The visual appearance of colonies of fungi can be very serious. Some colonies are entirely superficial and can be wiped away or removed by a buffer in a solvent or aqueous cleaner. Some fungi appear to live on the surface but occasional strands gouge troughs in the emulsion surface; this visual effect is known as "etching".
The troughs are permanent scars on the film and print clearly.
On occasions, [probably different species] the fungal hyphae "burrow" extensively forming a network of tunnels below the emulsion surface. It is probable that just one coating layer is being selectively fed on. The visual appearance of this type of damage is obvious and it is difficult or impossible to remedy.
The more diffuse bacterial colonies may also wipe off the surface or may have so damaged the emulsion that it too wipes away. It is important to look very closely at this type of damage before making any decisions. The emulsion may have deteriorated to such an extent that any mechanical contact with the emulsion will produce it's own damage.
Treatment to film with fungus or bacterial growth may be in several stages.