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Resistance seen among certain common herbicides, prescription drugs

Similarities exist between resistant weeds and drugs Continued, widespread use is catalyst for resistance New products, management systems needed

It’s an alarming pattern affecting both agriculture and human medicine, say two experts.

On one hand, farmers are left scratching their heads wondering why certain weeds have become resistant to common herbicides; on the other, physicians across the U.S. are seeing patients becoming resistant to common, prescription antibiotic drugs.

In both farming and human medicine, traditional tools used to solve routine problems may no longer be as effective as they used to be.

“In both agriculture and medicine, we may have been using too much of a good thing and it’s catching up with us,” said Dr. Paul Baumann, a Texas AgriLife Extension Service state weed specialist.

For example, Baumann said glyphosate (Roundup and other products) is highly effective, “because it binds a specific enzyme that’s needed to produce plant proteins.”

“It is a highly effective herbicide that controls a large number of weeds and can be used safely in crops that have glyphosate-resistant genes,” Baumann said.  “These positive features have led to continued, widespread use and in many cases as the only herbicide in the program.  This has been the foundation for what we are seeing as weed resistance in some parts of Texas, but predominantly in the Southeastern U.S.”

Herbicide-resistant weeds such as Palmer amaranth began to pop up in Georgia cotton fields in 2004 and have since continued to escalate due to the repetitive use of glyphosate herbicide and nothing else.

Mode of action

“If it is only killing weeds one specific way, eventually there’s going to be a genetic anomaly that will show up that is not sensitive to the herbicide,” Baumann said.

Once this happens and nothing is done to control it, seed production will spread it all over the field, he said.

“The key to all of this is the development of new chemistries that have different sites of action or simply the use of other products that have a different site of activity in the plant,” Baumann said. “In years past, we have used three or four herbicides that had different modes of action and attacked different sites in a plant.”

Meanwhile, Dr. Tom Wagner, clinic director of Scott and White Arrington Road Clinic in College Station, said in the 1960s, treatment of various skin infections would involve “a garden variety of penicillin.”

“But in the late 1960s, patients developed penicillin resistance and synthetic penicillins followed which that worked for a while,” he said. “Subsequently, the issue of antibiotic over-utilization emerged. This resulted from a combination of several factors. Medical providers would inappropriately prescribe an antibiotic for the common cold. At times, patients would pressure a provider in the exam room for antibiotic therapy when any nasal congestion, sore throat or cough was present.”

Antibiotic resistance

Rather than always taking the extra time in a hectic schedule to educate the patient between the common cold and sinus infection, Wagner said a doctor or provider would commonly prescribe an antibiotic.

“Once antibiotic therapy was started, in many instances, the patient would start to feel better and would stop the antibiotic before it was all completed.”

That’s led to the latest challenge in overcoming a multi-antibiotic resistant staph aureus (bacterium) called MRSA.

“I see this in my clinic nearly every day,” Wagner said. “It’s very difficult to treat. When you eliminate penicillin, erythromycin and cephalosporin classes, you are limited in your antibiotic options. For those not allergic, we’ve resorted to using an older sulfa antibiotic to treat MRSA because of these limitations.”

Just like glyphosate, Wagner says the synthetic penicillin worked for a while, but over time “it couldn’t easily reach the site of activity.”

“The staph’s penicillin-binding protein lowered its affinity, creating a layer of protection,” Wagner said. “Thus, the penicillin antibiotic doesn’t fully penetrate.”

“It’s very analogous to the way herbicides work,” Baumann said. “The site of activity for an herbicide may be altered in resistant species, causing the herbicide to be ineffective. For example, in an acre, there may be one seed in millions that has an altered binding site. When you control 99 percent of all of the other targeted weed species, the genetic anomaly flourishes and all heck breaks loose.”

Management systems

In the meantime, while research continues to develop new tools for fighting the problems, both Baumann and Wagner offer advice to their respective audiences in the farming and medical fields.

“From an agriculture perspective, that being farmers, we need to go back to some of the old chemistry or use new products with different sites of  action than the product in question," Baumann said. “We need to employ preventive instead of remedial approaches because remedial, post-emergence products may not be available to you. Rotation of herbicides is one recommendation. Or, at least use an alternative site of action herbicide somewhere in the program, in conjunction with glyphosate products to pick up the resistant biotype.”

Meanwhile, Wagner says both patients and medical doctors need to work together with treatment protocols involving both pharmacological and non-pharmacological measures.

“The first few days of nasal or sinus congestion may resemble a sinus infection, but more likely it is a viral upper airway infection,” he said.

“More times than not, patients are better off waiting seven to 10 days. Usually this type of viral infection improves over this time span.”

If the patient’s symptoms persist or worsen, consider antibiotic therapy at that time.

“By the same token, all medical providers need to take time in the exam room and educate their patients,” Wagner said.  “The common cold is caused by a virus which resolves on its own, and a sinus infection has a bacterial etiology and improves with antibiotic therapy.  Furthermore, there are many preventive practices a patient can employ to minimize the risk of contracting MRSA infection.”

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