A Study Analyses the Presence of Antibiotic-Resistant Bacteria in Wastewater from Baix Llobregat

Antimicrobial resistance has become one of the main challenges for global public health, as it hinders the effective treatment of infections and increases the risk of serious complications, including death. It is estimated that in 2021 antimicrobial resistance was associated with 4.71 million deaths worldwide. In this context, a study led by the Barcelona Institute for Global Health (ISGlobal), a centre supported by the ”la Caixa” Foundation, in collaboration with Aigües de Barcelona and conducted in Baix Llobregat (Barcelona), evaluated the role of wastewater and drinking water treatment plants in the persistence and spread of antibiotic-resistant bacteria. The results emphasize the importance of strengthening the monitoring of resistant bacteria throughout the entire water cycle and confirm that the water reaching our homes is completely safe.

The research, carried out during the winter and summer of 2023, analysed the presence of antibiotic-resistant bacteria as well as resistance genes in water samples from two wastewater treatment plants in the province of Barcelona (Gavà-Viladecans and Baix Llobregat) and from a drinking water treatment plant in Sant Joan Despí. The research team used a combined approach of metagenomics, cultures, and whole genome sequencing to characterize the bacterial diversity in these facilities.

The study, published in the International Journal of Hygiene and Environmental Health, is part of the collaboration agreement between ISGlobal and Aigües de Barcelona, initiated in 2021. With this project, the strategic alliance between the two institutions is strengthened, which began with a study on human exposure to nano- and microplastics through drinking water.

Nearly a Thousand Antibiotic-Resistant Strains

The study identified a total of 991 resistant bacterial strains in the water samples analysed. Of these, 57.3% were multidrug-resistant and 2.7% were classified as extremely resistant. However, the analysis of the water at the end of the treatment processes showed a marked reduction in antimicrobial resistance compared to the initial stages, with minimal presence of resistance genes and absence of highly resistant strains, highlighting the effectiveness of the treatment plants.

“The reduction was significant in the final stage of the wastewater treatment plants and complete in the drinking water plant, meaning that the water reaching our homes is safe,” notes Sara M. Soto, director of the Viral and Bacterial Infections Program at ISGlobal and senior author of the study. “However, resistant bacteria from wastewater treatment plants could, once released into the environment, transfer their resistance mechanisms to other bacteria. For this reason, all treated wastewater intended for environmental use or reuse is disinfected, and it is essential to maintain thorough control of all treatment processes.”

Therefore, according to the study’s authors, although wastewater treatment plants are essential for reducing fecal load and microbial pathogens, they could also act as reservoirs of antimicrobial resistance. “Our findings expand understanding of the spread of antimicrobial resistance in aquatic environments and highlight the need to strengthen its monitoring throughout the water cycle within the ‘One Health’ framework, which integrates the human, animal, and environmental domains. Furthermore, the study provides a solid foundation for developing new regulatory frameworks capable of addressing the challenges associated with antimicrobial resistance,” says Victoria Ballén, ISGlobal researcher and first author of the study.

Main bacteria and resistance genes detected

During the early stages of treatment, the most common bacteria identified were from the genus Aeromonas (44.3%), followed by Enterobacterales (27.9%), Pseudomonas (19.1%), Acinetobacter (4.8%), Shewanella (2.2%), and Stenotrophomonas (1%). The wastewater treatment plants of Baix Llobregat and Gavà-Viladecans concentrated most of the findings, with 531 and 414 strains, respectively, while the drinking water plant of Sant Joan Despí recorded only 46 strains. The analysis also revealed a marked seasonal variation, with a higher number of resistant strains identified in summer (67.8%) compared to winter (32.2%).

Regarding the resistance genes —that is, fragments of bacterial DNA that enable survival against antibiotics— it was observed that the most frequent conferred resistance to beta-lactams, widely used against common infections; to aminoglycosides, used in severe infections such as kidney or bloodstream infections; and to macrolides, used in respiratory, skin, and soft tissue infections. In addition, resistance genes related to tolerance to biocides—used in disinfectant and cleaning products—and to heavy metals were found.

Another key finding was the detection of 29 colistin (COL)-resistant strains, an antibiotic considered a last-resort option for severe infections. These strains were found in both wastewater treatment plants and the drinking water plant, with higher incidence in summer.
 

Reference

Ballén, V., Mondéjar, L., Gabasa, Y., Castellsagués, L., Alcalde-Rico, M., Pinar-Méndez, A., Vilaró, C., Galofré, B., Soto, S.M. Integrated metagenomic, culture-based, and whole genome sequencing analyses of antimicrobial resistance in wastewater and drinking water treatment plants in Barcelona, Spain. International Journal of Hygiene and Environmental Health, 2025. DOI: 10.1016/j.ijheh.2025.114664