1. The puzzle in the pipes
If a carbapenem resistance gene appears in wastewater, does it reflect people carrying resistant bacteria—or bacteria living along the sewer route? That distinction matters when wastewater measurements are used to interpret resistance in a community. A review of hospital wastewater surveillance also identifies methodological challenges and inconsistent reporting as barriers to interpreting resistance signals (2). Schussman and colleagues tested whether the sewer environment helps explain the genes detected at two treatment plants serving the Milwaukee area (1).
2. How the authors tested the signal
The researchers measured six carbapenem resistance genes in untreated wastewater. Their 122 samples comprised 72 collected from September 2022 through August 2023 and 50 collected over 25 days in July 2024. Droplet digital PCR measured gene copies; targeted sequencing checked whether the assays amplified the intended genes. The team also compared bacterial communities at upstream sewer sites and the treatment plants, tested relationships with temperature and flow, and cultured a selected set of wastewater bacteria. The two systems differed in average sewer travel time: approximately 7.5 versus 22 hours (1).
3. Abundant genes, uncertain origins
Four genes were quantifiable in every sample. Reported mean concentrations reached 3.55 million copies per liter for blaKPC and 1.11 million for blaOXA-24/40; blaNDM was below the quantification limit in 51% of samples. More than 99% of sequenced target amplicons matched the intended genes. These results establish substantial, largely target-specific DNA signals—not how many residents carried resistant bacteria, or whether the detected DNA came from living cells (1).
Human-associated bacterial markers generally fell threefold to fourfold between upstream sites and the plants, while some taxa associated with resistance genes rose by roughly twofold to sixfold. The authors infer that sewer-associated reservoirs may contribute to the signal. Cautiously interpreted, these are relative-abundance shifts: they cannot distinguish bacterial growth from faster loss of human-associated bacteria (1).
4. Temperature and cultured hosts
At one plant, flow-normalized blaOXA-48 increased with temperature (Spearman’s r = 0.71); at the other, blaOXA-24/40 decreased (r = −0.59). The contrasting results suggest that local sewer conditions matter, but correlations cannot establish that temperature caused either change. The same seasonal pattern was not demonstrated across both plants (1).
Of 51 selected cultured isolates, 20 carried either blaKPC or blaOXA-24/40. Fifteen of the 18 blaKPC-positive isolates were Aeromonas, rather than the clinical hosts commonly associated with that gene. This directly identifies an alternative host among the cultured isolates, not the host of every gene copy measured in wastewater (1).
5. Limitations and uncertainty
The main time series spans one year, so seasonal associations need testing across additional years. Selective culture and identification captured only a limited, selected set of organisms; sequencing short gene regions confirmed assay targets but did not connect every detected gene to its host. Relative abundance and temperature correlations also leave growth, survival and changing inputs difficult to separate. These limits matter before translating a plant-wide measurement into a claim about infections (1).
6. The takeaway
Wastewater can reveal resistance genes without revealing where they originated. Before treating a rising gene signal as rising human carriage, ask whether local sewer ecology—and the bacteria carrying the gene—have been examined (1).
7. References
(1) Schussman MK, Feng S, Schmoldt A, Bootsma MJ, Dennis K, Janssen KH, McLellan SL. Environmental reservoirs account for high levels of carbapenem resistance genes in wastewater. Microbiology spectrum. 2026;14(2):e01737-25. doi:10.1128/spectrum.01737-25.
(2) Lymperatou D, Konstantopoulou R, Mentsis M, Atzemoglou N, Diamanti C, Tzourtzos I, Naka KK, Mitsis M, Konstantina G, Milionis H, Ntzani E, Christaki E. Hospital Wastewater Surveillance and Antimicrobial Resistance: A Narrative Review. Microorganisms. 2025. doi:10.3390/microorganisms13122739.
8. Topics
#AntimicrobialResistance #WastewaterSurveillance #SewerMicrobiology #ResistanceGenes