​Coliform bacteria are an accumulation of different bacteria species, including both fecal and non-fecal bacteria. The fecal coliform bacteria are found in the intestines of warm-blooded animals, some fecal coliform species are disease-causing. When fecal coliform is present in water bodies it indicates possible contamination from human or animal fecal matter.

Enterococci are a different type of fecal bacteria found in warm-blooded animal intestines. They are a subgroup of fecal streptococcus that can live in salt water. Enterococci is an indicator organism at shellfish harvesting areas and coastal beaches. We use both types of bacteria as water quality indicators. 

Bacteria levels can fluctuate throughout different areas of Oyster Bay, Mill Neck Creek and Cold Spring Harbor depending on rainfall, the amount of runoff entering the water and the amount of tidal flushing. Bacteria levels can vary geographically, especially in areas of the bay bordered by streams or outfall pipes.

When a beach is closed for swimming after a heavy rainfall, it is because bacteria levels are too high, and it is a good idea to wait 48 hours before going swimming. Swimming standards are set by New York State and if enterococci levels are above 104 cfu (colony forming units) per 100 millileters it is unsafe to swim.