Final answer:
The correct answer is fimbriae, which are short, bristle-like fibers on the surface of some bacteria that aid in adherence to host cells and surfaces, crucial for colonization, infectivity, and biofilm formation.
Step-by-step explanation:
The stiff fibers on the outer surface of some bacteria that help them adhere to host cells are called fimbriae. While fimbriae and pili are often used interchangeably due to their structural similarities, the term fimbriae commonly refers to short, bristle-like proteins present in hundreds on the cell surface. These structures are essential for a cell to attach to surfaces and to other cells. In the case of pathogenic bacteria, adherence to host cells is critical for processes such as colonization, infectivity, and virulence, which are vital for the pathogen's lifecycle. Additionally, adherence plays a key role in biofilm formation, allowing bacteria to create complex and protective communities.