Abstract: OBJECTIVE The in vitro effect of Pseudomonas aeruginosa quorum sensing system(QS) on early polymicrobial biofilm of P aeruginosa - Aspergillus fumigatus(A.f) was investigated, in order to provide the basis for the treatment of clinical related polymicrobial microorganism infection. METHODS The wild-type P aeruginosa (PAO1)-clinical A fumigatus strain and the P aeruginosa quorum sensing defective strain (P.a ΔlasI/rhlI)-clinical A fumigatus strain were established in vitro and mixed to form the polymicrobial biofilms. Biofilms of solely the wild-type P aeruginosa (PAO1), P aeruginosa quorum sensing defective strain(P.a ΔlasI/rhlI) or A fumigatus were also developed. The RPMI-1640 group with pH 7 was used as a blank control. After incubated for 24 h, the biofilm formation ability was semi-quantitatively detected by crystal violet staining. The growth kinetics of biofilm in vitro was evaluated by 2,3-Bis-(2-methoxy-4-nitro-5-sulfophenyl) -2H-tetrazolium-5-carboxanilide (XTT) colorimetric method, and the ultrastructure of biofilm was observed under positive electron microscope and scanning electron microscope. RESULTS Crystal violet staining and semi-quantitative detection of 24h biofilm formation showed that the thickness of polymicrobial biofilm formed by the group of P aeruginosa quorum sensing defective strain (P.a ΔlasI/rhlI) + A fumigatus was higher than those of the other groups (P<0.05), whereas there was no significant difference between the thickness of biofilms of the PAO1+A.f group and the A fumigatus alone group. XTT colorimetric method test showed that the viability of P aeruginosa quorum sensing defective strain (P.a ΔlasI/rhlI) + A fumigatus (A.f) was significantly higher than the wild-type P aeruginosa (PAO1) + A fumigatus (A.f) (P<0.05). The electron microscope and scanning electron microscope observation of the biofilm ultrastructure suggested that the A fumigatus hypha density in the group of P aeruginosa quorum sensing defective strain (P.a ΔlasI/rhlI) + A fumigatus (A.f) was significantly higher than that of group PAO1+A.f . The hyphal density in the group of the wild-type P aeruginosa (PAO1) + A fumigatus was less than the group with A.f alone . CONCLUSION The quorum sensing system of P aeruginosa inhibited the formation of early polymicrobial biofilms of the wild-type P aeruginosa (PAO1) + A fumigatus.