Fimbriae are short hair-like projections across the surface of a bacterium or pathogen. They are shorter than flagellum or pilus, with a bacterial cell only have one or two, whereas it can have many fimbriae.
Fimbriae is important in the adhesion and colonisation of bacteria in host-pathogen interactions because they are 'sticky'. Some fimbriae have adhesions on their surface, allowing for efficient adhesion. Furthermore, fimbriae colonise into a capsule around the bacterium and allowing it to hide fom immune recognition while potentially causing host cell damage. Fimbriae also allow bacteria to grow in a community population.
A bacterial species consisting of fimbriae are most likely community based, whilst if they have flagella they are likely to be motile and planktonic rather than functioning to colonise. Both structures are used by pathogens, with transitions between the amount of each structure that each bacteria has.