The sulfonamide antibiotics sulfamethoxazole (SMX) has been frequently detected in the wastewater. It has been reported that part of SMX can be transformed by the co-metabolism of ammonia oxidizing bacteria (AOB) during nitrifying process. However, previous studies reported inconsistent or even contradictory results in terms of SMX degradation and/or transformation. Literature study revealed that nitrite may play certain role in SMX transformation, which has been neglected previously. In this study, the transformation behavior of SMX was investigated with and without the presence of nitrite in an enriched nitrifying culture. The results clearly show that the elimination of SMX occurred with the presence/accumulation of nitrite, and a linear regression was observed between SMX elimination efficiency and free nitrous acid (FNA) concentration, indicating that FNA was the major factor responsible for the SMX transformation. By reacting with FNA, SMX transformation products, such as 4-nitro-SMX, desamino-SMX and hydroxylated SMX, were detected. However, when FNA concentration decreased, these intermediates may be retransformed back to SMX. These findings improved our understanding on SMX transformation in a biological system and highlighted the role of nitrite/FNA in the sulfonamide antibiotics degradation.

在废水中经常检测到磺胺类抗生素磺胺甲恶唑（SMX）。据报道，在硝化过程中，SMX的一部分可以通过氨氧化细菌（AOB）的共代谢而转化。然而，先前的研究报道了就SMX降解和/或转化而言不一致甚至矛盾的结果。文献研究表明，亚硝酸盐可能在SMX转化中起一定作用，这一点以前已被忽略。在这项研究中，研究了富硝化培养物中有无亚硝酸盐存在下SMX的转化行为。结果清楚地表明，SMX的消除是由于亚硝酸盐的存在/积累而发生的，并且观察到SMX消除效率与游离亚硝酸（FNA）浓度之间存在线性回归，表明FNA是负责SMX转换的主要因素。通过与FNA反应，检测到SMX转化产物，例如4-硝基SMX，脱氨基SMX和羟基化SMX。但是，当FNA浓度降低时，这些中间体可能会重新转化回SMX。这些发现提高了我们对生物系统中SMX转化的理解，并强调了亚硝酸盐/ FNA在磺酰胺类抗生素降解中的作用。