Antibiotic Resistance from Unexpected Sources–Herbicides, Dust and MetalsApril 15th, 2015
by Judy Stone
More disturbing news was revealed this week on new sources of antibiotic resistance in the environment. First, in a troublesome report in mBio, the journal of the American Society for Microbiology, researchers showed that three commercial herbicides—Monsanto’s dicamba (Kamba) and glyphosate (Roundup), and 2,4-dichlorophenoxyacetic acid (2,4-D)—could make strains of Escherichia coli and Salmonella enterica serovar Typhimurium less sensitive to antibiotics. (The response varied with different combinations of antibiotic, herbicide, and bacterial strain).
This is hugely important for several reasons: Herbicides are fairly ubiquitous in the environment. Glyphosate (Roundup) has been found in the milk and meat of cows, and in human urine. According to German researchers, “Glyphosate residues cannot be removed by washing and they are not broken down by cooking. Glyphosate residues can remain stable in foods for a year or more, even if the foods are frozen, dried or processed.” Thus, there is great chance for interaction of herbicides with antibiotics. Interestingly, Roundup alone had once been considered as an antibiotic, but resistance was found to develop rapidly. Dr. Jack Heinemann, the study’s lead author and professor at the University of Canterbury in New Zealand explains that while a bacteria alone might have been killed by an antibiotic, when exposed to an herbicide, a resistance gene is turned on, in effect “‘immunizing’ the bacteria to the antibiotic.”
An additional problem is that some weeds have become resistant to Roundup. New genetically modified soybeans and corn have been engineered by Dow to resist the “Enlist Duo” herbicide, which is a combination of glyphosate and 2, 4-D. Monsanto also is looking to sell soybean and cotton GMO varieties resistant to both herbicides.
Herbicides don’t stay in the field or on your manicured lawn. They are tracked into homes from treated lawns as well as commercial agricultural application, and they can linger in carpets. Pets pick up the herbicides as well.
Antibiotic use has been soaring, with 80% of antibiotics being used in livestock production. In 2010, an estimated 63,151 tons of antibiotics were consumed by livestock; this is projected to increase by 67% by 2030. While there have been some recent inroads here, lobbying has kept the efforts of Rep. Louise Slaughter (D-N.Y., and the only microbiologist) to mandate restrictions on agricultural antibiotic use from being implemented.
Another new study just showed that antibiotic-resistant bacteria are being spread from cattle yards through dust laden with excrement. Given the winds and droughts in some areas, dust bowls could spread the resistant bugs for hundreds of miles. The Proceedings of the National Academy of Sciences (PNAS) study estimates the mass of particulate matter that can be inhaled, released from cattle yards in Colorado, Kansas, Nebraska, Oklahoma and Texas, exceeds 46,000 lbs. (21,000 kg) per day. Some of the antibiotics dispersed this way may stay active in the environment for weeks.
This bad news about antibiotic-resistant bacteria in the environment follows other earlier bad news on related factors. For example, these resistant bacteria have been shown to surround hog farms, both in the U.S. and in China, and are readily disseminated from there. Dr.Tara Smith, a Kent State microbiologist and epidemiologist and others have found that workers on hog farms carry a resistant Pig MRSA.
Another study showed that heavy metals, added to feed as growth promoters, can also select for antibiotic resistance. Pollution with heavy metals—even at very low levels— can promote bacteria with multiresistance plasmids (small bits of extrachromosomal DNA). This combination can fuel resistance, as the antibiotics can act synergistically with the heavy metals in selecting for resistance. In this Swedish study, investigators examined an extended-spectrum beta-lactamase (ESBL) plasmid from a hospital outbreak of Klebsiella pneumoniae and Escherichia coli. This plasmid carried resistance not only to beta-lactam and other antibiotics, but also to quaternary ammonium compounds, copper, silver, and arsenic. This is important as copper and silver are being explored for their properties that might treat infections without antibiotics and “quats” are a commonly used hospital disinfectant. Resistance to these metals has not yet been a problem.
These studies are important and relatively novel, as they look at multiple factors in the environment in combination, rather than at each isolated factor, and demonstrate the need to do so, looking at the interaction of antibiotics, heavy metals, and other waste products.
A few months ago I learned about the huge amount of antibiotics dumped into wastewater by drug companies, especially in India. As I explained here, this, combined with extraordinarily poor sanitation, is likely to greatly fuel the transfer and emergence of resistance genes.
Last week, Olya Keen, Ph.D., of the University of North Carolina at Charlotte, presented data at the American Chemical Society about the effects of chlorine on wastewater. Not only does the treatment process not remove antibiotics, but in the lab, chlorine can combine with antibiotics, changing its antibacterial activity and making new compounds. Keen used doxycycline for her tests. It is not yet known how other antibiotics might be affected by prolonged exposure to chlorine, but Keen is concerned that this might be another mechanism promoting antibiotic resistance. Keen recommends collecting and incinerating antibiotics, rather than dumping them into wastewater.
So it has been a rather gloomy two weeks in the land of antibiotic resistance. All this is occurring at a time when there are more than 2 million infections and 23, 000 deaths from antibiotic resistant organisms in the U.S. annually and pharmaceutical companies are hardly investing in drug development for new antibiotics.
On that note, I’ll leave you with Tom Lehrer’s advice:
“Just two things of which you must beware,
Don’t drink the water and don’t breathe the air.”