Sulfonamide resistance gene sul4 is hosted by common wastewater sludge bacteria and found in various newly described contexts and hosts including clinically relevant species

The introduction of the first broad-spectrum antibiotics, sulfonamide drugs fundamentally revolutionized medicine in the 1930s. Shortly after and ever since sulfonamide resistance genes (sul genes) have been widely detected. Still, the most recent variant of these genes sul4, was first described only in 2017 and its host range and transmission mechanisms are still largely unknown. Here we applied PacBio long-read metagenomic sequencing and bacterial methylation signals to investigate the genetic contexts and bacterial carriage of the sul4 gene in wastewater. Furthermore, we extended our description of sul4 carriers to previously published data sets. Our results indicate that sul4 is prominently found in sludge and hosted by various bacteria such as the species from the phyla Myxococcota and Chloroflexota and genera Trichlorobacter, and Desulfobacillus, which are commonly found in activated sludge. Moreover, worryingly, according to our results, sul4 has already spread into multiple strains of opportunistic human pathogen genera, such as Aeromonas and Moraxella in addition to the previously described Salmonella. The sequence region flanking sul4, which included an integron integrase gene and an ISCR20-like element, exhibited a high degree of conservation across the investigated sequences and suggested a potential for mobility. Also other mobility-related elements that could further increase the likelihood of sul4 mobilization were detected. Altogether, our results describing the sul4 hosts of bacteria from distant lineages indicate the efficient mobility of sul4 by genetic elements that traverse both clinical and environmental bacteria and that wastewater may provide favorable conditions for such horizontal gene transfer events.