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Factsheet: Faecal Indicators

What are faecal indicator bacteria?

Faecal indicator bacteria (FIB) are bacteria that come from the gut of warm-blooded animals (including people).  FIB are introduced to the environment through animal and bird droppings, effluent and waste water discharges, and stormwater run-off.  The presence of them in our waterways and groundwater indicates that other harmful pathogens may also be present.

To be safe from pathogens, any drinking water should be completely free of FIB.  Some faecal bacteria or pathogens can cause disease in people simply be coming in contact with them, so there are guidelines for acceptable amounts of FIB in recreational water.  Pathogens can also accumulate in some types of shellfish, posing a risk to people if eaten.

Regional councils monitor faecal contamination in water using faecal indicator bacteria.   The two most commonly measured FIB are 

E. coli (in freshwater) and Enterococci (in marine waters).

E. coli

What is E. coli?

E. coli (short for Escherichia coli) is a type of bacteria commonly found in the guts of warm-blooded mammals (including people) and birds.  E. coli can survive for up to four to six weeks outside the body in fresh water, making it a useful indicator of faecal contamination and the presence of disease-causing organisms.  Common sources of E. coli bacteria are human wasterwate discharges, animal waste, bird droppings and stormwater run-off.

Why is E. coli a problem?

The presence of E. coli indicates that the water has been contaminated with faecal matter, and it may therefore contain other pathogens that can cause illness.  

Drinking water should be completely free of E. coli to ensure that it is safe. To reflect this, the New Zealand Ministry of Health (2018) has set a Maximum Acceptable Value (MAV) for E. coli at less than 1 in 100 millilitres (mL) of sample. In other words, E. coli should not be detected at all in a 100 mL sample.

In water used for swimming and other recreation, low counts of E. coli are acceptable, but too much faecal contamination can cause gastroenteritis or infections of ears, eyes, nasal cavity, skin, and the upper respiratory tract. When E. coli in rivers and lakes are detected above 550 per 100 mL, health authorities put up signs warning that swimming or collecting shellfish is not recommended.

How to test for E. coli?

Water samples are collected in sterile bottles and sent to laboratories to be tested using nationally approved methods.

Many laboratories offer drinking water testing services and can provide guidance on the analysis of drinking water for well owners. Public water suppliers also test for E. coli and can be contacted regarding test results. Regular testing for E. coli is recommended.  

Which unit is it given in?

E. coli counts are reported as Colony Forming Units (cfu) or Most Probable Number (MPN) per 100 ml. The different units reflect different test methods – counting directly under a microscope versus a statistical presence/absence approach – but they give essentially equivalent results. Both represent the number of viable bacteria in a 100 millilitre sample of water.

For drinking water testing, if no bacteria are found, the result is reported as <1 cfu or MPN per 100 mL.  It would not be appropriate to report a result of zero, because it is not possible to give absolute assurance that an entire water supply is free of bacteria on the basis of a single 100 mL sample. The New Zealand drinking water standard, the MAV, for E. coli is <1 cfu or MPN per 100 mL sample.

 

Enterococci

What are enterococci?

Enterococci are bacteria that naturally occur in the gut of humans and animals as well as birds, fish and reptiles. These are the preferred biological indicator for faecal contamination of coastal swimming sites. Enterococci are the bacterial indicator for marine water.

How to test for enterococci?

Water samples are collected in sterile bottles and sent to laboratories to be tested using nationally approved methods.

Water samples are collected weekly from popular swimming sites by regional, unitary or local councils during the swimming season (usually November – March), and these results are shown in the 'Can I Swim Here?' section on LAWA.

Which unit is it given in?

Enterococci are counted under the microscope as Colony Forming Units (cfu) per 100 mL.

FAQs

Why not monitor for pathogens?

It is simply not practical, or even possible, to test for every potential pathogen that might be present in water.  Each type of pathogen would require a separate test, and there are many pathogens for which effective tests have not yet been developed. The most effective way to test for pathogens is to use indicators, FIBs, to let us know if faecal material is present in the water. We know when levels of FIBs may start to cause illness in people using the water, which makes them a robust, reliable indicator.

 Why is there a delay in getting FIB results?

Water samples are collected and then sent to the laboratory.  At the lab a sample of water is put onto a special substance (selective media) which encourages the growth of any FIB present in the sample.  After 24 hours, any FIB organisms present will have formed a colony, which are counted.  This is why there is a delay in taking a water sample and obtaining a water quality result.

How does E. coli get into groundwater?

Groundwater comes from rain that infiltrates downward through the soil and into aquifers. If the soil is contaminated with E. coli, the E. coli can also be carried downward by the infiltrating water. Aquifers act as natural filters and can reduce microbial contamination of groundwater, but their filtration ability varies widely. Many factors influence their filtration effectiveness, including permeability and the speed that groundwater travels through the aquifer. In places where water flows rapidly through the soil and aquifers, E. coli can be transported considerable distances and end up in aquifers that people use for water supply. A poorly sealed or open well can also provide a potential pathway for contaminated runoff from the land to directly enter a groundwater supply, bypassing the filtration provided by aquifer materials. If a homeowner has a private well, E. coli can be pumped directly to the tap in the home.

How do I avoid contamination of my groundwater supply?

All wells should be constructed to specifications set out in the Environmental Standard for Drilling of Soil and Rock (NZ4411:2001). The area immediately surrounding your well should be fenced off to prevent stock access and kept clear of any potential contaminants. Well casing should be elevated above ground and above stormwater and flood levels. The top of the well should be securely sealed or capped, and any hoses or cables should also be securely sealed. Backflow prevention devices should also be installed.

Where do I find more information?

Microbial Water Quality Guidelines for Marine and Freshwater Recreational Areas (2003). New Zealand Ministry for Environment and Ministry of Health. 

Drinking-water Standards for New Zealand 2005 (revised 2018) (2018). New Zealand Ministry of Health. 

Well Water Health and Safety Guide. Hawkes Bay Regional Council.