Measuring Zeta Potential in Bacterial Strains

Overnight cultures were started as described in Etest section above. Since media composition (salt concentration and pH) can affect zeta potential, [67 (link)], 0.5 mL of each culture was washed in 1X PBS and resuspended in 1 mL 0.1X PBS diluted in sterile nanopure water to minimize media influence on these measurements. We also report conductivity measurements to show that salt concentration has been normalized by this procedure. The Zeta potential was measured on a Malvern Zetasizer Nano ZS90 device (Malvern) at room temperature 25°C. Zeta potential standards were obtained from Malvern (DTS1235, –42 mV ± 4.2 mV) and used according to the manufacturer’s instructions. Measurements were performed in Zetasizer folded capillary cells (DTS1070, Malvern) in triplicate reads (n = 3). Values given are mean ± standard deviation. Zeta potential assays were repeated on at least two separate occasions. GraphPad Prism version 8 was used to perform a two-way ANOVA with Tukey’s multiple comparison test to compare UGA strains 10 and 14 to the antimicrobial susceptible strain ATCC 13311 (*P < 0.05, **P < 0.01, ***P < 0.001).