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All of the various surface components of a bacterial cell are important in its ecology since they mediate the contact of the bacterium with its environment. The only "senses" that a bacterium possesses result from its immediate contact with its environment. It must use its surface components to assess the environment and respond in a way that supports its own existence and survival in that environment. The surface properties of a bacterium are determined by the exact molecular composition of its membrane and cell envelope, including capsules, glycocalyx, S layers, peptidoglycan and LPS, and the other surface structures, such as flagella and pili or fimbriae.

The surface of Streptococcus pyogenes. High magnification electron micrograph of an ultra-thin section by Maria Fazio and Vincent A. Fischetti, Ph.D. with permission. The Laboratory of Bacterial Pathogenesis and Immunology , Rockefeller University. At this magnification, especially in the cell on the left, the cell wall and cell surface fibrils, consisting mainly of M protein, are well defined. The interdigitaion of these fibrils between neighboring cells of different chains can also be seen.

Bacterial surface components may have a primary biological function that has nothing to do with pathogenicity. Thus, the function of the LPS in the outer membrane of Gram-negative bacteria has to do with its permeability characteristics, rather than its toxicity for animals. However, there are endless examples wherein a bacterial surface component plays an indispensable role in the pathogenesis of infectious disease. Bacterial surface structures may act as (1) permeability barriers that allow selective passage of nutrients and exclusion of harmful substances (e.g. antimicrobial agents);  (3) adhesins used to attach or adhere to specific surfaces or tissues; (3) enzymes to mediate specific reactions on the cell surface important in the survival of the organism; (4) protective structures against phagocytic engulfment or killing; (5) antigenic disguises to bypass activation of host immune defenses; (6) endotoxins, generally cell wall components, that cause an inflammatory response in the host; (7) "sensing proteins" that can respond to temperature, osmolarity, salinity, light, oxygen, nutrients, etc., resulting in a molecular signal to the genome of the cell that will cause expression of some determinant of virulence (e.g. an exotoxin).

In medical situations, the surface components of bacterial cells are major determinants of virulence for many pathogens. In animals, they may be used to colonize tissues, resist phagocytosis and immune responses, and  to induce inflammation, complement activation and harmful immune responses.

The surface of Bacillus anthracis. From Mesnage, et al. Journal of Bacteriology (1998) 180, 52-58. http://www.pasteur.fr/recherche/unites/scme/Biblio/capsulea.htm. The bacterial membrane is evident as the innermost layer surrounding the cytoplasm. P denotes the peptidoglycan cell wall.S refers to the S-layer which consists of two proteins including the major antigen.C denotes the poly-D-glutamic acid capsule that is exterior to and completely covers the S-layer proteins.

The Structure of the Bacterial Surface

Structurally, a bacterial cell has three architectural regions: appendages (proteins attached to the cell surface) in the form of flagella and fimbriae; a cell envelope consisting of a capsule, cell wall and plasma membrane; and a cytoplasmic region that contains the cell genome (DNA) and ribosomes and various sorts of inclusions. The surface components of a bacterium are the constituents of its cell envelope and appendages.

Flagella are filamentous protein structures attached to the cell surface that provide swimming movement for most motile bacterial cells. The diameter of a bacterial flagellum is about 20 nanometers, well-below the resolving power of the light microscope. The flagellar filament is rotated by a motor apparatus in the plasma membrane allowing the cell to swim in fluid environments. Bacterial flagella are powered by proton motive force (chemiosmotic potential) established on the bacterial membrane.

Bacteria are known to exhibit a variety of types of tactic behavior, i.e., the ability to move (swim) in response to environmental stimuli. For example, during chemotaxis a bacterium can sense the quality and quantity of certain chemicals in its environment and swim towards them (if they are useful nutrients) or away from them (if they are harmful substances). During aerotaxis, bacteria swim toward or away from O₂.

For a few pathogens motility is known to be a determinant of virulence. In the case of Vibrio cholerae, the vibrios apparently swim (laterally) into the intestinal mucosa to avoid being flushed out by the peristaltic action of the gut. Flagella are antigenic, and therefore, vulnerable to attack by host antibody molecules. Antibody molecules directed against flagellar antigens can agglutinate and/or immobilize bacterial cells, or possibly opsonize them from phagocytosis, which presumably would aid in host defense.

Vibrio cholerae. Liefson's flagellar stain (CDC). Bacterial flagella are below the resolving power of the light microscope. In order to be visualized, the bacteria must be reacted with a stain that precipitates along the flagellar filaments, which increases their effective diameter to the point of resolution. Vibrio cholerae is motile by means of  a single polar flagellum inserted into one pole of the cell.

Fimbriae and Pili are interchangeable terms used to designate short, hair-like structures on the surfaces of bacterial cells. Fimbriae are shorter and stiffer than flagella, and slightly smaller in diameter. Like flagella, they are composed of protein. A specialized type of pilus (always called a pilus), the F or sex pilus, mediates the transfer of DNA between mating bacteria, but the function of the smaller, more numerous common pili is quite different. Inasmuch as many bacteria are able to exchange genes for virulence by means of conjugation, the sex pilus which confers the ability to conjugate, may well play a role in the their assembly of virulence determinants.

Common pili or fimbriae are often involved in adherence (attachment) of bacterial cells to surfaces in nature. In medical situations, they are major determinants of bacterial virulence because they allow pathogens to attach to (colonize) tissues and, sometimes, to resist attack by phagocytic white blood cells. As surface structures on the bacterial cell, the functions of fimbriae overlap with those of capsules discussed below. Fimbriae are also antigenic and secretory antibodies (IgA) will often block bacterial colonization, while circulating antibodies (IgG or IgM) will opsonize bacterial cells for phagocytosis.

Neisseria gonorrhoeae. Electron micrograph by David M. Phillips, Visuals Unlimited, with permission. This pathogen utilizes its fimbriae in order to initially colonize the urethral or cervical epithelium.

Most bacteria contain some sort of a polysaccharide layer outside of the cell wall or outer membrane. In a general sense, this layer is called a capsule. A true capsule is a discrete detectable layer of polysaccharides deposited outside the cell wall. A less discrete structure or matrix which embeds the cells is a called a slime layer. Slime layers are equivalent to biofilms (below) A type of capsule found in bacteria called a glycocalyx is a thin layer of tangled polysaccharide fibers which is a observed on the surface of cells growing in nature. Some microbiologists consider all types of exopolysaccharides to be glycocalyx. Capsules, slime layers, and glycocalyx are known to mediate specific or non specific adherence of bacteria to particular surfaces. The also protect bacteria from engulfment by predatory phagocytes and from attack by antimicrobial agents.

In nature, and in many medical situations, colonies of bacteria construct and live in a biofilm, made up principally of capsule material. A biofilm usually consists of a consortium (mixture) of bacteria living in a matrix of slime which is secreted by one of the bacterial members. Dental plaque is an example of a natural biofilm, as is a slimy mass of bacteria attached to a rock in a mountain stream. In medical situations, bacteria in a biofilm may have certain advantages over planktonic counterparts. For example, biofilm bacteria may be less susceptible to phagocytosis, drugs, or neutralizing antibodies.

Many polysaccharide capsules possess an antigenic epitope so they will induce and react with host antibodies. Where the capsule is a main determinant of virulence of a pathogen (e.g. Streptococcus pneumoniae) antibodies against the bacterium neutralize its virulence.

Bacterial capsules visualized by various techniques. Left. Streptococcus pneumoniae -India ink capsule outline (K.Todar); Middle. Bacillus anthracis -fluorescent-tagged antibody (CDC); Right. Streptococcus pyogenes -transmission electron micrograph by Maria Fazio and Vincent A. Fischetti, Ph.D. with permission. The Laboratory of Bacterial Pathogenesis and Immunology , Rockefeller University. S. pneumoniae capsular material is composed of polysaccharide. The capsule is the pathogen's most important determinant of virulence because it allows the bacterial cells to escape phagocytes in the lung. The B.anthracis capsule is composed of  poly-D-glutamic acid. Its capsule is antiphagocytic, and it protects the bacteria from complement- mediated lysis in serum or blood. The capsule of S. pyogenes is composed of hyaluronic acid, the same polymer as found in human connective tissue. The capsule is an antigenic disguise that prevents recognition of the streptococci by  phagocytes or the immune system.