Determination of crAssphage in water samples and applicability for tracking human faecal pollution

In recent decades, considerable effort has been devoted to finding microbial source-tracking (MST) markers that are suitable to assess the health risks of faecally polluted waters, with no universal marker reported so far. In this study, the abundance and prevalence of a crAssphage-derived DNA marker in wastewaters of human and animal origins were studied by a new qPCR assay with the ultimate aim of assessing its potential as an MST marker. crAssphage showed up to 10⁶ GC/ml in the sewage samples of human origin, in both the total DNA and the viral DNA fraction. In wastewaters containing animal faecal remains, 39% of the samples were negative for the presence of the crAssphage sequence, while those showing positive results (41% of the samples) were at least 1 log₁₀ unit lower than the samples of human origin. Noteworthy, the log₁₀ values of the ratio (R) crAssphage (GC/ml)/Escherichia coli (CFU/ml) varied significantly depending on the human or animal origin (R > 1.5 for human samples and R < -1.5 for animal wastewater samples. This study opens the way for further research to explore if different specific animal variants of crAssphage exist and whether other zones of the crAssphage genome are better suited to source discrimination.