When doing a microscopic exam at 100x or 400x magnification, the bacteria do not appear particularly distinctive. In fact, free bacteria are the small round, oval, or rods that appear to be bouncing around in the water. Bacteria in biofilm or floc appear as larger aggregates that should be evaluated for size and density. However, what we usually note in microscopic exams are the protozoa that are associated with the bacteria. The observed protozoa are highly dependent upon water quality which is why frequent use of microscopic exam can establish a baseline and any changes is readily apparent.
The organisms pictured above is a stalk ciliate. This protozoa is found in systems with low soluble BOD, good floc formation that allows for stalk attachment, and sufficient dissolved oxygen. Any toxic shocks will cause stalk ciliates to detach from the floc or disappear completely.