Do all bacteria form colony?

Yes, but recall the different types of bacteria: archaebacteria and eubacteria.

Somebody told me that some bacteria don't grow in solid medium. Can you explain why?

If you've microbiology, you'll know that bacteria either are grown by culturing them on solid media as individual colonies, or in liquid culture. The good things about the later is that the nutrient broth for liquid culture allows rapid growth up to a maximum density. Liquid culture is easy and cheap. On the contrary, solid media use the same nutrient broth as liquid culture, solidifying it with agar. Agar a polysaccharide derived from seaweed that most bacteria can’t digest. If it can digest it, then we have a problem and have to use another means to grow the species of choice. The purpose of growth on solid media is to isolate individual bacterial cells, then grow each cell up into a colony. This is the standard way to create a pure culture of bacteria.  All cells of a colony are closely related to the original cell that started the colony, with only a small amount of genetic variation possible. Solid media are also used to count the number of bacteria that were in a culture tube.

Take care, Duddz