Final answer:
Researchers use bacteriological culture methods such as membrane filtration and lactose broth tubes to collect and estimate bacterial presence in potable water sources. These methods allow for the concentration and detection of bacteria, with subsequent calculations of cell density or estimations of bacterial numbers based on observed growth and colour changes.
Step-by-step explanation:
To collect bacteria from a potable water source, researchers use several bacteriological culture methods. One effective method is the membrane filtration technique, where known volumes of water are filtered through a membrane that retains microorganisms due to its small pore size.
After filtration, this membrane is transferred to a Petri plate containing a nutrient-rich medium appropriate for bacterial growth. After incubation, colonies that form on the plate are counted, and the calculation of cell density is made by dividing the cell count by the volume of filtered liquid.
Another common method of estimating bacterial presence involves using lactose broth tubes that change color when lactose is fermented by coliforms, which indicates possible faecal contamination in the water sample. Different dilutions of the water sample are prepared, and the growth of bacteria is checked by observing the colour change from red to yellow in these tubes after incubation. The resulting numbers are then compared with a probability model to estimate the number of bacteria per unit volume.