Effect of sunlight on the efficacy of commercial antibiotics used in agriculture

Abstract

Antibiotic stewardship is of paramount importance to limit the emergence of antibiotic-resistant bacteria in not only hospital settings, but also in animal husbandry, aquaculture, and agricultural sectors. Currently, large quantities of antibiotics are applied to treat agricultural diseases like citrus greening disease (CGD). The two commonly used antibiotics approved for this purpose are streptomycin and oxytetracycline. Although investigations are ongoing to understand how efficient this process is to control the spread of CGD, to our knowledge, there have been no studies that evaluate the effect of environmental factors such as sunlight on the efficacy of the above-mentioned antibiotics. We conducted a simple disc-diffusion assay to study the efficacy of streptomycin and oxytetracycline after exposure to sunlight for 7- or 14-day periods using Escherichia coli and Bacillus subtilis as the representative strains of Gram-negative and Gram-positive organisms respectively. Freshly prepared discs and discs stored in the dark for 7 or 14 days served as our controls. We show that the antibiotic potential of oxytetracycline exposed to sunlight dramatically decreases over the course of 14 days against both E. coli and B. subtilis. However, the effectiveness of streptomycin was only moderately impacted by sunlight. It is important to note that antibiotics that last longer in the environment may play a deleterious role in the rise and spread of antibiotic-resistant bacteria. Further studies are needed to substantively analyze the safety and efficacy of antibiotics used for broader environmental applications. IMPORTANCEAlthough antibiotics have been used for agricultural purposes for decades, due to the rapid rise in antibiotic resistance this usage needs to be revisited. Questions remain on the appropriate mode of application of antibiotics and the actual benefits of using antibiotics for treating the infections caused by plant pathogens, especially for the ones that are intracellular in nature. Here we show that the two commonly used commercial antibiotics, oxytetracycline and streptomycin, lose their efficacy at different rates in the presence of sunlight. While the former loses its potency within days the latter remains active for many days. Thus, oxytetracycline may not be active long enough to produce desired effect and streptomycin may persist in the environment and as a side effect due to its selective pressure, may force the rise of streptomycin-resistant pathogens.