Some bacteria adhere to red blood cells by attachment pili and cause the blood cells to clump, a process called hemagglutination.
In Neisseria gonorrhoeae strain with pili are highly infectious because they attach to epithelial cells of the urogenital system and also attach to sperm cells and thereby spread to the next individual.
The attachment pili allow the organisms to remain in the broth, from which the take nutrients, while they congregate near air, where the oxygen concentration is greatest.