Question

What part of the bacteria cell helps it stick to surfaces?

Answer 1

any of three pili, fimbrae, flagella. but it is not the only way

Explanation:

there are various discrepancy on how bacteria stick to surfaces but new studies have started to reveal how it is done.

following is quote from a paper available at sciencedirect
http://www.sciencedirect.com/science/article/pii/S1002007108002049

"the accumulation of microorganisms on a collecting surface as a process of three stages: (1) adsorption, or the accumulation of an organism on a collector surface i.e. substrate (deposition); (2) attachment, or the consolidation of the interface between an organism and a collector, often involving the formation of polymer bridges between the organism and collector; (3) colonisation, or growth and division of organisms on the collector’s surface"

so flagella, pilli and fimbrae may be involved in adhesion of the bacteria on the surface. but you can not overlook the possibility that they can use polymer to stick on various surfaces.

here is figure showing pilli, flagella and fimbrae.