The two factors that make biofilm in an intravenous line especially dangerous to the patient is infection and the difficulty of treatment. While the microorganisms are within the biofilm, they cannot be eradicated with antibiotics. However, if the biofilm were to detach from the IV line, the access to the circulatory system would potentially cause a massive infection.

Longevity.

Sterile or dry conditions

Biofilm forms when bacteria sticks to surfaces in moist environments by excreting a slimy substance. Examples of this would be the slippery rocks on the bottom of a stream or plaque on teeth.

It depends on the type of biofilm, what surface the biofilm is on, and what information you want to get by looking at the biofilm under a microscope.

If you just want to look at how much of a surface is covered by a biofilm, you can use normal light trasmission microscopy (as long as the surface is transparent e.g. glass). Alternatively you could use epifluorescent microscopy in combination with a fluorescent stain.

If you want to look at the structure of the biofilm, confocal laser scanning microscopy is probably the best as you can get a 3D image.

Other useful types of microscopy include phase contrast and DIC, which allow you to look at the biofilm without staining it first.