Many correct indications were ...
Published by Giovanni Pavanello, Director at ALVIM Biofilm Monitoring Technologies
Many correct indications were provided with the previous answers. I would just add that also biofilm contributes to protect coliforms and other pathogens from chemicals. This means that bacteria attached to surfaces (forming a layer usually known as "biofilm") are more protected from chemicals than free-floating bacteria, thus they can survive even very high concentrations of chlorine. Then, when parts of biofilm detach from pipe/tank surfaces and float away, you can find those survived pathogens also in the water.