How Do Antibiotics in Agriculture Affect Human Clinical Infectious Diseases?

Nov. 15, 2018

World Antibiotic Awareness Week is a week dear to the hearts of all microbiologists. Antibiotics play a direct role in almost all research studies (even fungal and viral studies rely on antibiotics, if only to prevent bacterial contamination). More importantly, antibiotics affect all sectors of microbiology: from the obvious application to clinical infectious disease to the preventative use in livestock to the effect of runoff antibiotics on the soil microbiota, these small molecules play a big role in the microbial world. 

The growing issue of antibiotic resistance means that the drugs that once were effective treatments for infectious diseases are now sometimes ineffective. This is largely due to resistance acquired through genetic changes that allow bacteria to counteract the drug effects. Widespread drug-resistant infections have many terrible effects, including 23,000 deaths annually in the U.S., and are in part due to the overuse of antibiotics—but not only where you might expect it. 
 
Agriculture has historically been one of the common avenues for dissemination of antibiotics into both health and environmental systems. For decades, low levels of clinically important antibiotics were given to promote animal growth, a practice that the Food and Drug Administration recently curtailed. Constant exposure to subinhibitory drug concentrations selects for resistant bacteria in both in vitro and in vivo experiments.

While some countries still permit nonmedical use of antibiotics in livestock animals, the United States lagged behind other countries, such as Denmark, in regulating their use:

 

How Do Antibiotics in Agriculture Affect Human Clinical Infectious Diseases?

Farm animals require caretakers. Because of their exposure to animals harboring drug-resistant microbes, animal caretakers are often at higher risk of becoming colonized with resistant bacteria, such as drug-resistant Salmonella or methicillin-resistant Staphylococcus aureus (MRSA),than the rest of the community. This isn’t a new discovery; scientists first described transfer of resistance plasmid-carrying bacteria from chickens to people in the 1970s.

Similarly, scientists have long associated drug-resistant infections in people with strains circulating in livestock. These associations have been made in patients suffering gastrointestinal disease, which might arise from consuming undercooked meats, but also in patients with urinary tract infections (UTIs). The first U.S. E. coli isolate with resistance to the last-resort antibiotic colistin, and which was also resistant to aminoglycosides, β-lactams, chloramphenicol, fluoroquinolones, rifampin, sulfonamides and tetracycline, was found in a UTI. This isolate illustrates another danger of drug-resistant infections: strains can harbor multiple resistance genes, making it difficult to quickly determine the best course of therapy.
 
What is new is the ability to directly connect bacteria causing disease in humans to their livestock source. While pulsed-field gel electrophoresis was previously used to demonstrate similar genetic patterns between strains, scientists working in the genomic era can trace an isolate’s history by tracking every nucleotide difference. This technology allowed scientists to more directly demonstrate that an E. coli strain causing UTIs in people had originated in chickens, rather than alternatively having jumped from people into chickens. Strains jumping into new hosts offer 2 dangers: the possibility of causing a variety of diseases, including those more life-threatening than UTIs, and the possibility of introducing new resistance genes into the human microbiota population.
 
Stay tuned to later installments of our World Antibiotic Awareness Week blog posts, where we'll address the questions, "what happens when antibiotics stop working?" and "what can we do to stop the spread of drug-resistant infections?"

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Author: Julie Wolf

Julie Wolf
Dr. Julie Wolf is in science communications at Indie Bio, and was a former ASM employee. Follow Julie on Twitter for more ASM and microbiology highlights at @JulieMarieWolf.