What indicates the presence of fecal coliform in water samples?

The presence of fecal coliform in water samples is indicated by gas formation during a fermentation process. Fecal coliform bacteria, which are commonly found in the intestines of warm-blooded animals, can ferment lactose to produce gas, particularly in the form of carbon dioxide. This characteristic is utilized in laboratory testing, such as the multiple-tube fermentation technique, where a sample of water is inoculated into a lactose broth. The production of gas in the broth suggests the presence of fecal coliform, confirming contamination from fecal sources.

Gas formation is a direct and reliable indicator, as it signifies metabolic activity of fecal coliform bacteria specifically. Other options, while they may relate to water quality or biological processes, do not specifically indicate the presence of fecal coliform. For example, a change in color might indicate various chemical reactions or the presence of different organisms but is not specific to fecal coliform. Similarly, an increase in turbidity can result from sediment or other microorganisms, and the presence of algae is indicative of nutrient loading, such as nitrogen and phosphorus, rather than fecal contamination. Thus, gas formation is the most definitive indicator of fecal coliform presence in water samples.