The coronavirus disease (COVID-19) pandemic has triggered a major increase in the use of various medical facilities and drugs, which has exacerbated pollution in wastewater biochemical treatment systems. Environmental rare earth elements (REE) pollution has increased due to the widespread use of REE in medical applications.
Gadolinium (Gd) is commonly used in contrast agents and is released as a monomer that is toxic to organisms. Studies have shown the presence of Gd in both the influent and effluent of wastewater treatment plants (WWTPs).
In addition, different types of antibiotics were detected in WWTPs, with some antibiotic concentrations being detected at mg/L levels. The accumulation of antibiotics in WWTPs is detrimental to on microorganisms and negatively impacts the performance of WWTPs. Therefore, the combined pollution of REE and antibiotics in wastewater cannot be ignored, especially in the context of the COVID-19 pandemic.
Rising demand
Notably, due to the increased risk of bacterial infections, medical institutions need more sulfamethoxazole (SMX) to treat infected patients in the pandemic. On the other hand, magnetic resonance imaging (MRI) enables visualization of pulmonary structures and assessment of COVID-19-related lung damage, inflammation, and complications like thrombosis and myocarditis.
Gd, as a key component of contrast agent in MRI, the increased demand for MRI detection leads to an increase in the use of Gd. Therefore, Gd and SMX have been extensively used in this pandemic. However, the impact of co-occurring Gd and SMX in wastewater on bacterial resistance in WWTPs remains unclear.
Long-term use of antibiotics in medical, agricultural, animal husbandry, and aquaculture industries eventually lead to the dissemination of antibiotic resistance genes (ARGs). ARGs pose a serious threat to human health and environmental safety. Therefore, many researchers have studied the effects of antibiotics on ARGs in wastewater treatment systems.
Resistance genes
Studies have shown that a 55%–81% decrease in the concentration of antibiotics resulted in a 13-order of magnitude decrease in the relative abundance of ARGs in activated sludge systems. Furthermore, scientists noted that heavy metals drive the co-selection of ARGs and heavy metal resistance genes (MRGs). Studies have reported an increased relative abundance of ARGs and MRGs in heavy metal-polluted environments, and high concentrations of metals could promote multi-metal and multi-antibiotic resistance.
Moreover, studies have investigated on the co-selection of ARGs and MRGs under combined pollution of antibiotics and heavy metals. However, only a few reports have investigated the effects of Gd on ARGs and MRGs, and the succession and transmission characteristics of resistance genes under combined Gd and antibiotics exposure remain unclear.
Gene mobility
Quantitative Polymerase Chain Reaction (qPCR) and 16S rRNA gene high-throughput sequencing can quantify some known ARGs and MRGs, but the mobility of ARGs and the correlation with host bacteria are unelucidated. In addition, 16S rRNA sequencing is limited by microbial isolation and culture, and cannot detect and describe the diversity and function of complex microbial communities.
As a more advanced sequencing technology, metagenomics enables the assessment of uncultured microbial genes, which greatly expands the applications of microbial resources. Moreover, metagenomic sequencing can also be used for the in-depth study of genes and functions. Researchers have used metagenomics to explore ARGs, MRGs and host bacteria in an aquatic environment, demonstrating the broad-spectrum diversity of bacteria in water and the related resistance genes.
Some researchers used metagenomics to explore the selection process of antibiotic resistance in WWTPs. However, few studies focused on core genomes and the interaction between ARGs and MRGs in WWTPs. In addition, previous reports revealed the impact of antibiotics and heavy metals on ARGs and MRGs in an aquaculture environment and discussed the interaction between ARGs, MRGs, and bacteria. Nevertheless, these studies mostly ignored the interaction between ARGs, MRGs, and mobile genetic elements (MGEs), which play essential roles in the proliferation and transmission of resistance genes. The work of Professor Kangping Cui’s team fills this gap.
Co-occurrence in wastewater
In this study, the research team investigated the co-occurrence of Gd and SMX in wastewater pollution by applying metagenomics to analyze the mechanisms of changes in ARGs, MRGs, MGEs, and genera in an activated sludge system.
This study offers an in-depth and new understanding of the mechanisms underlying the changes and interactions between ARGs and MRGs in activated sludge, providing technical support for the removal of to ARGs and MRGs in WWTPs.
To help reach the overarching aim, the following specific goals were set by the team of Professor Kangping Cui: 1) to explore the dynamic change in characteristics of ARGs, MRGs, and MGEs, and identify the core genome; 2) to evaluate the joint effect of Gd and SMX on ARGs, MRGs, MGEs, and abundant genera; 3) to reveal the interactions and shifts in ARGs, MRGs, and MGEs.
Patterns found
The effects of SMX and Gd(III) on ARGs and MRGs were studied by metagenomic sequencing in an activated sludge system. The findings demonstrated that single SMX alone and co-occurrence of SMX and Gd(III) resulted in an increase in the abundance of ARGs, while most MRGs decreased in abundance.
Furthermore, the co-occurrence of SMX and Gd(III) significantly promoted the HGT of ARGs and MRGs. Gd(III) alone caused a decrease in the abundance of ARGs and MRGs, whereas the abundance of Hg MRGs was increased. Compared to core MRGs, core ARGs exert a greater negative effect in the presence of Gd or SMX alone.
Streptomyces, Pseudomonas, and Thauera were abundant under SMX exposure and may be potential hosts for ARGs and MRGs. The bacterial community was sensitive to single Gd(III) stress. Moreover, the correlations among ARGs, MRGs, MGEs, and the bacterial community were discussed in this study, suggesting a positive relationship between internal ARGs and MGEs, while positive and negative relations were found in MRGs. Moreover, most ARGs and MRGs were closely related to MGEs.
Prof. Kangping Cui’s team include Xinrui Yuan, Yihan Chen, Shiyang Wu, Yao Zhang and Tong Liu. They are affiliated with Hefei University of Technology. This work is supported by the National Key R&D Program of China.
Their analysis is published in the journal Frontiers of Environmental Science & Engineering on December 20, 2023.
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