There are essentially two methods, both indirect, for gaining information about aerobic microbial populations: Growth and Biochemical.

The traditional manner (Growth method) by which the contamination level of microbes is determined, is to take a sample of the fluid to be tested, dilute it appropriately, streak it onto a nutrient plate, incubate for 24-72 hours, manually count the colonies that grew, then back-calculate (from the known dilution ratio) the concentration in the original sample. This was the method of Pasteur and it has changed little; the principal modern improvements coming in the nutrient selections available. The assumption when using growth methods is that each viable organism will multiply sufficiently during incubation to produce one (and only one) colony visible to the naked eye. The result of this examination is expressed in COLONY FORMING UNITS PER MILLILITER (CFU/ML). Growth methods are called COUNTS. They are, in fact, not really counts but are estimates of the density of organisms with a published precision of around + 30 percent.

Biochemical tests (HMB) measure the level of some marker molecule that is produced in metabolic activity or in reproductive activity. They are, therefore, measures of ACTIVITY. Activity and counts are not the same measurement. Both are useful, often complementary, but, in a given system there may or may not exist any useful correlation (important).