AMR Monitoring and Surveillance
Knowledge of removal efficiencies of technology employed at WWTPs is essential in AMR surveillance, whether effluents are intended for discharge into the environment or reuse for irrigation purposes.
To determine the presence of antibiotic resistant E. coli
in a conventional WWTP in Georgia, Aslan et al. [36
] isolated E. coli
from post-secondary, post-UV and post-chlorination effluent and performed antibiotic susceptibility tests on the isolates. They reported that ~5.2 log removal of E. coli
and an additional 1.1 log reduction post chlorination was obtained. However, the MICs of E. coli
isolated in the finished water were higher than those at the other treatment stages. The selection of more resistant organisms in the finished water underscores the urgent need to evaluate the health risks of using reclaimed water for downstream irrigation. In a qPCR assessment of ARGs of a full-scale tertiary water reclamation plant, Quach-Cu et al. [39
] showed that tertiary-stage WWTPs with disinfection had superior removal of ARGs (sul1
), ~3–4 logs compared to reliance of secondary treatment alone where the removal was only ~1–3 logs. To assess the impact of treated effluent on receiving environments, Lambirth et al. [35
] measured the removal efficiency of ten antibiotics and assayed resistomes upstream, downstream and within various treatment steps of two urban WWTPs (secondary and disinfection treatments). The authors found elevated concentration of all 10 antibiotics surveyed in downstream receiving waters compared to waters upstream of the WWTP. The relative abundance of ARG signatures encoding for resistance to carbapenems and ESBL antibiotics were much lower than those detected upstream and sampling points within the WWTP, which debunks the notion that the wastewater treatment process selects for ARG resistance. Instead, the authors hypothesized that antibiotics discharged from treated wastewater effluent into the downstream environment may have an effect on natural microbial communities.
Jumat et al. [37
] conducted an ESBL study of diversity and transcriptional activity of bacteria in a WWTP in Saudi Arabia using metagenomics, metatranscriptomics and real-time qPCR. They found an increase in the relative abundance of Acinetobacter junii
in MBR- and chlorinated treated effluent. Survival and predominance of A. junii
was explained by metatranscriptomics data that showed an upregulation of gene associated with active cell repair, resistance, virulence (efflux transporters involved in metal and antibiotic resistance) and cell signaling. These adaptive cellular mechanisms enable A. junii
to withstand depletion of nutrients and counter the effects of chlorination. However, the authors indicate that the low concentrations of viable A. junii
isolated from MBR effluents may not present that huge a risk. The varying results from WWTP studies covered in this issue makes it challenging to establish whether WWTPs are indeed hotspots of AMR dissemination. Rather, it is likely that differences in global and plant operating process contribute to variation in antibiotic resistance elements detection.
In countries with high agricultural productivity, water reuse is commonly practiced to meet the high water demands of the industry [43
]. On a global scale, there are no clear guidelines implemented on assessment of water quality for reuse purposes, although a few countries have drawn up recommended microbiological parameters to monitor the quality of recycled water [44
]. A recent review by Hong et al. [44
] highlights the urgency of understanding the risks of microbiological and ARG contamination linked to water reuse. In the Philippines, surface waters contaminated with fecal coliform are frequently used for irrigating urban farms in densely populated cities [45
]. Vital et al. [33
] evaluated the antibiotic resistance profiles of 212 E. coli
strains isolated from irrigation water, soil, and vegetables from six urban agricultural farms. Of the total isolates, 36.5% were resistant to more than three antibiotics tested, with the most multidrug-resistant (MDR) isolates being detected in irrigation water, followed by soil and vegetables. Of the MDR E. coli
isolated from irrigation water, 7 of them were ESBL producers that carried either blaTEM
genes, which raises public health concerns in primary production environments such as agricultural soils and fresh produce grown in these areas.
The use of antimicrobials in food animals is widespread, and runoffs originating from animal waste may carry unmetabolized antibiotics or ARB and ARGs that have direct impact on surrounding water bodies [46
]. Tsai et al. [32
] linked significantly higher concentrations of A. baumannii
along the Puzi River in China to sampling sites of livestock wastewater channels and tributaries adjacent to livestock farms. Further testing of 20 A. baumannii
isolates against 7 antibiotics (ciprofloxacin, cefepime, gentamicin, imipenem, ampicillin-sulbactam, sulfamethoxazole/trimethoprim, tetracycline) by the Kirby-Bauer disk diffusion test showed that only 10% had resistance to sulfamethoxazole/trimethoprim and 5% had resistance to tetracycline. Although the A. baumannii
isolated by Tsai et al. [32
] did not display MDR patterns which are regarded a serious AMR threat by WHO, their epidemiological potential warrants further studies on prevalence and AMR developmental trends in the environment.
In China, the Yellow River in the North serves as an important source of drinking water and is flanked by cities along its banks and watersheds. Previous studies have reported high turbidity and concentrations of antimicrobials in the Yellow River Catchment [47
]. This prompted Lu et al. [38
] to investigate the distribution and abundance of ARGs in Sand Settling Reservoirs (SSR) and Drinking Water Treatment Plants (DWTP) along the Yellow River. By targeting 17 ARGs as a proxy for AMR removal through the treatment process, the total concentrations of ARGs decreased from 104
copies/mL in influent river waters to 103
copies/mL in SSR effluent to 102
copies/mL in finished water. The 2 MGE targets decreased by at least an order of magnitude, from 106
copies/mL in influent river waters to 105
copies/mL in finished waters.
Horizontal gene transfer (HGT) of ARGs in the mammalian gut through the ingestion of contaminated food or water generally poses a low risk due to harsh conditions in the gut. However, this may present a greater risk for propagation in the environment through transformation and transduction [44
]. In another study by Xu et al. [34
], the authors studied the relationship of 16 antibiotics with environmental water quality parameters and the impact of antibiotic concentrations on the microbial community structure along Qingcaosha Reservoir, the largest estuary reservoir in China. This reservoir has a similar function to the Yellow River, in that it compensates for drinking water shortages in Shanghai. From the study, the authors concluded that upstream runoffs and anthropogenic activity along the river contributed to the concentrations of antibiotics measured within the reservoir, and that tylosin, penicillin G and erythromycin-H2
O showed significant correlations with variations in bacterial community structure. Further to this study, Jiang et al. [40
] studied seasonal variations of antibiotics in surface waters of Qingcaosha Reservoir. By using risk quotients (RQs) based on the European technical guidance documentation (TGD) on risk assessment, they showed that out of the 17 antibiotics monitored, four antibiotics (doxycycline, penicillinV, norfloxacin, ciprofloxacin) posed a high risk to relevant sensitive aquatic organisms, as well as imposed selective stress on microbial communities. In another study, of a subtropical river-reservoir system located in the Headwater Region of the Dongjiang River which supplies drinking water to three major cities in China, Chen et al. [41
] conducted an ecological risk assessment which showed that concentrations of ciprofloxacin and norfloxacin posed a moderate risk, while tetracycline posed a higher risk to the aquatic ecosystem.
Finally, through screening a local antimicrobial peptide (AMP) library, Chin et al. [42
] identified LG21, an AMP that specifically binds to exopolysaccharide PsI of P. aeruginosa
that has a functional role of biofilm formation, which provides a protective environment for tolerance and resistance towards antibiotic treatment. This strategy of exploring AMP to target specific biofilm matrix components to disrupt formation and development of biofilms is a promising line of treatment to eradicate antibiotic resistant biofilms in both environmental and clinical settings.