Microbial proliferation cannot occur in fuel without the presence of some free water. The microbes first develop in this water and later spread into the fuel. Few fuel systems with water associated with them are completely free of micro-organisms but the level of sensitivity of the test is such that normal and acceptable levels of microbial contamination are not detected. A positive result in the test indicates an abnormal level of microbial contamination in the water tested and by implication strongly suggests unacceptable microbial proliferation in some of the fuel/water system sampled. Conversely a negative result in the test strongly suggests that the fuel/water system sampled will be unlikely to be operationally affected by microbial spoilage. Microbes in large volumes of fuel tend to settle towards the bottom water and are detectable by the Sig Rapid WB™ test. Upper fuel may then not be heavily infected, however, movements of fuel will probably disturb and redistribute bottom contamination.