(Beyond Pesticides, July 12, 2017) In a move that threatens to further the spread of antibiotic resistance, researchers at the University of Western Australia (UWA) mining a collection of antimalarial drugs for potential new herbicides. Joshua Mylne, PhD, a senior lecturer in the School of Molecular Sciences (SMS) who directs the project, got started based on work he did while in the Australian Army Reserve. There he discovered that “malarial parasites are actually closely related to plants.” Due to widespread resistance of weeds to the popular herbicide glyphosate, Dr. Mylne began investigating antimalarial drugs as potential replacements.
Malaria parasites are actually protozoans in the genus Plasmodium. Their crucial connection with plants is that the parasite contains a plastid similar to the chloroplast in plants. Like the chloroplast in plants, this plastid is critical for the survival of the parasite.
Along with organic chemist Keith Stubbs, PhD, associate professor in SMS, Dr. Mylne began screening chemicals in the “Malaria Box,” an open source collection of potential anti-malarial drugs that have never been commercialized. Of the 20 chemicals tested on the small weed Arabidopsis thaliana, 11 were found to have some herbicidal activity. They were then compared to the herbicides glyphosate, glufosinate, asulam, and atrazine. The researchers found that ciprofloxacin (LD50 = 0.45 µg/mL), clindamycin (LD50 = 0.9 µg/mL), sulfadiazine (LD50 = 0.86 µg/mL), and sulfadoxine (LD50 = 1.29 µg/mL) had greater herbicidal potency than glufosinate and glyphosate.
Ironically, the herbicide these researchers seek to replace –glyphosate— is also an antibiotic with antimalarial activity. As with glyphosate, an issue that has yet to be addressed with these potential new herbicides is the extent to which the broadcasting of an antibiotic over agricultural fields promotes antibiotic resistance.
The spread of antibiotic resistance is a health care crisis of major proportions. The Centers for Disease Control and Prevention (CDC) call it “one of the world’s most pressing public health problems.” Many bacterial infections are becoming resistant to the most commonly prescribed antibiotics, resulting in longer-lasting infections, higher medical expenses, and the need for more expensive or hazardous medications, and inability to treat life-threatening infections. The development and spread of antibiotic resistance is the inevitable effect of the use of antibiotics. Microorganisms evolve quickly, and antibiotics provide strong selection pressure for those strains with genes for resistance.
The use of antibiotics in agriculture can contribute to resistance to the antibiotic in human pathogens. The human pathogenic organisms themselves do not need to be sprayed by the antibiotic because movement of genes in bacteria is not solely “vertical” –that is from parent to progeny– but can be “horizontal” –from one bacterial species to another. So, a pool of resistant soil bacteria or commensal gut bacteria can provide the genetic material for resistance in human pathogens.
Herbicide resistance is a serious problem because it results in increased use of hazardous chemicals when growers rely on chemical to meet their management objectives. Soil fertility depends on microbial life in the soil. Spraying a chemical that is toxic to soil microbes is counterproductive. Organic no-till is an example of a successful non-chemical approach to weed management that also builds fertile soil with healthy microbial life.