Impact of Storage at -80℃ on the Quantification of Bacterial Targets in Beach Water Samples

Many environmental microbiology research studies involve the archiving of water samples for days to years, at which point molecular analyses may be performed. However, few research studies have evaluated the impacts of sample storage on quantitative polymerase chain reaction (qPCR) analyses of microbial targets. As a result, the extent to which results of qPCR analyses from archived samples accurately reflect results that would have been obtained immediately after sample collection is unknown. To address this knowledge gap, we compared qPCR results from 205 pairs of sample analyses, with one conducted immediately after collection and one conducted after archiving at -80℃ for up to six years. As part of an ongoing beach water monitoring program in Chicago, water sample aliquots have been filtered and analyzed by qPCR for enterococci within two hours of collection and other aliquots have been filtered and stored at -80℃. There was no significant difference in qPCR results between samples analyzed immediately and those archived for 1 month to 5 years. Additionally, the correlation between original qPCR results and those obtained up to 6 years after archiving was moderate to strong. These findings suggest that bacterial target concentration data generated using samples stored for up to 5 years at -80℃ are a reasonable reflection of the results that would have been obtained from the immediate qPCR analyses of water samples.