Which method is commonly used for testing the presence of bacteria in water?

The membrane filtration method is a widely recognized technique for testing the presence of bacteria in water because it effectively separates microorganisms from the water sample for subsequent analysis. This method involves passing the water sample through a membrane filter with a pore size small enough to retain bacteria while allowing the liquid to pass through. After filtering, the membrane is placed on a growth medium and incubated, allowing any viable bacteria to grow into visible colonies. The colonies can then be counted and identified, providing an accurate indication of bacterial contamination levels in the water.

Other methods mentioned, such as Gram staining, are primarily utilized for identifying and classifying bacteria that have already been isolated, rather than determining their presence in a bulk water sample. The sedimentation method is mainly focused on separating solids from liquids by gravity and does not specifically target bacteria. Gas chromatography is a technique more suited for analyzing volatile compounds in liquids or gases and is not used for detecting bacterial contamination in water.