The sul1 gene encodes an alternate dihydropteroate synthase that is not inhibited by sulfonamides. sul1 is classically located on class 1 integrons in Gram-negative bacteria, often alongside other resistance genes. Detection of sul1 predicts resistance to sulfonamides and typically to trimethoprim-sulfamethoxazole (which relies on sulfonamide activity). Its presence signals that trimethoprim-sulfamethoxazole therapy will likely fail (Partridge et al. 2018, Sköld 2000, Sköld 2010).
Partridge, S. R., Kwong, S. M., Firth, N., & Jensen, S. O. (2018). Mobile Genetic Elements Associated with Antimicrobial Resistance. Clinical microbiology reviews, 31(4), e00088-17. https://doi.org/10.1128/CMR.00088-17
Sköld O. (2000). Sulfonamide resistance: mechanisms and trends. Drug resistance updates : reviews and commentaries in antimicrobial and anticancer chemotherapy, 3(3), 155–160. https://doi.org/10.1054/drup.2000.0146
Sköld O. (2010). Sulfonamides and trimethoprim. Expert review of anti-infective therapy, 8(1), 1–6.
