Phototransformation of sulfamethoxazole under simulated sunlight: Transformation products and their antibacterial activity toward Vibrio fischeri
Graphical abstract
Introduction
For the last few decades the occurrence and impact of xenobiotics such as personal care products, pharmaceuticals or endocrine disruptors in the aquatic environment have given cause for concern. Sulfonamide antibiotics are one of the oldest class of antimicrobial agents among the administered antibiotic compound classes and frequently used in European countries and Korea (Białk-Bielińska et al., 2011, Kim et al., 2007, Thiele-Bruhn, 2003). These derivatives of sulfanilamide are commonly used in aquaculture, animal husbandry and human medicine to treat bacterial infections (García-Galán et al., 2008). Sulfamethoxazole (SMX) is one of the most widely prescribed sulfonamide drugs (Hu et al., 2007). It has been frequently detected in surface and drinking waters as well as in effluents of wastewater treatment plants (WWTPs) (Bahnmüller et al., 2014, Białk-Bielińska et al., 2011, García-Galán et al., 2008, Gao et al., 2014, Hu et al., 2007, Kim et al., 2007). Jiang et al. (2014) suggested that SMX is the most common antibiotic with a detection frequency of up to 73%. The concern regarding the occurrence of SMX in the environment is due to two aspects: i) the transfer, acquisition and promotion of antibiotic resistance in environmental bacteria (Gullberg et al., 2011) and ii) the possibility of toxic effects for numerous exposed aquatic organisms (Białk-Bielińska et al., 2011, García-Galán et al., 2008, Park and Choi, 2008). Several studies demonstrated that SMX is poorly and slowly biodegradable causing low removal efficiencies during conventional biological treatment (García-Galán et al., 2008, Ingerslev and Halling-Sorensen, 2000, Jiang et al., 2014, Larcher and Yargeau, 2011, Perez et al., 2005).
Therefore, phototransformation of SMX under sunlight can represent an important transformation process in natural waters. It should be noticed that a lot of studies focused on SMX photolysis by UV light, while natural systems are exposed to atmospheric sunlight. Furthermore, transformation products (TPs) were mostly not considered (Andreozzi et al., 2003, Bahnmüller et al., 2014, Boreen et al., 2004, Lam and Mabury, 2005, Ryan et al., 2011). There are only few articles available about the identification of photolysis TPs of SMX and their toxicity (Bonvin et al., 2013, Trovó et al., 2009a).
Previous work showed that some of the SMX derived TPs still exhibit antibacterial activity (Majewsky et al., 2014). For instance, the growth inhibition of 4-nitro-sulfamethoxazole (NO2-SMX) and 4-hydroxy-sulfamethoxazole (4-OH-SMX) toward Vibrio fischeri was greater than that of SMX. Moreover, the mixture toxicity of derivative TPs was observed to be additive. Therefore, the elucidation of the SMX transformation pathway is essential to assess its environmental impact in natural waters.
In this context, the aim of the presented study is to investigate the TPs formed during SMX photolysis and to determine their antibacterial toxicity in mixture. Two approaches are used to achieve this: a) reference standards of TPs are applied for quantitative analysis where available and b) TPs are tentatively identified by interpretation of product ion spectra using LC–MS/MS and UHPLC-Q/ToF. To simulate natural conditions, a solar simulator was used as light source. Monitoring of antibiotic activity in mixture during simulated sunlight irradiation was realized by bacterial toxicity tests using V. fischeri with the two endpoints growth and luminescence inhibition.
Section snippets
Chemicals
3-Amino-5-methylisoxazole (3A5MI), sulfanilic acid (SA), sulfacetamide (SFAA) and sulfamethoxazole (SMX) were purchased from Sigma-Aldrich (Seelze, Germany). N4-acetyl-sulfamethoxazole (acetyl-SMX), 4-nitroso-sulfamethoxazole (NO-SMX), N4-hydroxy-sulfamethoxazole (N-OH-SMX), and 4-nitro-sulfamethoxazole (NO2-SMX) were purchased from Toronto Chemicals (Toronto, ON, Canada). 4-Hydroxy-sulfamethoxazole (4-OH-SMX) and N4-hydroxy-acetyl-sulfamethoxazole (OH-acetyl-SMX) were synthesized as previously
Photolysis experiments
First, a control experiment in the dark was carried out to investigate the possible contribution of hydrolysis to SMX decomposition. Results showed that there is no decomposition (~ 2%) of SMX in the dark during 7 days (Fig. 1). Simulated sunlight within the range of 290–700 nm only modestly transformed SMX in pure aqueous solution, which is not surprising given that the maximum absorbance of SMX is between 250 and 300 nm (Lam and Mabury, 2005). Over the course of 1 week exposure, approximately 50%
Conclusions
During long-term SMX photolysis experiments, nine TPs could be identified and quantified by reference standards (3A5MI, SA, N-OH-SMX, NO2-SMX, NO-SMX, 4-OH-SMX, SFAA, acetyl-SMX and OH-acetyl-SMX). Some of the TPs (e.g. N4-acetyl-SMX) were observed to be formed during photolysis for the first time. Furthermore, a TP with [M + H]+ ion at m/z 271 was tentatively confirmed as 4-,x-dihydroxylated SMX.
Six further TPs of phototransformation of SMX with the molar mass 281 g mol− 1 (m/z 282, [M + H]+), 238 g mol
Acknowledgments
M.G. acknowledges the support from the Foundation for Polish Science within the START scholarship (START25.2015) and is grateful to the Deutscher Akademischer Austauschdienst (DAAD) for an exchange fellowship at KIT.
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