The presence of which microorganism in water indicates past or intermittent pollution?

Clostridium perfringens is a spore-forming bacterium that is commonly associated with the environment, particularly in soil and the intestines of humans and animals. The presence of Clostridium perfringens in water is used as an indicator of past or intermittent pollution because it is more resistant to environmental stresses than many other pathogens. This characteristic allows it to persist longer in the environment than fecal coliforms, which indicates recent contamination.

Since Clostridium perfringens does not multiply in water and can survive longer than other indicators, its detection often suggests that contamination may have occurred at some point in the past, rather than from a recent or continuous source. This quality makes it a valuable microbiological marker for assessing water quality and understanding pollution events over time.

In contrast, other microorganisms listed may indicate more immediate pollution or may not survive as long in the environment, which is why they are not as suitable as indicators of past pollution events.