What are they?
Measuring the concentration of all microorganisms that can potentially affect the health of swimmers is both difficult and very expensive. A more cost effective approach is the use of indicator bacteria. These are comparatively easily measured and are generally present when water is contaminated with harmful organisms such as Salmonella, Campylobacter, Giardia or Cryptosporidium. High concentrations of indicator bacteria are a sign that the water is potentially contaminated with human sewage or animal faeces, which results in an increased health risk associated with the use of the water body for recreational activities, such as swimming.
Two different indicator bacteria are used depending on the type of water that is being monitored. Freshwater samples are analysed for the concentration of E. coli. For coastal samples, Enterococci are the preferred indicator bacterium.
How are they measured?
A sample from the water at a site is placed on ice and taken to the local laboratory. In the laboratory the number of E. coli and Enterococci in a sample is measured by allowing them to grow for 18 to 24 hours on either a solid growth medium (agar) or in a liquid broth. If the bacteria were grown on a solid medium, the results are reported in units of cfu/100ml (cfu = colony forming units). If the bacteria were grown in a liquid broth, the unit of the result will be MPN/100ml (MPN = most probably number).
There is currently no way of measuring the concentrations of indicator bacteria at the site in real time. There is always a delay of at least 18 hours before the concentration is known.
Guidelines for individual results
In 2003, the Ministry for the Environment and the Ministry of Health released recreational water quality guidelines that are based on a reasonable risk approach. The document provides two guideline values for each of the two indicator bacteria. Based on these guidelines, sample results are categorised into three 'modes', which allow a decision to be made on whether the water can be considered safe for swimming. The table below outlines these 'modes' and their meaning as well as the actions that need to be taken as a result.
See Ministry of Health Recreational Water Quality Guidelines