Biofilm is an aggregate of microorganisms and microbial agents attached to a surface, and to each other. A biofilm often contains bacteria, viruses, and other parasites, which can be dangerous sources of microbial contamination in pools and spas.
Biofilms form due to various factors that induce metabolic changes in their member cells, producing colonial behavior among multiple species. They tend to form in cracks in the plumbing or on the pool's walls, but may also be present as floating mats on the water's surface. Their cells usually produce a protective matrix of extracellular polymeric substance, or EPS; this is the "slime" one feels when touching the film. This EPS is resistant to most sanitizers, and can only be destroyed through concentrated spot chlorination. Sometimes, draining the pool before applying spot chlorination is necessary.