Introduction:Thehost intestinal epithelial cell layer and extracellular mucusplaya key role inpreventing infection fromthe vast numbers of enteric microorganisms.Pseudomonas aeruginosa can penetrate these barriers and set the stage for the development of invasive infections suchas gut-derived sepsis in immunocompromisedpatients. Wehave demonstrated that a type III effector, ExoS, facilitates P. aeruginosa penetration through the epithelial barrier by impairing the defense of tight junctions. However, how P. aeruginosa penetrates the mucin layer to reach the epithelial cell surface remains largely unknown. Objectives: Here, we found that two efficient mechanisms, flagellar motility and mucin degradation, are needed for penetration of P. aeruginosa through the mucin barrier. Methods: (1) Association assay: Bacterial association was evaluated by human epithelial colorectal adenocarcinoma (Caco-2) cells at 1 h and 2h after infection; (2) Mucin penetration assay: Mucin, from bovine submaxillary glands, was prepared in Transwell®. Bacteria were loaded on the top of the mucin layer, and the numbers of bacteria in the basolateralmediumwere counted at 1 h; and (3)Mucin degradation assay: Following infection, the mucus layer of the Caco-2 cells was stained with the Periodic Acid-Schiff (PAS) Staining and the MUC2 protein was analyzed by immunoblot with anti-MUC2 antibody. Results: Deletion of the flagellar motility-related gene, the filament protein gene fliC and the motor complex protein genes motABCD , from P. aeruginosa PAO1 decreased association of P. aeruginosa with the apical surface of Caco-2 cells and penetration to an artificial mucin layer. Additionally, the presence of P. aeruginosa decreased the total mucin, including the secreted mucin protein MUC2 on the surface of the Caco-2 cell monolayer, regardless of flagellar motility. Construction of the PAO1 mutant series knockedout 12 putative serine protease genes and identified the mucD gene, which participated in degradation of total mucin including MUC2. Furthermore, decreased association with the Caco-2 cell monolayer was observed in the mucD mutant, and the decrease was synergistically amplified by double knock-out with fliC . Conclusion: P. aeruginosa can penetrate the mucin layer using flagellar motility and mucin degradation, which is dependent on the MucD protease or the mucD related-gene protease. This information, in addition to further analysis of the mechanisms of P. aeruginosa penetration of the epithelial cell layer followingmucin layer penetration, will lead to a greater understanding of P. aeruginosa infection via translocation in the human intestinal tract. P2 Genetic research of leprosy F. Zhang1 *, H. Liu1, S. Chen1, J. Liu2. 1 Shandong Provincial Institute of Dermatology and Venereology, Jinan, China , 2 Human Genetics, Genome Institute of Singapore, Singapore, Singapore E-mail address : hongyue2519@hotmail.com