Combining direct viable count (DVC) and fluorescent in situ hybridisation (FISH) to enumerate viable E. coli in rivers and wastewaters

In this study, a combination of the direct viable count procedure (DVC) and the FISH method was used to monitor by epifluorescence microscopy the abundance of viable E. coli in river water and wastewater samples. The DVC procedure consisted of exposing bacterial cells to a resuscitation medium containing antibiotics preventing cellular division and, thus, inducing an elongation of the viable cells. The FISH was performed using the “Colinsitu” probe specific for E. coli 16S r-RNA. Accuracy and detection limit of the epifluorescence microscopic DVC-FISH procedure were investigated. The method was then applied to river-water and wastewater samples. A good correlation was found in a log-log plot between the abundance of E. coli enumerated by a classical culture-based method (MPN method) and the DVC-FISH procedure. However, the DVC-FISH procedure gave consistently higher numbers. The ratio between both enumerations (DVC-FISH/MPN), which also indicated the ratio between viable and culturable E. coli, ranged between 2 and >30. It increased with decreasing abundance of culturable E. coli.