Effect Of IFN-γ On The Formation Of Pseudomona Aeruginosa Biofilm

Biofilms are highly structured, surface attached communities of bacterial cells enclosed in self-produced polymeric matrix. Their formation reflects an adaptive behavior that enables the bacterial cells to survive in the hostile environment. Biofilm formation by opportunistic pathogens is devastating because the bacteria in these structured communities can withstand host immune responses, and become more resistant to antibiotics (100-1000 folds higher than planktonic bacteria). According to the public announcement of National Institutes of Health (NIH), biofilms may be involved in up to 80 percent of human infections. Consequently, controlling of biofilms will play a crucial role in cure of bacterial infections.Pseudomonas aeruginosa, a versatile Gram-negative opportunistic human pathogen, is the most prevalent bacterial pathogens in infectious diseases, such as respiratory tract infections, bacteremia, pneumonia, urinary tract infections, cystic fibrosis, infections of burn, catheter infections and infections of heart valve. P. aeruginosa has shown multi-drug resistance that always causes failure in clinical therapy and persistent and chronic infections. So, infections with P. aeruginosa are a major health problem worldwide, being associated with significant morbidity and mortality. The host-pathogen interaction has become a hot-spot for research of medicine and microbiology. At present, most of the studies are involved in how hosts sense pathogen's invading and regulate immune system to clear the bacteria away, whereas it is not clear how pathogens sense host immune response and alter itself phenotype to evade immune system. More recently, it is reported that IFN-γbinds to an outer membrane protein in P. aeruginosa, OprF, resulting in the expression of a quorum-sensing dependent virulence determinant, the PA-I lectin by triggering QS system of itself. This study is aimed to clarify whether P. aeruginosa can sense IFN-γand enhance the formation of biofilm to escape from host defense. To determine whether IFN-γis capable of influcening P. aeruginosa biofilm formation, biofilm assay of P. aeruginosa under static and dynamic culture condition with or without IFN-γwere carried out. The results indicated that IFN-γcan promote the formation of P. aeruginosa biofilm dramatically. Furthermore, there was distinct influence on P. aeruginosa biofilm formation with various concentrations of IFN-γ, incubation time and cell densities. The results of the static biofilm assay indicated that the formation of P. aeruginosa biofilms was increased by 40% when the concention of IFN-γwas 10 ng/ml, cultured 6 hours and at high cell densities (OD600=1.8). And in the dynamic flow-cell conditions, significant enhancement of biofilm development, 4.0-fold as assessed by total biomass, 3.0-fold as assessed by average thickness, was observed when cultivated with 10 ng/ml of IFN-γfor 72 h. Moreover, IFN-γin concentrations used for this study had no effect on growth of P. aeruginosa PAO1. Therefore, IFN-γworked specifically on biofilm formation and not indirectly by disruption of primary metabolic functions.Previous studies have suggested that quorum-sensing (QS) is involved in both the initiation and the maturation of P. aeruginosa biofilm. Quorum-sensing system is the behavior of the entire community that tune gene expression and regulation according to bacteria cell densities, which is widespread and involves complex networks that serve as fine-tuner of the performance of diverse behaviors. To further investigate the mechanism underlying the effect of IFN-γon the formation of P. aeruginosa biofilm, PAO1 and three strains with mutations in different QS system (PAO-JP1, PDO100, PAO-MW) were used to study the distinction among the effect of IFN-γon their biofilm formation in this study. The results showed that this influence was abrogated in strains PDO100 and PAO-MW, which are rhlI-deficient mutants of P. aeruginosa PAO1, suggesting that the rhl QS system plays a crucial role in biofilm formation response to exogenous IFN-γ. Furthermore, the results showed that IFN-γhad no effect on the formation of oprF-deficient strain biofilm, suggesting that OprF is the receptor of IFN-γ.In conclusion, P. aeruginosa can sense the IFN-γproduction of host cells using the major outer-membrane protein OprF specifically, and then IFN-γ-OprF signal activate rhl QS system to promote the formation of P. aeruginosa biofilm. This study clarified the mechanism underlying pathogens how to sense host immune response and alter itself lifestyle to evade immune system, which may provide new insight into the mechanisms of P. aeruginosa chronic infection and may also provide a new therapeutic strategy.