Sublethal Exposure to Commercial Formulations of the Herbicides Dicamba, 2,4-Dichlorophenoxyacetic Acid, and Glyphosate Cause Changes in Antibiotic Susceptibility in Escherichia coli and Salmonella enterica serovar Typhimurium

DOI: 10.1128/mBio.00009-15

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FIG 1
Killing curves. On the x axis, antibiotic concentrations (μg/ml) are plotted. Blue bars, no herbicide; orange bars, herbicide present. Herbicide concentrations used were as follows: for Kamba, 1,830 ppm/1,950 ppm (E. coli/S. Typhimurium); for 2,4-D: 1,830 ppm/1,950 ppm; and for Roundup, 1,240 ppm/1,240 ppm. Error bars are standard errors of the means (SEM). Asterisks indicate P values (see Materials and Methods for details). *, P < 0.05; **, P < 0.01; ***, P < 0.001; ns, not significant.
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FIG 2
Dose-response curves. Antibiotic concentrations used (in μg/ml) were as follows: with 2,4-D, Amp at 4/40 (S. Typhimurium/E. coli), Cip at 0.003/0.003, Cam at 4.4/-, Kan at 6/2.5, and Tet at 0.75/-; with Kamba, Amp at 2/0, Cip at 0.03/0.05, Cam at 4/20, Kan at 2/1, and Tet at 0.75/3; and with Roundup, Amp at -/20, Cip at 0.05/0.07, Cam at 2/10, Kan at 12/10, and Tet at 0.45/1. Error bars are SEM, and asterisks indicate P values (see Materials and Methods for details). *, P < 0.05; **, P < 0.01; ***, P < 0.001; ns, not significant; -, not tested.

TABLE 2

Fold changes in antibiotic concentration to cause a 1,000-fold change in the EOP in the presence of a herbicide

Antibiotic	Herbicide	Fold change in antibiotic concn with:
Antibiotic	Herbicide	E. coli^a	S. Typhimurium
Amp	Kamba	0	2.33
	2,4-D	1.5	2
	Roundup	NA	0
Cam	Kamba	2	2.2
	2,4-D	0	2.33
	Roundup	1.5	2.5
Cip	Kamba	1.66	2.66
	2,4-D	1.66	1.66
	Roundup	1.8	5.8
Kan	Kamba	2.5	2.5
	2,4-D	1.5	1.2
	Roundup	NA	5
Tet	Kamba	2	3.33
	2,4-D	1.66	2.5
	Roundup	3	1.66

↵a NA, not applicable (the EOP did not drop below 0.001 at the highest tested concentration).

TABLE 3

Influence of PAβN exposure on Kamba- and Roundup-induced tolerances in E. coli^a

↵a Values are means of results from 3 independent experiments (SEM). Cam was at 10 μg/ml, Kan was at 5 μg/ml, Kamba was at 1,380 ppm, Roundup was at 1,250 ppm, and PAβN was at 25 mM.
↵b Below the detection limit.

TABLE 4

Strains of E. coli and S. enterica serovar Typhimurium used

Figure S1
Results of blind repeats of the killing curves. On the x axis, antibiotic concentrations (μg/ml) are plotted. Blue bars, no herbicide; orange bars, herbicide present. Herbicide concentrations used were as follows: Kamba, 1,830 ppm/1,950 ppm (E. coli/S. Typhimurium); 2,4-D, 1,830 ppm/1,950 ppm; and Roundup, 1,240 ppm/1,240 ppm. Data are the averages from two independent experiments conducted in duplicate. Download Figure S1, PDF file, 0.5 MB.
Copyright © 2015 Kurenbach et al.
This is an open-access article distributed under the terms of the Creative Commons Attribution-Noncommercial-ShareAlike 3.0 Unported license, which permits unrestricted noncommercial use, distribution, and reproduction in any medium, provided the original author and source are credited.
Table S1
Conversion of ppm to ppm ae of herbicide stock solutions. Table S1, DOCX file, 0.04 MB.
Copyright © 2015 Kurenbach et al.
This is an open-access article distributed under the terms of the Creative Commons Attribution-Noncommercial-ShareAlike 3.0 Unported license, which permits unrestricted noncommercial use, distribution, and reproduction in any medium, provided the original author and source are credited.