Microbiological agents range from single cell organisms to small living things such as Bacteria, Protozoans, Fungi, Molds, and Viruses. This section deals with bacteria, which may or may not be in the water itself, which have an indirect effect on the water quality.
There are certain conditions, like a broken pipe, that cannot be regulated under some kind of standard. How would you test a water sample for ‘broken pipe’? Such a condition could make itself known by its effect on the drinking water, an effect such as bacterial contamination and loss of water pressure. In a similar fashion, there are some bacteria for which there are no standards. It could be that the water source such as an aquifer, a well, and/or the water distribution system have considerable quantities of a particular bacterium but there is little or none of that bacteria in the water that comes out of the tap. Like the broken pipe, such bacteria are known by their indirect effects on the water quality, effects which are regulated under primary or, more likely, secondary standards. It may happen that there is a significant amount of the bacteria in the water itself, enough to be revealed in a water test sample but even in that case the water would have to be tested for that particular bacterium. Even then, about all you could say is that this particular bacterium is present as there is no good way to correlate this amount of bacteria with that amount of effect.
Common effects include: elevated iron and manganese levels (Iron-reducing bacteria), hydrogen sulfide gas (sulfate-reducing bacteria), and reduced water flow (slime bacteria). Moist areas in a bathroom or basement may have what appears to be a pink mold (not a mold; it is pink bacteria).