By: Heather Williams

A recent article in the American Society for Microbiology explains that antibiotics promote resistance in crops both directly and indirectly.  “Antibiotic resistance is a global threat to human and animal health with bacteria now resistant to the last-resort antibiotics, including carbapenems and polymyxins,” explains Ed Topp, PhD and Principal Research Scientist of the study group. This particular study was conducted by a collaboration of scientists from Agriculture and Agri-Food, Canada, London, and Ontario.  In this study, researchers looked at crops exposed to antibiotics over periods up to 16 years to determine if antibiotic usage created antibiotic resistance in the crops.  The results were overwhelmingly “yes.”  This problem is becoming very big prompting many health organizations to get involved in reducing this practice to minimize harmful side-effects.  The American Medical Association, American Public Health Association, Infectious Disease Society of America, and the World Health Organization are calling for a significant reduction in the use of antibiotics for food animal production.

What is Antibiotic Resistance?

Antibiotics are medicines used to treat and even prevent bacterial infections.  Looking at the name you can understand where it is coming from.  “Anti” meaning against.  “Biotics” meaning bacteria.  Antibiotics literally fight bacteria.  This is a good thing, right?  In moderation and for treatment purposes, yes.  Unfortunately, antibiotics are used frequently outside of the scope of treatment and more in the scope of prevention.  An alarming 80% of antibiotics sold in the United States is used in meat and poultry production, not to treat illness, but to promote growth or prevent disease in overcrowded or unsanitary living conditions.

Antibiotic resistance occurs when the bacteria are exposed to antibiotics over time and develop an immunity to the antibiotics as a life-saving mechanism, creating a super bug that cannot be killed with typical antibiotics. This can occur directly by giving humans or animals antibiotics either ineffectively to treat a bacterial infection or preventatively to avoid bacterial infection or promote growth.  Antibiotic resistance can also occur indirectly by using manure of antibiotic resistant animals to fertilize crops or runoff into another habitat creating additional antibiotic resistant organisms.

Genetically Modifying Organisms?

Many people are familiar with the term genetically modified organism (GMO), where the genetic makeup of a plant or animal is intentionally modified to achieve a particular result.  This could be to make it naturally resistant to certain pests, make the crops grow more consistently to aid in easier harvest, or create larger or faster growing crops and animals.  But did you know that as a result of environmental factors, an organism can genetically modify itself?

An organism can genetically modify itself by environmental factors.  For example, when a bacterial organism is not completely killed by antibiotics it has an opportunity to adapt to what was trying to kill it.  Since bacteria replicate so quickly, several generations of trial and error with varying mutations can occur in a very short time period.  In some cases this means the bacteria develops an efflux pump identified for a specific drug or multiple drugs.  These efflux pumps are embedded in the cell membrane and removes certain molecules from the bacterial cell.  In this case, the efflux pump would identify a particular antibiotic and remove it from the cell before the antibiotic could do its job and kill the bacteria.  There are many other ways that bacteria can use to survive and change when it is exposed to antibiotics but does not die.

What Did Researchers Discover?

Researchers collected soil samples from farm plots that were exposed to antibiotics.  DNA was extracted from the soil and cloned into a strain of E. coli that is sensitive to most antibiotics.  This method uses a particular E. coli that can take in other DNA while maintaining its own DNA.  Since this E. coli is sensitive to antibiotics, it should not grow on an antibiotic media.  Media infused with antibiotics from classes including: macrolides, β-lactams, and sulfonamides was used to grow these cultures. When researchers attempted to grow the composite E. coli on the antibiotic infused media, they found growth.  “Any colonies that grew would presumptively contain a cloned fragment that conferred resistance,” said Topp.  “We obtained a total of 36 antibiotic resistance genes from the antibiotic-resistant E. coli.”

Researchers sequenced the colonies that grew and discovered that some of the resistance genes were similar or the same as known resistance genes.  But in a few of the colonies that grew on the antibiotic media, sequencing exposed new genes unlike any known resistance genes.

“Results from these studies will establish the risk of crop contamination, potential enrichment of antimicrobial resistance in environmental bacteria, and effects of antibiotics on non-target micro-organisms that underpin many of the services provided by ecosystems,” said Topp.  He cites that if runoff from animal manure or pharmaceutical manufacturing plants are creating antibiotic resistant bacteria in the environment, it may justify treating those waste streams prior to being used on crops.

Pro Antibiotic Rationale

In a letter to Congressperson Slaughter in 2012, the American Farm Bureau Federation explains that “animal use contributes little, if anything, to the burden of human antibiotic resistance.”  Those who use antibiotics on farm animals or on crops claim that antibiotics are a way to maintain healthy stock and has little to no negative impact on the environment or people.  The industry argues that what happens on the farm has no impact outside of the farm.  Many do not deny that some resistant bacteria may be there, but claim that they do not affect people.  The industry also says that 40% of the antibiotics that are used are in the ionophore class and not used in human medicine, so bacterial resistance does not affect humans.

Anti-Antibiotic Rationale

Those opposing the routine use of antibiotics explain that the resistant bacteria do end up leaving the farm.  This can be from a direct route from resistant bacteria on the meat that will be consumed by humans.  Or indirect route through runoff of water exposing other environments to the resistant bacteria.  Those who oppose antibiotic use explain that while the resistant bacteria in the environment may not always create an immediate illness, they are equipped to exchange genetic material through plasmids spreading the resistance to other bacteria.

Farm employees exposed to resistant bacteria may also become sick and spread the resistant bacteria or track the resistant bacteria out of the farm on their shoes or clothing.  This directly impacts people and environments that the employee comes in contact with.

Historical Study

In 1995, the FDA approved the use of fluoroquinolones for animal use.  This class of antibiotics includes ciprofloxacin, also known as Cipro.  By 1999, studies indicated that 20% of the sampled chicken breasts contained ciprofloxacin-resistant Campylobacter. By the time it was fought in courts and the FDA banned the drug, in 2005 the statistics rose to 30% containing ciprofloxacin-resistant bacteria.  The study was repeated five years later in 2010 and ciprofloxacin-resistant bacteria found in sampled chicken breasts had dropped to 13.5%.

What Do You Think?

The routine use of antibiotics in farms is a continued debate, with legislation going back and forth.  What are your thoughts on the outcome of use of antibiotics and their contribution to resistant bacteria?

For more information on Antibiotic Resistance, you can visit our latest series starting here.

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