## 3 Results and Discussion
### Host Tracking of ARGs  
Identifying the potential hosts of ARGs is crucial for the risk assessment of the spread of antibiotic resistance [47]. Potential ARG hosts in HX WWTP were classified into seven phyla with _Proteobacteria_ accounting for more than 70% (Figure 3A). This correlated with previous studies showing that ARGs in WWTP sludge are mainly carried by _Proteobacteria_, and this phylum contains a variety of human pathogens [26; 48]. A total of 91 genera carrying ARGs were identified, with the majority represented by _Acinetobacter_ (21.1%), _Escherichia_ (11.6%), and _Klebsiella_ (7.71%) (Figure 3A). Figure 2: Summary of ARGs in the influent (INF), activated sludge (AS), and effluent (EFF) of HX WWTP. (**A**) Relative abundance of the ARG types. (**B**) Relative abundance and distribution of persistent ARGs. ppm: one read in one million reads. The classifiable ACCs belonged to 119 bacterial species (Table S2) with six of the top 10 species carrying ARGs classified as pathogens, including _Bacteroides fragilis_, _Acinetobacter johnsonii_, _Escherichia coli_, _A. baumaniii_, _K. pneumoniae_, and _P. aeruginosa_ (Figure 3A). The last three species mentioned belong to ESKAPE pathogens and carry a variety of ARGs (Table 2). These ESKAPE pathogens are the key microorganisms for high-risk ARGs to spread in the environment, and they are the main cause of nosocomial infections worldwide [41; 49]. _E. coli_ was the most abundant potential host (12.9%) of ARGs and carried the most diverse ARG subtypes (40) in HX WWTP followed by _A. johnsonii_ (8.94%, 13 ARG subtypes), and _K. pneumoniae_ (5.30%, 16 ARG subtypes) (Figure 3A; Table 2). _E. coli_ was the most prevalent resistant pathogen in a hospital study in Xinjiang [50], and this organism easily acquires multidrug resistance genes [51], including _acrA_, _mdt_HO, _mdt_H, and _mdt_L [36; 52], which is consistent with our results (Table 2). _E. coli_ and _K. pneumoniae_ carried ARGs from at least seven ARG types and expressed resistance to multiple antibiotics (Figure 3B). _A. johnsonii_ carried over 80% of multidrug resistance genes harboring _abeS_, _adeL_, _adeK_, _enrB_, _mdf_A, _mdt_K, _mex_B, _mex_T_, _oprM_, and _TolC_ in this study (Table 2), and _Acinetobacter_ is a potential host for _adeK_[53]. Approximately 84.5% of _B. fragilis_ carried _emt_F and _tet_Q genes, which confer resistance to MLS and tetracycline antibiotics (Figure 3B; Table 2). This type of resistance was similarly observed in _Bacteroides_ from human intestinal and pig manure samples [6]. _Bacteroides_ mainly carry _tet_Q [36], with the proportion of _tet_Q in _Bacteroides_ increasing sharply from 30% to over 80% because of HGT [54]. More than half of _P. aeruginosa_ carried _class A beta-lactamase_ and _OXA_-101 (Figure 3B; Table 2). _P. aeruginosa_ is a well-known and  
Figure 3: The potential host information of ARGs in HX WWTP. (**A**) Phylogenetic tree representing the composition and relative abundance of the top 10 ARG potential hosts from the domain level (D), to the phylum (P), class (C), genus (G), and species (S) levels. Branch width represents the relative abundance of the corresponding ARG potential hosts. Larger bold fonts represent antibiotic-resistant pathogens (ARPs). (**B**) The bar chart shows the diversity and relative abundance of ARG types carried by potential hosts at the species level in (**A**). MLS: macrolide-lincosamide–streptomycin. difficult-to-treat pathogen that may cause serious infection and disease [55] and is the most dominant ARP among agricultural WWTP, carrying 20 ARG subtypes [19]. Out of the six widespread PARGs in Xinjiang WWTPs, 60% of _class A beta-lactamase_-carrying contigs belong to _P. aeruginosa_ (Table S2). Additionally, about half of _qac_Edelta1-carrying contigs and all _sul_1-carrying contigs were derived from _K. pneumoniae_ (Table S2). Similar results were obtained in CJ WWTP (Figure S4; Tables S2 and S3).