Evaluating the Antimicrobial Efficacy of Copper-Based Pesticides Against Salmonella Under Simulated Tank-Mixing Conditions

Abstract

Copper-based pesticides are used to manage plant diseases in organic produce production. When mixed with contaminated agricultural water, these pesticides may serve as a vehicle for foodborne pathogen contamination of pre-harvest produce. Reoccurring multistate outbreaks associated with fresh produce further suggest persistent environmental contamination may come from common sources. This study evaluated the antimicrobial efficacy of four copper-based pesticides (Cueva Fungicide Concentrate, Cuprofix Flex, Kocide 3000-O, and Nordox 75 WG) against five rifampicin-resistant Salmonella serovars (S. Infantis MS3, S. Javiana, S. Newport, S. Poona, and S. Saintpaul) (~5.0 log CFU/mL) under simulated laboratory conditions. Minimum bactericidal concentrations (MBCs; mg Cu/mL), determined using a microtiter plate assay, ranged from 0.60–0.96 for Cueva, 1.75–2.50 for Kocide, 20–25 for Cuprofix, and 50 for Nordox. For tank-mixing simulations, pesticides were standardized to 0.38 mg Cu/mL and reconstituted in deionized water under constant agitation (n = 6 per serovar-pesticide combination). Salmonella populations were enumerated on non-selective media after 0, 15, 30, 45, 60, 90, and 120 min of mixing. Reductions at 15 min, representing a practical pesticide mixing timeframe, were analyzed using linear regression and Tukey's HSD. Under tank-mixing conditions, copper-based pesticides reduced Salmonella populations; however, antimicrobial efficacy differed significantly by pesticide and serovar (P < 0.05). Exposure to Cueva and Nordox resulted in the greatest reductions at 15 min, with average decreases of 4.28±0.67 to 5.29±0.15 log CFU/mL and 2.17±0.13 to 3.64±0.31 log CFU/mL, respectively, across serovars. In contrast, Cuprofix and Kocide reduced Salmonella the least and did not exhibit significantly different reductions in Salmonella comparatively at 15 min (P > 0.05), and Salmonella populations remained above the limit of detection for the duration of the study (120 min). Serovar-dependent differences in survival were observed, with S. Infantis MS3 and S. Saintpaul exhibiting greater persistence compared to other serovars evaluated in Cueva and Nordox, respectively (P < 0.05). These findings demonstrate antimicrobial efficacy under tank-mixing conditions differs by pesticide and serovar, despite equivalent copper concentrations. Selection of copper-based pesticide formulations may therefore influence microbial reductions in agricultural spray water.