Surface Proteins on Gram-Positive Bacteria (original) (raw)

2019, Microbiology spectrum

As arms, legs, hair, and fur are used in higher species for their survival in the environment, surface appendages are used by bacteria for similar purposes. Surface molecules in bacteria range from complex structures such as flagella that propel the organism in aqueous environments, to less sophisticated polysaccharides and proteins. All of these molecules serve to benefit the organism for survival in a hostile environment, such as the waters of a rushing stream, the blood of an infected animal, the surface of an object, or the surface of a mucosal epithelium. Although it was previously believed that bacteria were simple single-cell organisms with little complexity, it is now apparent that they are highly evolved, advanced particles that possess a wide array of surface molecules that serve to manipulate the organism in its environment. For human pathogens, surface molecules have been finely tuned to allow adherence and colonization of host surfaces, invasion of cells, evasion of the host's immune response, and persistence in infected tissues. In an effort to emphasize the complexity of bacterial surface molecules and their use in the everyday life of the bacterium, this chapter will focus on those surface proteins found on grampositive bacteria. For an extensive review of the subject, see references (1, 2). GRAM-POSITIVE CELL WALL From electron microscopic analysis of the gram-positive cell envelope (see chapter 1, this volume) and a number of elegant chemical, structural and immunological analyses, a picture of the gram-positive cell wall has emerged (Fig. 1). The structure differs significantly from the gram-negative cell wall in two ways: (i) the presence of a thicker and more cross-linked peptidoglycan and (ii) the lack of an outer membrane. Because of these differences, surface molecules on gram-positive organisms vary from those on gram-negative organisms, requiring specialized systems to transport and anchor molecules through the outer membrane(3, 4). In general, surface proteins in gram-positive bacteria can be separated into three categories: (i) those that anchor at their C-terminal ends (through an LPxTG motif), (ii) those that bind by way of charge or hydrophobic interactions, and (iii) those that bind via their N-terminal region (lipoproteins) (Fig. 1).