Examining relationships between total silver concentration and Sil silver resistance genes in domestic wastewater...

The overarching aims of this study were to determine if microbes found in wastewater treatment plants (WWTPs) carry silver resistance genes and ascertain whether a relationship exists between silver loading and sil silver resistance gene copy numbers.

Activated sludge samples were collected from 17 WWTPs across the United States and characterized for chemical oxygen demand (COD) and total silver concentration. Gene copy numbers were quantified using qPCR for four common silver resistance genes (silC, silP, silR and silS). Spearman correlation analyses were performed to examine associations between three WWTP characteristics (i.e. size, COD and total silver concentration) and sil gene copy numbers. sil genes were found in all activated sludge samples, however the quantity of gene copy numbers was inconsistent. Of the sil genes screened, silS was the most abundant, followed by silR. Gene copy numbers for silC and silP were generally similar and overall lower than silS and silR.

Neither individual or combined sil gene copy numbers correlated significantly with total silver concentration, suggesting either the presence of alternate mechanisms of silver resistance or that a critical silver concentration threshold within WWTPs was not met that would require specific resistance against silver.

To the authors' knowledge, this is one of the first studies to measure the prevalence of sil genes in engineered systems. Results from this study suggest that at least four silver resistance genes are commonly found within WWTP-activated sludge, however it is unclear if these genes are being expressed under current total silver loadings.