Bradley DE. 1980. A function of Pseudomonas aeruginosa PAO polar pili: twitching motility. Can. J. Microbiol. 26: 146-54 Norsker NH, Nielsen PH, Hvitved-Jacobsen T. 1995. Influence of oxygen on biofilm growth and potential sulfate reduction in gravity sewer biofilm. Water Science and Technology 31: 159-67 h*****[email protected] Thanks a lot.