Microbes floating in process streams attach on surfaces. While on there, some species produce extracellular polymers that soon form a visible layer of slime on the surface. This does not happen in places where shear forces are high, e.g. in pipes with strong flow of stock, but is potentially possible everywhere else.
Typically these formations of microbes, polymers they produce and various fine matter are most harmful in those paper/board machine areas that are directly above the fiber web. Eventually these formations will grow so big that they no longer adhere to the surface and they detach. In the wrong place this kind of loose piece of biomatter can cause the web to break. This leads to increased production costs due to downtime and spots and holes in the product.