Epidemiological data reveal that over 1 million individuals aged 15–49 worldwide are infected with curable sexually transmitted infections (STIs) daily. In 2020, the World Health Organization reported an estimated 374 million new cases of four major curable STIs: chlamydia (129 million), gonorrhoea (82 million), syphilis (7.1 million), and trichomoniasis (156 million) [1]. The frequent mild or asymptomatic presentation of early-stage STIs poses significant challenges for timely detection and treatment [2,3]. Delayed or inaccurate identification of pathogens can lead to latent, chronic, or persistent infections, substantially elevating the risk of severe complications. These include increased susceptibility to HIV, pelvic inflammatory disease, infertility in women, and congenital malformations or neonatal mortality [[4], [5], [6], [7]]. To address this, a novel patient management strategy named doxycycline post-exposure prophylaxis (doxy-PEP) has been proposed for high-risk populations. This strategy advocates for the oral administration of 200 mg doxycycline, a second-generation tetracycline with higher bioavailability, within 24 hours (and no later than 72 hours) post-sexual exposure to prevent infections caused by Treponema pallidum, Chlamydia trachomatis, and Neisseria gonorrhoeae (NG) [8]. Three large-scale randomized controlled trials have demonstrated that doxy-PEP reduces syphilis and chlamydia infections by over 70% and gonorrhoea infections by more than 50% [[9], [10], [11]]. These findings highlight doxy-PEP as an innovative biomedical intervention that significantly lowers the risk of bacterial STIs.

The introduction of doxy-PEP provides a novel preventive strategy against STIs, offering a potential solution to reduce their global burden. However, its widespread implementation raises concerns regarding antimicrobial resistance (AMR). The major challenge lies in maintaining doxy-PEP efficacy while preventing the emergence of resistance in STI pathogens and commensal bacteria such as Staphylococcus aureus [12]. Among STI pathogens, NG is of particular concern due to its high genetic diversity and rapid adaptability to selective pressure, which have led to the global emergence of multidrug-resistant strains [13]. Tetracycline resistance in NG is already widespread globally [[14], [15], [16]]. Since doxy-PEP is primarily recommended for populations at high risk of STIs – who also have a higher baseline risk of gonococcal infection – the introduction of doxycycline could accelerate the selective proliferation of tetracycline-resistant gonococcal strains, potentially compromising the long-term effectiveness of this prophylactic measure [17]. Clinical data support these concerns. Two studies from the United States [10] and France [18] reported higher proportions of high-level tetracycline resistance in doxy-PEP than in standard care (38% vs. 12%; 35.5% vs. 12.5%).

Moreover, doxy-PEP may facilitate a selective advantage to tetracycline-resistant genogroups (e.g. G14769 and G1407), which often harbour a high prevalence of resistance-associated mutations to other antimicrobials [19]. This could indirectly promote the emergence of multidrug-resistant strains. Supporting evidence from a European surveillance study conducted in 2018 involving 2375 NG isolates showed that among 520 isolates carrying tetM, 518 also possessed penA mutations, indicating that tetracycline and ceftriaxone resistance-associated mutations often coexisted within the same isolates [19]. Recent surveillance data reinforce this epidemiological linkage. In China, an analysis of 1559 NG isolates collected from 2019 to 2021 found that among 384 isolates with reduced susceptibility to ceftriaxone (minimum inhibitory concentration [MIC] ≥ 0.125 mg/L), 91.1% (350/384) were also tetracycline-resistant [20]. A more comprehensive study of 357 ceftriaxone-resistant isolates collected from eight provinces during 2002–2022 also reported a high tetracycline resistance rate of 92.7% (319/344) [21]. Similarly, in New South Wales, Australia, 97% (64/66) of ceftriaxone-resistant or reduced-susceptibility isolates collect from 2015 to 2022 were resistant to tetracycline [17]. These findings indicate that NG strains resistant to ceftriaxone often exhibit concurrent resistance to tetracyclines. Additional evidence supports a correlation between tetracycline and β-lactam MICs in NG [22], and results from the DoxyVAC trial also suggested that doxy-PEP reduce gonococcal susceptibility to cephalosporins [18]. Therefore, the implementation of doxy-PEP could potentially co-select for ceftriaxone resistance, leading to the emergence of dual-resistant strains. Since ceftriaxone is the first-line therapy for gonorrhoea, this would pose a significant threat to current treatment strategies.

To address these challenges, it is essential to enhance AMR surveillance for NG among doxy-PEP users, focusing on regular monitoring of gonococcal susceptibility to doxycycline and ceftriaxone. Given the correlation between doxycycline and tetracycline MICs, tetracycline resistance is commonly used as a proxy for doxycycline resistance [23]. High-level tetracycline resistance is often mediated by the tetM gene, which can serve as an indicator of doxycycline resistance during doxy-PEP implementation. The ceftriaxone-resistant FC428 clone, first identified in Japan, has rapidly spread globally and is now a priority for public health surveillance [[24], [25], [26]]. The mosaic penA-60.001 allele plays a key role in high-level ceftriaxone resistance of FC428-like strains, making it a crucial target for detecting ceftriaxone resistance [27]. Consequently, we developed the high-resolution melting (HRM)-doxyPEP, a molecular surveillance method based on HRM analysis that simultaneously targets the species-specific gene porA for identification of NG, as well as penA for tracking ceftriaxone resistance and tetM for detecting tetracycline resistance. By incorporating advanced molecular techniques into surveillance systems, HRM-doxyPEP enables real-time monitoring of key resistance determinants, supporting ongoing AMR surveillance during doxy-PEP implementation. This approach provides early warnings and facilitates rapid public health responses (Fig. 1).