practice) kills MRSA. Don't prepare hamburger then pick your nose people. Clean surfaces on which meat has been prepared etc. However, the packaging, your cutting board, your trash can, all are likely to get contaminated if the meat was surface contaminated. Three, realize MRSA is not pathogenic in normal healthy people. But, something as simple as a cut can introduce staph and create a serious infection. Staph is everywhere, and the human body generally has no problem handling it. But when those defenses are down, MRSA reduces our therapeutic options. You don't want that. Fourth, this is just one bug we may be exposed to, we're also training the animals e. coli and enterobacter to become resistant too, and with poor food prep and exposure, you can get colonized with these bugs as well.

From a public-health standpoint it's important that we reduce the prevalence of resistant bacteria we're exposed to, so fewer of our infections will require the big-gun antibiotics. There is good news though, and we shouldn't just develop a fatalistic attitude towards this problem. As we stop the overuse of antibiotics, selective pressure on the bacteria will cause some of them to shed the resistance genes, and there won't be a reason for the bacteria to maintain and improve their antibiotic resistance genes. Without consistent exposure to antibiotics, they have far less selective pressure to produce proteins and maintain plasmids that provide them no advantage. While the resistance genes will still be out there (always have, always will), we can still benefit from common-sense measures that decrease their prevalence, and thus our individual risk of exposure to resistant organisms. And, the less we have to take out the big guns to treat infections, the fewer multiply-resistant organisms we'll see.