Source: MICROBIOLOGY SOCIETY, 11/2/17


Herbicides are frequently released into both rural and urban environments. Commercial herbicide formulations induce
adaptive changes in the way bacteria respond to antibiotics. Salmonella enterica sv. Typhimurium and Escherichia coli were
exposed to common co-formulants of formulations, and S. enterica sv. Typhimurium was exposed to active ingredients
dicamba, 2,4-D and glyphosate to determine what ingredients of the commercial formulations caused this effect. Coformulants Tween80 and carboxymethyl cellulose induced changes in response, but the pattern of the responses differed
from the active ingredients, and effect sizes were smaller. A commercial wetting agent did not affect antibiotic responses.

Active ingredients induced changes in antibiotic responses similar to those caused by complete formulations. This occurred
at or below recommended application concentrations. Targeted deletion of efflux pump genes largely neutralized the
adaptive response in the cases of increased survival in antibiotics, indicating that the biochemistry of induced resistance was
the same for formulations and specific ingredients. We found that glyphosate, dicamba, and 2,4-D, as well as co-formulants
in commercial herbicides, induced a change in susceptibility of the potentially pathogenic bacteria E. coli and S. enterica to
multiple antibiotics. This was measured using the efficiency of plating (EOP), the relative survival of the bacteria when
exposed to herbicide and antibiotic, or just antibiotic, compared to survival on permissive media. This work will help to
inform the use of non-medicinal chemical agents that induce changes in antibiotic responses.

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