Sulfonamide resistance as a global one health challenge

Antimicrobial resistance (AMR) has been recognized as a major global One Health threat by the World Health Organization, arising largely from the widespread and often inappropriate use of antibiotics across human medicine, agriculture, and animal production systems [1, 2]. From a One Health perspective, the dissemination of antimicrobial resistance genes (ARGs) across interconnected human, animal, and environmental reservoirs highlights the need for coordinated intervention strategies [3]. In response, global policy frameworks including the WHO Global Action Plan on AMR and collaborative initiatives led by the Food and Agriculture Organization and the World Organization for Animal Health, under the broader Quadripartite alliance, have emphasized integrated monitoring of AMR and antimicrobial use across sectors [3]. These efforts are further strengthened by international recommendations such as the O’Neill Report, which advocate for improved stewardship, reduced environmental contamination, and the development of alternative strategies. Within these frameworks, coordinated surveillance systems increasingly utilize ARGs as molecular indicators of anthropogenic resistance dissemination, with sulfonamide resistance genes such as sul1 and sul2 widely recognized as robust environmental markers due to their association with class 1 integrons and human-driven pollution [4]. Although emerging variants such as sul4 are not yet formally incorporated into global monitoring schemes, their expanding detection across environmental and clinical settings highlights the evolving complexity of the sulfonamide resistome [4,5,6].

Sulfonamide resistance continues to pose a serious threat to global public health, extending beyond the realm of clinical medicine to veterinary and environmental domains [7,8,9,10]. Although traditionally viewed as a consequence of antibiotic misuse in human healthcare, emerging evidence suggests that the reality is much more multifaceted. Sulfonamide resistance genes, specifically the sul gene family, including sul1, sul2, sul3 and sul4, are now widely detected across a spectrum of reservoirs ranging from humans to livestock, companion animals, and down to environmental sinks [5, 11, 12]. The efficacy and low cost of these antibiotics have historically made them widely indispensable, while their long-term use cultivated a widespread resistome, particularly among Gram-negative bacteria [11, 13,14,15]. Here, we argue that sulfonamide resistance represents one of the clearest examples of a One Health failure, where environmental and veterinary reservoirs are not secondary, but central drivers of resistance persistence. As depicted in Fig. 1, under a One Health lens, it is evident that sulfonamide resistance is a shared risk where its use in one sector, such as a farm or a clinic, inevitably impacts all others [3, 11, 16].

The distribution of these resistance factors globally illustrates an alarming level of interrelation between humans and animals (Table 1). For instance, livestock, particularly pigs, show a high frequency of sul1 and sul2 genes, which are often carried on mobile genetic elements like class 1 integrons and conjugative plasmids [11, 17]. Furthermore, poultry serves as a significant reservoir, with 58% of E. coli from broiler farms exhibiting sulfonamide resistance. Perhaps more concerning is the recent emergence of the sul4 gene variant in companion animals such as cats and dogs, which suggests a direct potential route for transmission to humans [11]. This resistance is not limited to industrial settings; even remote populations, such as Tibetan yak herders [13], show high rates of sulfamethoxazole resistance, supporting the idea that these genes can spread to the most isolated ecological niches.

In this context, environmental reservoirs play a dual role as both sinks and amplifiers for sulfonamide resistance. Although advanced wastewater treatment systems, such as constructed wetlands, can remove nearly all sulfamethoxazole residues, they frequently fail to eliminate the corresponding resistance genes. The sul1 gene, in particular, often remains at high levels in treated effluent, suggesting that water systems may act as recyclers of resistance back into the environment, most frequently through horizontal transmission [7, 18, 19]. In aquatic ecosystems, exposure to even low concentrations of sulfonamides can lead to a dramatic increase in resistance genes and a sharp decline in survival rates for species such as milkfish [9]. Additionally, the presence of heavy metals such as copper and mercury in soil and water can co-select for sulfonamide resistance, as these metal-resistance genes are often linked to sul genes on the same mobile genetic elements [20, 21].

At the molecular level, resistance is driven by sul genes that produce altered dihydropteroate synthase (DHPS) enzymes. These enzymes possess a unique structural insertion (Phe-Gly motif) that prevents sulfonamides from binding while allowing the bacteria to maintain normal folate metabolism [14, 22, 23]. Although sul1 and sul2 have long been established on plasmids and transposons, the novel sul4 gene has evolved from a chromosomal gene in marine bacterium to a mobile element detected in human-associated bacteria, including Salmonella enterica, with emerging evidence in opportunistic pathogens [6, 24]. Initially, the sul4 gene is present in 45% of sulfonamide-resistant marine isolates, making it relatively more prevalent than sul1-3 in this environment and its genetic context suggested it might be a conserved gene involved in folate metabolism rather than a transferable resistance gene [5]. This “resistome acceleration” is further complicated by fitness trade-offs and compensatory mutations, where bacteria adapt to the cost of carrying resistance genes, often contribute to cross-resistance to multiple other antibiotics [15, 25].

The sul4 gene was first identified in river sediment metagenomic samples from India [6]. It has since been increasingly identified across diverse ecological and biological reservoirs, including marine bacteria, wastewater systems, and companion animals [5, 11, 26]. However, the host range of sul4 remains largely unknown. In particular, sul4 has now been widely detected in sludge environments, where it is hosted by diverse bacterial taxa, notably members of the phyla Myxococcota, Chloroflexota, Proteobacteria and Bacilli, as well as genera such as Trichlorobacter and Desulfobacillus [5, 26]. In addition to environmental reservoirs, emerging evidence indicates that sul4 has already disseminated into opportunistic human-associated pathogens, such as Aeromonas and Moraxella, in addition to reported occurrences in S. enterica [26]. However, serotype-level characterization of sul4-carrying Salmonella remains limited in current literature, representing an important knowledge gap. Together, the current findings highlight the ongoing ecological transition of sul4 into human-associated microbiomes and its potential significance within a One Health context.

In order to mitigate the impact of the sulfonamide resistome requires an integrated approach that combines improved surveillance with innovative interventions. Integrating high-throughput detection platforms, such as quadruplex droplet-digital PCR, are essential for tracking the spread of sul1-4 genes across different sectors [27]. Simultaneously, the development of new sulfonamide derivatives, like sulfa-salicylimine compounds, shows promise in overcoming existing resistance by shifting the drug’s action from bacteriostatic to bactericidal [28]. Equally, environmental management strategies, including thermophilic composting of manure and optimized wastewater oxidation, are also crucial for breaking the transmission cycle [29]. Ultimately, addressing sulfonamide resistance requires a comprehensive One Health framework that integrates medical, veterinary, and environmental expertise to prevent the further re-entry of these genes into human pathogens.

In conclusion, sulfonamides have transcended their role as a simple antimicrobial agent to become significant ecological driver that influence bacterial evolution across interconnected human, animal, and environmental systems. Despite decades of clinical use, they present a persistent and evolving threat characterized by the widespread dissemination of sul genes and the emergence of mobile variants like sul4 in unexpected hosts. Addressing this challenge effectively requires the adoption of a comprehensive One Health framework that moves beyond clinical stewardship to include integrated surveillance, the development of innovative bactericidal derivatives, and the optimization of waste treatment technologies. By fostering collaboration among medical, veterinary, and environmental experts, we can translate these insights into practical actions that limit the spread of the sulfonamide resistome and protect the future of antimicrobial therapy.