Physicochemical properties of bacterial surfaces.

Streptococcal M protein is one of the best characterized anti-phagocytic surface molecules. The protein has a coiled fibrillar structure extending outwards about 600 nm from the streptococcal cell wall. The anti-phagocytic nature of the M protein appears to be related to two properties: ( a ) its ability to bind fibrinogen and the D family of its degradation products, thereby preventing deposition of C 3b on the bacterial cell surface (Whitnack et al., l988), and ( b ) its ability to prevent contact between the streptococcus and the phagocyte through electrostatic repulsion (Fischetti et al., 1988). The structural features of the M protein responsible for the above features have been partially characterized. The fibrinogen-binding region of the M protein probably lies in an extended portion of the fibril near its centre (Whitnack el al., 1988), whereas electrostatic repulsion probably results from a high concentration of negatively charged amino acids, predominantly glutamic acid, at the distal tip of the structure (Fischetti et al., 1988). Since phagocytes have a net negative surface charge, the negative charge in this region of the M protein may prevent cell-to-cell contact by electrostatic repulsion. In addition to phagocytosis, many Gram-negative bacteria are susceptible to the direct bactericidal action of serum complement (Hammond et ul., 1984). This depends upon later components of the complement sequence. When activated, the terminal complement component C 5-C 9 form a membrane attack complex (MAC) which eventually results in the formation of a channel through the outer and cytoplasmic membranes. The channel causes leakage of cytoplasmic constituents from the cell and dissipates the membrane potential derived from the proton gradient across the cytoplasmic membrane. These events result in bacterial death. In contrast to Gram-negative organisms, the terminal complement sequence does not directly kill Gram-positive organisms probably because the thick peptidoglycan wall restricts access to the cytoplasmic mcnibranc. Although many Gram-negative bacteria are serum sensitive, a number are serum resistant. Several mechanisms of serum resistance have been described (Joiner, 1988) and the molecular basis of these phenomena is beginning to be defincd. A n intvresting example IS provided by the work of Joiner (1988) who showed that serum resistance in Salmonella montevideo results from failure of the CSb-9 complex to insert properly into the outer membrane, so that it is shed and is not bactericidal. The 0 antigens of lipopolysaccharide provide protection by sterically hindering access of the CSb-9 complex to the outer membrane with a critical density (approximately 20%) of long 0 antigen chains (> 14 0 antigen units/molecule) being necessary for protection. Conclusion