Occurrence of Antibiotic-Resistant Genes and Bacteria in Household Greywater Treated in Constructed Wetlands

Heterogeneous Antibiotic Resistance Gene Removal Impedes Evaluation of Constructed Wetlands for Effective Greywater Treatment

Microorganisms carrying antimicrobial resistance genes are often found in greywater. As the reuse of greywater becomes increasingly needed, it is imperative to determine how greywater treatment impacts antimicrobial resistance genes (ARGs). Using qPCR and SmartChip™ qPCR, we characterized ARG patterns in greywater microbial communities before, during, and after treatment by a recirculating vertical flow constructed wetland. In parallel, we examined the impact of greywater-treated irrigation on soil, including the occurrence of emerging micropollutants and the taxonomic and ARG compositions of microbial communities. Most ARGs in raw greywater are removed efficiently during the winter season, while some ARGs in the effluents increase in summer. SmartChip™ qPCR revealed the presence of ARGs, such as tetracycline and beta-lactam resistance genes, in both raw and treated greywater, but most abundantly in the filter bed. It also showed that aminoglycoside and vancomycin gene abundances significantly increased after treatment. In the irrigated soil, the type of water (potable or treated greywater) had no specific impact on the total bacterial abundance (16S rRNA gene). No overlapping ARGs were found between treated greywater and greywater-irrigated soil. This study indicates ARG abundance and richness increased after treatment, possibly due to the concentration effects of the filter beds.

Distribution and dynamics of antibiotic resistance genes in a three-dimensional multifunctional biofilm during greywater treatment

Antibiotic resistance genes (ARGs) have been identified as serious threats to public health. Despite the widespread in various systems, dynamics of ARGs in three-dimensional multifunctional biofilm (3D-MFB) treating greywater are largely undefined. This work tracked the distributions and dynamics of eight target genes (intI1, korB, sul1, sul2, tetM, ermB, bla_CTX-M and qnrS) in a 3D-MFB during greywater treatment. Results showed that hydraulic retention times at 9.0 h achieved the highest linear alkylbenzene sulfonate (LAS) and total nitrogen removal rates at 99.4% and 79.6%, respectively. ARGs presented significant liquid-solid distribution feature, but non-significant with biofilm position. Intracellular ARGs (predominant by intI1, korB, sul1 and sul2) at bottom biofilm were 210- to 4.2 × 10⁴- fold higher than that in cell-free liquid. Extracellular polymeric substances (EPS)-attached LAS showed linear relationship with most of ARGs (R² > 0.90, P < 0.05). Sphingobacteriales, Chlamydiales, Microthrixaceae, SB-1, Cryomorphaceae, Chitinophagaceae, Leadbetterella and Niabella were tightly bound up with target ARGs. Key is that EPS-attached LAS considerably determines the occurrence of ARGs, and microbial taxa play an important role in the dissemination of ARGs in the 3D-MFB.

Constructed wetlands as hotspots of antibiotic resistance genes and pathogens: Evidence from metagenomic analysis in Chinese rural areas

In rural China, many constructed wetlands (CWs) have been developed to treat rural wastewater sustainably. However, due to the scarce information on those rural CWs, it is difficult to analyze the biological contaminants within those systems, such as antibiotic resistance genes (ARGs) and pathogens. Based on the data collected from two pilot-scale, one-year-observed CWs, for the first time, this study explored the accumulation of ARGs and pathogens using the metagenomic sequencing approach and SourceTracker analysis under different hydraulic loading rates. The Shannon index of ARGs in the effluent surpassed the level found in the influent. The DESeq2 analysis showed that up to 21.49% of the total pathogen species had increased relative abundance in the effluent compared with the influent. By combining the contribution of substrate and rhizosphere, the CW became a more influencing factor for ARGs and pathogens contamination than the influent. The network analysis revealed a critical but latent fact that the development of antibiotic-resistant pathogens is highly likely to be triggered by the co-occurrence of ARGs and pathogens. Collectively, from the aspect of biological risk, our study showed that CWs alone might not be an ideal solution for improving wastewater treatment in rural China.

Organic Compounds and Antibiotic-Resistant Bacteria Behavior in Greywater Treated by a Constructed Wetland

Laundry greywater is considered as an alternative source of non-potable water, as it is discharged in approximately 70% of homes. Because this water contains compounds such as biodegradable and recalcitrant organic matter, surfactants, and microbiological compounds, it must be treated prior to reuse. Therefore, the objective of this study was to assess the behavior of organic matter and antibiotic-resistant bacteria (ARB) in greywater treated by a constructed wetland (CW). The results show that the organic matter removal efficiencies were 67.19%, 50.15%, and 63.57% for biological oxygen demand (BOD₅), chemical oxygen demand (COD) and total organic carbon (TOC), respectively; these efficiencies were not significant (p > 0.05). In addition, the CW allows the distribution of TOC and ionic compounds in the fractions below 1000 Da to increase by 5.03% and 13.05%, respectively. Meanwhile, the treatment of microbiological compounds generated non-significant removals (p > 0.05), along with increases in bacteria resistant to the antibiotics ciprofloxacin (CIP) and ceftriaxone (CTX) of 36.34%, and 40.79%, respectively. In addition, a strong association between ARB to CIP, CTX, cationic and non-ionic surfactants was determined, indicating the role of surfactants in ARB selection. It is suggested that disinfection systems should be employed prior to the reuse of the treated water.

Transmission of tetracycline resistance genes and microbiomes from manure-borne black soldier fly larvae frass to rhizosphere soil and pakchoi endophytes

Manure treatment with black soldier fly larvae (BSFL) and BSFL frass application in crop land is a sustainable strategy; however, whether residual antibiotic resistance genes (ARGs) and their transmission risk are related to the manure BSFL treatment process is still unknown. In this paper, the effect of BSFL addition density on residual tetracycline resistance genes (TRGs) and transmission from frass to pakchoi was determined. The results showed that BSFL frass can provide sufficient nutrients for growth, improve the economic value of pakchoi, and reduce the risk of transmission of TRGs in chicken manure regardless of BSFL density. The potential hosts of the TRGs we detected were found in BSFL frass (Oblitimonas and Tissierella), rhizosphere soil (Mortierella and Fermentimonas), and pakchoi endophytes (Roseomonas). The present study concluded that BSFL frass produced by adding 100 BSFL per 100 g of chicken manure has the advantages of high value and low risk. These findings will provide important strategic guidance for animal manure disposal and theoretical support for preventing the transmission of TRGs in BSFL applications.

Antibiotics and antibiotic-resistant bacteria in greywater: Challenges of the current treatment situation and predictions of future scenario

The current work reviews the quantitative microbiological risk assessment of antibiotic-resistant bacteria (ARB) in greywater and discusses the international strategies currently used for reducing antimicrobial resistance. The work highlights the countries that have a plan for the treatment and reuse of greywater and the current guidelines used in these countries. The paper also investigates the role of greywater in the distribution of antimicrobial resistance because of antibiotics and ARB. A bibliometric analysis was conducted for the studies on greywater, pathogenic bacteria, and antibiotics. The studies obtained from Scopus database were screened and compared to obtain the data for global antimicrobial resistance in 2000 and 2021. The strategies used by developed countries that led to the reduction in the recorded antimicrobial resistance are also listed. The challenges and limitations associated with the current plans adopted by several countries to minimise the spreading of the antimicrobial resistance are highlighted, while proposed solutions are provided. Two main issues associated with the distribution of antimicrobial resistance are (1) the absence of a plan in developing counties and presence of antimicrobial agents and ARB in the environment and (2) the difficulties in the current treatment technologies used for the removal of these antimicrobial agents from the water and wastewater. Based on the review and discussion, it was concluded that more advanced technologies are required to ensure total elimination of the antimicrobial agents and ARB from the environment. In addition, a new international standard should be drafted for the ARB in the environment, as they differ from the one currently used for medical applications.