Structure and Functions of Bacterial Cell Envelope
The outer layer or cell envelope provides a structural and physi-ological barrier between the protoplasm (inside) of the cell and the external environment. The cell envelope protects bacteria against osmotic lysis and gives bacteria rigidity and shape. The cell envelope primarily consists of two components: a cell wall and cytoplasmic or plasma membrane. It encloses the proto-plasm, which consists of (i) cytoplasm, (ii) cytoplasmic inclu-sions (mesosomes, ribosomes, inclusion granules, vacuoles), and (iii) a single circular DNA (Fig. 2-7).
The surface appendages of the bacteria include flagella and fimbriae or pili.
Bacterial flagella are thread-like appendages intricately embedded in the cell envelope. These structures are responsible for conferring motility to the bacteria. The arrangement of flagella varies between different bacterial species. Depending on the arrangement, flagella can be of the following types:
· Monotrichous (single polar flagellum), e.g., Vibrio cholerae.
· Lophotrichous (multiple polar flagella), e.g., Spirilla.
· Peritrichous (flagella distributed over the entire cell), e.g., SalmonellaTyphi, E. coli, etc.
· Amphitrichous (single flagellum at both the ends), e.g., Spirillum minus (Fig. 2-11).
Structure: The flagella are 3–20mm in length and 0.01–0.03mmin diameter. The main part of the filament is made up of protein subunits called flagellin arranged in several helices around a cen-tral hollow core. The flagellum is attached to the bacterial cell
body by a complex structure consisting of a hook and a basal body. The basal body bears a set of rings, one pair in Gram-positive bacteria and two pairs in Gram-negative bacteria, through which the bacteria rotates either in a clockwise or an anticlockwise direction. Above the base of filament is the hook, a short curved structure between the external filament and basal body. This part produces a propeller-like repulsion from the revolving flagellum (Fig. 2-12).
Spirochetes are motile bacteria but without any external flagella. They are motile due to the presence of an axial fila-ment. Axial filament consists of a bundle of flagellum-like structures that lie between the cell surface and an outer sheath, and connects one end of the cell to the other. They are some-times called the endoflagellates.
Function: Flagella have the following functions:
· They are primarily responsible for motility of bacteria by chemotaxis.
· They may play a role in bacterial survival and pathogenesis.
· They are highly antigenic, they possess H antigens, and some of the immune responses to infection are directed against these proteins. The flagella of different bacteria differ antigenically. Flagellar antibodies are not protective but help in serodiagnosis.
Demonstration of flagella: The flagella can be demonstratedby direct and indirect methods. The direct methods include direct demonstration of capsule by electron microscope. These also include demonstration of capsule after staining by special staining methods, such as Ryu’s method and Hugh–Leifson’s method. Since flagella are very thin structures, these staining methods are used to demonstrate flagella by increasing their thickness by mordanting with tannic acid.
Indirect methods of demonstration of flagella include demonstration of motility of the bacteria by (a) dark-ground microscopy, (b) hanging drop method, or (c) observing spread-ing type growth on semisolid media, such as mannitol motility medium.
Pili or fimbriae are synonymous for most purposes. They are hair-like filaments that extend from cell surface and are found almost exclusively on Gram-negative bacteria. They are composed of structural protein subunits termed pilins. Minor proteins termed adhesins are located at the tips of pili and are responsible for the attachment properties.
Structure: The pili are shorter and straighter than flagella,although the basic structure is same. Like flagella, it consists of helics of protein called pilins, arranged around a hollow core but without a motor. They are 0.5 mm long and 10 nm thick. They are antigenic in nature. Pilihemagglutinate RBCs of guinea pigs and are specifically inhibited by mannose, on the basis of which they are classified into four types as follows:
o Type 1: These occur inE. coli, Klebsiella, Shigella,andSalmonella.They are mannose sensitive.
o Type 2: These are present inSalmonellaGallinarum andSalmonella Pullorum, devoid of any hemagglutinating oradhesive properties.
o Type 3: These are present in some strains ofKlebsiella, Serratia,etc. They agglutinate RBC only after heating and are man-nose resistant.
o Type 4: These are mannose resistant and occur inProteus.
Sex pili: A specialized kind of pili called sex pili is responsiblefor the attachment of donor and recipient cells in bacterial conjugation. These pili are longer (10–20 mm) and vary 1–4 in number. The sex pili are of two types:
o F pili: They specifically adsorb male specific RNA and DNAbacteriophages. They are encoded by sex factor F and fertil-ity inhibition–positive resistance factors (fi 1 R factors).
o I pili: They adsorb male specific filamentous DNA phages,encoded by col factor and fi 2 R factor.
Function: Pili play a major role in the adherence of symbioticand pathogenic bacteria to host cells, which is a necessary step in initiation of infection. Transfer of bacterial DNA takes place through sex pili during the process of conjugation.
Demonstration of pili: The pili can be detected:
o Directly by electron microscope and
o By agglutination of RBCs of guinea pigs, fowl, horses, and pigs. They agglutinate human and sheep RBCs weakly. The hemagglutination can be specifically inhibited by D-mannose.
Some of the Gram-positive bacteria do not possess typical pili but instead possess a fine fibrillar arrangement of proteins on their surfaces known as fibrils. These fibrils bind to the host surfaces. M-protein of S. pyogenes is an example of Gram-positive bacteria possessing fibrils.