# 5.7.5 Problems caused by large numbers of microorganisms Large numbers of aerobic heterotrophic bacteria in treated water can interfere with the interpretation of test results for the coliform group. The method employed for coliform group measurement and enumeration influences the type of interferences that may occur, for example: * where membrane filtration methods are employed, non-target organisms may overgrow target organisms, masking their presence and yielding false-negative results. * where defined enzyme substrate methods are employed, large numbers of non-target organisms may give false positive reactions (e.g. the presence of high levels of *Aeromonas* spp. in water samples, which may have a low level of galactosidase production, can lead to false positive results). Most of these organisms can be controlled relatively easily by water treatment processes, including disinfection. Nutrient-rich raw water should be avoided if water treatment cannot be applied. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://guidelines.nhmrc.gov.au/australian-drinking-water-guidelines/part-2/chapter-5/5.7-nuisance-organisms/5.7.5-problems-caused-by-large-numbers-of-microorganisms.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.