The Threat of Colistin Resistance: A Global Concern
The rise of multidrug-resistant (MDR) bacteria poses a significant threat to clinical care and global public health. Among these, carbapenem-resistant Enterobacteriaceae (CRE) have been identified as critical priority pathogens due to their limited treatment options. The emergence of plasmid-mediated colistin resistance genes such as mcr-1 has further exacerbated this issue, undermining colistin’s effectiveness as a last-resort antibiotic.
The Spread of MCR Genes
The mobile colistin resistance gene, mcr-1, was first identified in Escherichia coli from a pig in China in 2015. Since then, it has been reported globally across different bacterial species and hosts, including humans, animals, and environmental sources. The horizontal transfer of mcr genes via plasmids allows for the rapid spread of colistin resistance.
To date, ten different variants of the mcr gene family (mcr-1 to mcr-10) have been identified. These genes reflect the diversity of mechanisms contributing to colistin resistance. The horizontal transfer of these genes through various plasmids can also spread carbapenem resistance, as seen with the NDM-5 carbapenemase gene found in some E. coli strains.
Klebsiella Pneumoniae Complex: A Key Player
The Klebsiella pneumoniae complex, comprising species like K. pneumoniae, K. quasipneumoniae, K. variicola, and K. Africana, are opportunistic pathogens often implicated in infections in hospital settings. They frequently carry mcr genes, contributing to the dissemination of colistin resistance.
Colistin Resistance in Wastewater
Recent studies highlight the emergence of colistin-resistant strains in various environments, including wastewater treatment plants. For example, in Algeria, researchers identified MCR-1 producing Gram-negative bacteria in aquatic settings. In South Africa, colistin-resistant Aeromonas spp. containing mcr-3 and mcr-5 were found in water sources. In Croatia, mcr-4.3 positive Klebsiella spp. were isolated from treated wastewater.
Wastewater plants play a crucial role in the dissemination of antibiotic resistance by integrating antibiotics excreted in urine and feces from both human and veterinary sources. This integration ultimately facilitates the transfer of resistance genes into the environment, making colistin resistance a global concern.
China’s Unique Challenges
China presents a unique case due to several factors. The widespread agricultural use of antibiotics, especially in animal farming, significantly contributes to the development and spread of antibiotic-resistant bacteria. Rapid urbanization and heavy industrialization increase the volume of wastewater, further exacerbating the challenge.
Public health concerns regarding the rising prevalence of resistant bacteria in both human populations and environmental ecosystems underscore the importance of monitoring colistin resistance in China’s wastewater systems. Understanding these local challenges is crucial for developing targeted interventions.
Research Aim
A recent study aimed to investigate the prevalence, genomic characteristics, and horizontal gene transfer potential of mcr-carrying Enterobacteriaceae isolates from the influent of the Yangzhou Wastewater Treatment Plant in China. Researchers sought to better understand their epidemiology and transmission dynamics within this environmental context.
Methodology
From March 2022 to January 2024, 366 sewage samples were collected from the Yangzhou Wastewater Treatment Plant in Jiangsu Province. Samples were enriched, streaked onto Eosin-Methylene Blue Agar plates supplemented with colistin, and colonies with distinct morphologies were selected and purified. Bacterial species were identified using matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF MS).
Antimicrobial susceptibility testing and PCR detection of mcr genes were performed. Conjugation experiments and whole-genome sequencing were conducted to study the horizontal gene transfer potential and genomic makeup of the isolates.
Results
Thirty-three colistin-resistant strains were isolated from the 366 sewage samples. Among these, three were identified as mcr-positive strains, representing an isolation rate of 0.82%. These included one isolate harboring mcr-1 and two isolates harboring mcr-10.
Species identification confirmed that the mcr-1 isolate was E. coli, while the mcr-10 isolates were Klebsiella quasipneumoniae subsp. similipneumoniae and Klebsiella variicola subsp. variicola. Resistance profiles showed distinct differences between E. coli and Klebsiella strains, with E. coli demonstrating a broader range of antibiotic resistance.
Genomic Analysis
Genomic DNA from mcr-positive isolates was sequenced. Genomic lineage analyses revealed that the mcr-1 isolate clustered with a K. pneumoniae strain isolated from human feces in Hangzhou, China, while the mcr-10 isolates belonged to novel sequence types.
The mcr-1 gene was located in a conserved genetic environment, with xerC upstream and ISEc36/IS26 downstream. The genetic context of the mcr-10 gene in the isolates was consistent with this conserved arrangement.
Discussion
Previous studies indicate that mcr-10-carrying plasmids are primarily found in Enterobacter spp., suggesting limited genus specificity. The presence of mcr-10 in Klebsiella pneumoniae complex strains is noteworthy, as these bacteria are frequently associated with hospital-acquired infections.
The findings align with domestic and international reports of mcr-1-carrying isolates in wastewater, with E. coli being the main carrier. Various sequence types (STs) of mcr-1-harboring E. coli have been identified in different sources, including animals, foods, humans, and environmental samples. E. coli ST167, found in this study, has been reported as a common carrier of mcr-1 in China.
The role of carbapenem resistance, particularly NDM-5, in the E. coli isolate YZ22WS208 is significant. This carbapenemase gene, located on the IncHI2 plasmid, is consistent with other studies reporting NDM-5-mediated resistance in Enterobacteriaceae.
Conclusion
This study detected mcr-1-positive E. coli and mcr-10-positive K. pneumoniae complex strains in Yangzhou wastewater. The presence of mcr-positive Enterobacteriaceae in wastewater highlights the potential risk of gene spread, even in environments with limited colistin exposure.
Early detection and ongoing monitoring are crucial, and wastewater surveillance remains a valuable tool for tracking antibiotic resistance across ecosystems.
The genome sequences from this study have been deposited into NCBI GenBank under PRJNA1162414.
Support for this work was provided by the fifth phase of the “333 Project” scientific research project in Jiangsu Province and other institutions.
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