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Antibiotic Resistance Found in Insecticides Commonly Used in California

August 18, 2021
Professor Mark Sistrom
Professor Mark Sistrom. Photo by Veronica Adrover

The over the counter, “safe,” organic-compliant insecticides people purchase at home-improvement stores could be causing a problem that goes far beyond the vegetable garden or farm field — antibiotic resistance.

A new study from evolutionary geneticist Professor Mark Sistrom, postdoctoral researcher Mo Kaze and their colleagues examines Bacillus thuringiensis (Bt), a soil-dwelling bacterium that produces a toxin that is fatal to certain herbivorous insects. That toxin has been used as an insecticide for about 100 years and is common in organic farming and as a replacement for dichlorodiphenyltrichloroethane (DDT). Bt is also the source of genes used to genetically modify some food crops to produce the toxin on their own to deter pests.

More than 8 million pounds are sprayed commercially in California every year.

Bt is closely related to a bacterium that causes food poisoning and another that causes anthrax, which is also soil-borne.

“You can spray Bt pesticides out of planes, and they do it all the time over the Central Valley,” said Sistrom, who’s with the Department of Life and Environmental Sciences in the School of Natural Sciences and is a member of the Health Sciences Research Institute. “If the Bt being sprayed carries drug resistance on mobile genetic elements, it could spread that resistance to human pathogens living in the soil.”

Because Bt colonizes in humans, it can make many kinds of bacterial infections drug resistant.

“It is generally not considered a contributor to antibiotic resistance,” Sistrom said.

Mo KazeHe, Kaze and their fellow researchers discovered the resistance accidentally while trying to culture a Staphylococcus aureus from wastewater. They noticed the resistance and, in looking closer, found that the bacteria in these readily available products are resistant to multiple antibiotics and carry genes encoding resistance to multiple antibiotics.

Theirs is the first to explicitly look for antibiotic resistance in these products, which are classified as safe by the FDA and organic by the USDA.

“It raises the possibility that they can spread these genes to other bacteria — including pathogens — and identifies their use as a previously unidentified activity that may spread antibiotic resistance, particularly in farmworker communities,” Sistrom said.

He and his colleagues sequenced the genomes in the Bt bacteria and found they are resistant to several antibiotics, including tetracycline, ceftazidime, cefazolin and ertapenem.

Now they want to develop strains of Bt using genes that cannot acquire resistance to antibiotics or share their resistance with other bacteria by using CRISPR technology, which is a inexpensive and relatively easy way of finding a specific bit of DNA inside a cell and altering it.

Kaze also has another goal.

“Multiple agricultural processes are routinely monitored and assessed because of their contributions in the generation and spread of antibiotic-resistant bacteria,” she said. “My hope is that our results will lead to the inclusion of live bacterial biopesticide spraying to the list of practices being surveilled. Large-scale live bacterial biopesticide use should also be considered when evaluating the health conditions of farm and feedlot laborers.”