National University for Science and Technology Microbiology Theory Lecture 2 PDF

National University for Science Microbiology \ theory and Technology Second stage College of Pharmacy Lecture 2...

National University for Science Microbiology \ theory and Technology Second stage College of Pharmacy Lecture 2 Assist. Prof. Dr. Riyad E. Abed Anatomy of bacteria The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. Some organisms, such as most bacteria, are unicellular (consist of a single cell). Other organisms, such as humans are multicellular. (Humans have an estimated 100 trillion or 10 14 cells, a typical cell size is 10μm and a typical cell mass is 1 nanogram.) The largest known cell is an unfertilized ostrich egg cell. Cells are mainly of two types: Prokaryotic cell (e.g. bacteria, virus) and eukaryotic cell (e.g. Plant cell and animal cell). Bacteria are Prokaryotic, unicellular that do not contain chlorophyll. Size of bacteria may range from 0.2-1.5 micrometer in diameter and 3-5 micrometer in length. Bacteria are microscopic single-celled organisms that prosper in diverse environments. They can live within soil, in the ocean and inside the human gut. Humans' relationship with bacteria is complex. Bacterial Cell Components: These can be divided into: A. Cell envelope and its appendages. 1. The outer layer or cell envelope consists of two components: Cell wall and Cytoplasmic or plasma membrane. 2. Cellular appendages: Capsule, fimbriae, and flagella B. Cell interior: Those structures and substances that are bounded by the cytoplasmic membrane compose the cell interior and include cytoplasm, cytoplasmic inclusions (mesosomes, ribosomes, inclusion granules, vacuoles) and a single circular chromosome of deoxyribonucleic acid (DNA). The plasma membrane  The cytoplasm and its contents are surrounded by a plasma membrane, which can be thought of as a bilayer of phospholipid arranged like a sandwich, together with associated proteins (Figure 1).  Phospholipids comprise a compact, hydrophilic (water-loving) head and a long hydrophobic (water-hating) tail region.  Also included in the membrane are a variety of proteins; these may pass right through the bilayer or be associated with the inner (cytoplasmic) or outer surface only. These proteins may play structural or functional roles in the life of the cell. Many enzymes 1  associated with the metabolism of nutrients and the production of energy, are associated with the plasma membrane in prokaryotes.  The majority of bacterial membranes does not contain sterols but contain molecules called hopanoids which thought to assist in maintaining membrane stability. The function of the plasma membrane is to: a- keep the contents in b- allowing the selective passage of certain substances in and out of the cell (it is a semipermeable membrane) Figure 1: Structure of bacterial cell membrane The bacterial cell wall o Bacteria have a thick, rigid cell wall, which maintains the integrity of the cell, and determines its characteristic shape. o The major component of the cell wall, which is responsible for its rigidity, is a substance unique to bacteria, called peptidoglycan (murein). This is a high molecular weight polymer whose basic subunit is made up of three parts: Nacetylglucosamine, N-acetylmuramic acid and a short peptide chain (Figure 2). The latter comprises the amino acids l-alanine, d-alanine, d-glutamic acid and either l-lysine or diaminopimelic acid (DAP). DAP is a rare amino acid, only found in the cell walls of prokaryotes. Figure 2: Peptidoglycan structure 2 Although all bacteria (with a few exceptions) have a cell wall containing peptidoglycan, there are two distinct structural types. These are known as Gram-positive and Gram-negative. The names derive from the Danish scientist Christian Gram, who, in the 1880s developed a rapid staining technique that could differentiate bacteria as belonging to one of two basic types. 1- Gram-positive cell walls are relatively simple in structure, comprising several layers of peptidoglycan (as many as 40 sheets of peptidoglycan, comprising up to 50% of the cell wall) connected to each other by cross-linkages to form strong, rigid scaffolding. In addition, they contain acidic polysaccharides called teichoic acids; these contain phosphate groups that impart an overall negative charge to the cell surface (Figure 3). Figure 3: Gram positive cell wall 2- Gram-negative cell wall (Figure 4), is multilayers and consists of a thin, inner wall composed of: 1. Peptidoglycan; is just 2-3 layers and generally 2-3 nm thick. Chemically, only 10 to 20% of the Gram-negative cell wall is peptidoglycan. 2. An outer membrane; is a lipid bilayer about 7 nm thick. It is composed of phospholipids, lipoproteins, lipopolysaccharides (LPS), and proteins.  Phospholipids are located mainly in the inner layer of the outer membrane,  The lipoproteins connect the outer membrane to the peptidoglycan.  The lipopolysaccharides, located in the outer layer of the outer membrane, consist of a lipid portion called lipid A embedded in the membrane and a polysaccharide portion extending outward from the bacterial surface. The LPS portion of the outer membrane is also known as endotoxin.  In addition, pore-forming proteins called porins span the outer membrane. The porins function as channels for the entry and exit of solutes through the outer membrane of the Gram-negative cell wall. The outer membrane of the Gramnegative cell wall is studded with surface proteins that differ with the strain and species of the bacterium. 3   The periplasm is the gelatinous material between the outer membrane, the peptidoglycan, and the cytoplasmic membrane. This periplasmic space is about 15nm wide and contains a variety of hydrolytic enzymes for nutrient breakdown, periplasmic binding proteins for transport via the ATP-binding cassette (ABC) system, and chemoreceptors for chemotaxis. Figure 4: Gram negative cell wall Functions of the cell wall: 1. It imparts shape and rigidity to the cell. 2. It supports the weak cytoplasmic membrane against the high internal osmotic pressure of the protoplasm. 3. It maintains the characteristic shape of the bacterium. 4. It takes part in cell division. 5. It also functions in interactions (e.g. adhesion) with other bacteria and with mammalian cells. 6. It provides specific protein and carbohydrate receptors for the attachment of some bacterial viruses. Beyond the cell wall A number of structural features are to be found on the outer surface of the cell wall; these are mainly involved either with locomotion of the cell or its attachment to a suitable surface. Perhaps the most obvious extracellular structures are: 1. Flagella (sing: flagellum), thin hair-like structures often much longer than the cell itself, and used for locomotion in many bacteria. There may be a single flagellum, one at each end, or many, depending on the bacteria concerned (Figure 5). 4  Figure 5: Types of flagella arrangement Each flagellum is a hollow but rigid cylindrical filament made of the protein flagellin, attached via a hook to a basal body, which secures it to the cell wall and plasma membrane (Figure 6). The basal body comprises a series of rings, and is more complex in Gram-negative than Gram- positive bacteria. Rotation of the flagellum is an energy-dependent process driven by the basal body, and the direction of rotation determines the nature of the resulting cellular movement. Figure 6: Structure of bacterial flagellum 2. Pili (sing: pilus) are structures that superficially resemble short flagella (Figure 7). They differ from flagella, in that they:  do not penetrate to the plasma membrane,  are not associated with motility. Their function, rather, is to anchor the bacterium to an appropriate surface. Pathogenic (disease-causing) bacteria have proteins called adhesins on their pili, which adhere to specific receptors on host tissues. Attachment pili are sometimes called fimbriae, to distinguish them from another distinct type of pilus, the sex pilus, which as its name suggests, is involved in the transfer of genetic information by conjugation. 5  Figure 7: Cell-surface appendages of a bacterial cell 3. Outside the cell wall, most bacteria have a polysaccharide layer called a glycocalyx, which could be as slime layer or as capsule (Figure 8). Slime layer is a diffuse and loosely bound. It helps protect against desiccation, and is instrumental in the attachment of certain bacteria to a substratum (the bacteria that stick to your teeth are a good example of this). Capsule is a better defined and generally thicker. It offers protection to certain pathogenic bacteria against the phagocytic cells of the immune system. Figure 8: Bacterial polysaccharide layer (glycocalyx); a, slime layer; b, capsule 6