Which of the following represents an instance of commensalism?
1) Salmonella colonizes the gallbladder without any effect on the gallbladder.
2) E. coli lives in the host's intestine and releases vitamins that are absorbed by the host.
3) Pseudomonas aeruginosa grows in the upper respiratory tract, inducing difficulty in breathing.
4) Clostridium difficile damages the cells lining the intestine.