Staphylococcus Aureus Responds To Immunoglobulin And Initiates A Clumping State To Evade Phagocytosis

Staphylococcus aureus is a widely distributed opportunistic human pathogen that is responsible for several organic infections such as chronic otitis, naso sinusitis, septicemia, endocarditis, and osteomyelitis. The human pathogen expresses a series virulence factors like cytotoxins, capsule, MSCRAMMs, staphylokinase, superantigen toxins, coagulase, lipase, CHIP, and Efb. Many of these virulence factor coding genes are located in the mobile genetic elements such as prophages, pathogenicity islands, and transposons. The virulence of Staphylococcus aureus mainly depends on the number of the virulence factors it carries and their expression level.S. aureus may form highly organized bacterial community, termed biofilm, in certain occasions. Its biofilm-forming ability is widely accepted as an important virulence that causes chronic infections. It is believed that a formed biofilm would help to cope with the variable microenvironment. S. aureus biofilm formation proceeds in three stages:primary attachment, biofilm maturation, and dispersal of the cells from the biofilm. The cell-wall associated MSCRAMMs play an important role in matrix adhesion during the primary attachment, while PIA mainly contributes to the maturation of a biofilm, and accumulation protein Aap is also involved. The detachment stage is crucial for bacteria dissemination to other colonization site, for instance when the nutrient is limited. In staphylococci, this mechanism is controlled by the quorum-sensing system agr.We have observed severe aggregation of S. aureus cells in the cultivation induced by mammalian serum. The clumps were somehow similar to biofilms. The phenomenon of aggregation was not cause by direct interaction between bacterial cells and adhesive proteins such as fibrinogen. Later we have found that protein A sefinose purified immunoglobulin would also induce S. aureus cells to form clumps during the cultivation. Real-time quantitative polymerase chain reaction showed that the mRNA levels of several cell-wall associated genes including ica, spa, clfA, clfB, fnbA,fnbB, and sasG were significantly increased in the Ig induced cells, similar to that in the serum induced group. These results indicated that Ig would efficiently induce the aggregation of S. aureus during the growth, while this procedure probably involved a switch in cell physiological conditions.The expression of S. aureus cell-wall associated adhesins is precisely regulated by several global regulators. Two component systems are usually responsible for bacterial perception of environmental changes. However, we have found that 15 tested two component systems, including the impotant global agr system, did not contributed to the clumping process induced by immunoglobulin. SarA is another important global regulator that modulates the biofilm formation. The real-time quantitative PCR showed that the transcriptional level of sarA in S. aureus MW2 was significantly increased after the induction of immunoglobulin. Previous study showed that sarA gene can be transcribed by three promoters. One of them isσB-dependent and the others areσA-dependent. Our Northern blot assay showed that the transcript produced from theσB-dependent promoter was gradually increased during the induction of immunoglobulin while the amount of transcripts produced from the other twoσA-dependent promoters remained unchanged. These results indicate that the alternative sigma factorσB was activated by the stimulation of immunoglobulin. The deletion mutation of the sigB gene weakened the aggregation ability induced by immunoglobulin. Besides, the phagocytosis assays showed that immunoglobulin induction would help the S. aureus cells to evade the phagocytosis by macrophage and slightly enhance the bacterial survival ability in macrophage.Here we have showed that the induction of mammalian immunoglobulin would cause severe aggregation of S. aureus cells during the bacterial growth. When stimulated by immunoglobulin, S. aureus alternative sigma factorσB was activated to elevate the transcription of several genes including the global regulator SarA, and further enhanced the transcription of various genes encoding cell-wall associated adhesins. The aggregation of the bacterial cells results in the efficient evasion of phagocytosis by macrophage. We postulated that the existence of mammalian immunoglobulin may provide S. aureus with specific environmental signals for organic invasion. The strategy is probably a result of the long time interactions between the S. aureus and its host during the evolution. Our study may help to understand how S. aureus receives specific environmental signals for organic invasion, and how this signal is transducted into the bacterial cell.