Bacterial Cell Structure PDF

## Bacterial Cell Structure ### Capsule - A true capsule is a discrete detectable layer of polysaccharides deposited outside the cell wall. - A type of capsule found in bacteria called a glycocalyx or microcapsule, is a very thin layer of tangled polysaccharide fibers on the cell surface. - Capsul...

## Bacterial Cell Structure ### Capsule - A true capsule is a discrete detectable layer of polysaccharides deposited outside the cell wall. - A type of capsule found in bacteria called a glycocalyx or microcapsule, is a very thin layer of tangled polysaccharide fibers on the cell surface. - Capsules are generally composed of polysaccharide. They contain amino sugars or peptides. - The capsule mediates adhesion of cells to surfaces. - Capsules also protect bacterial cells from engulfment by predatory protozoa or white blood cells (phagocytes). - Capsules mediate adherence of cells to surfaces. - Capsules in certain soil bacteria protect cells from perennial effects of drying or desiccation. ### Cell Wall - The cell wall provides strength and rigidity to the cell, and it is permeable to solutes. - Bacterial murein is a unique type of peptidoglycan. - Peptidoglycan is a polymer of sugars (a glycan) cross-linked by short chains of amino acids (peptides). - All bacterial peptidoglycans contain N-acetylmuramic acid, which is the definitive component of murein. - Gram-positive bacteria retain the purple crystal violet dye when subjected to the Gram staining procedure. The cell wall is thick (15-80) nanometers consisting of several layers of peptidoglycan. - Running perpendicular to the peptidoglycan sheets are a group of molecules called teichoic acids, which are unique to the gram-positive cell wall. - In gram-negative bacteria (which do not retain the crystal violet in the Gram-stain procedure), the cell wall is relatively thin (10 nanometers) and is composed of a single layer of peptidoglycan surrounded by a membranous structure called the outer membrane. - The outer membrane of gram-negative bacteria invariably contains a unique component, lipopolysaccharide (LPS or endotoxin), which is toxic to animals. - In *E. coli* and other gram-negative bacteria, the glycan backbone of peptidoglycan is made up of alternating molecules of N-acetylglucosamine (G) and N-acetylmuramic acid. - The N-acetylmuramic acid (M) attaches a peptide side chain that contains the amino acids: L-alanine (L-ala), D-glutamate (D-glu), diaminopimelic acid (DAP) and D-alanine (D-ala). - The glycan backbone of the peptidoglycan molecule can be an enzyme called lysozyme that is present in animal tissue and secretions and in phagocyte granules. - The exact site of lysozymal cleavage is the beta 1-bond between N-acetylmuramic acid (M) and N-acetylglucosamine (G). ### Cytoplasmic membrane - The cytoplasmic membrane of bacterial cells is a delicate and plastic structure that completely encloses the cell cytoplasm (or protoplasm). - The bacterial membrane is composed of 40% phospholipids and 60% protein. - Phospholipids are amphoteric molecules, meaning they have a water-soluble hydrophilic region (the glycerol head) attached to two insoluble hydrophobic fatty acid tails. - The cytoplasmic membrane helps in cellular growth, metabolism, replication. - The cytoplasm is the storehouse for all types of chemicals and components that are used to sustain the life of a bacterium. ### Bacterial DNA and Plasmid - Unlike eukaryotes, the bacterial DNA is not enclosed inside of a membrane-bound nucleus, but instead resides inside the bacterial cytoplasm. - The bacterial DNA is not packaged using histones to form chromatin as in eukaryotes. ### Plasmid - The structure of a plasmid is often stranded and circular. - It can contain a varying length of genetic information and there are several different kinds of plasmids which can perform different functions, such as, heavy metal resistance, virulence, environmental adaptability and persistence, and metabolic functions that allow utilization of different nutrients. - Another important feature includes plasmid-mediated biodegradation of a variety of toxic substances, such as, toluene and other organic hydrocarbons, herbicides, and pesticides. ### Ribosomes and other multiprotein complexes - The site of protein synthesis in all living organisms. - All prokaryotes have 70S (where S = Svedberg units) ribosomes, while eukaryotes contain larger 80S ribosomes in their cytosol. - In prokaryotes the ribosome subunits are 30S (Svedberg units) and 50S, yielding intact 70S ribosomes. Each subunit is itself a ribonucleoprotein complex, made up of specific ribosomal -RNAs and ribosomal proteins. - The 30S subunit contains 16S rRNA and about 21 proteins, while the 50S subunit contains 5S and 23S rRNA and about 34 proteins. ### Intracellular Membranes - **Mesosomes**: They are the folding present inside the plasma membrane mesosome plays a vital role in cellular respiration, replication of DNA, cell division, separation of chromosomes during cell division, and also performs the role of Golgi bodies and mitochondria. - **Chromatophores** are intracellular membranes found in phototrophic bacteria, used primarily for photosynthesis. They contain bacteriochlorophyll pigments and carotenoids. ### Fmbriae - Fimbriae (sometimes called "attachment pili") are protein tubes that extend out from the outer membrane in many members of the Proteobacteria. - They are generally short in length and present in high numbers about the entire bacterial cell surface. - Fimbriae usually function to facilitate the attachment of a bacterium to a surface. ### Pili - Pili are hair-like structures composed of single self-aggregating protein (pilin). - Pili are shorter, thinner, and straighter than flagella. - Pili are similar in structure to fimbriae, but are much longer and present on the bacterial cell in low numbers. - Pili are involved in the process of bacterial conjugation where they are called conjugation pili or "sex pili." ### Flagella - Flagella are filamentous protein structures attached to the cell surface that provide the swimming movement for most motile prokaryotes. - Prokaryotic flagella are much thinner than eukaryotic flagella. The diameter of a prokaryotic flagellum is about 7-15 µm long and 20 nm diam. - The arrangement of flagella about the bacterial cell is unique to the species observed. Common forms include: - **Monotrichous**: Single flagellum. - **Lophotrichous**: A tuft of flagella found at each of two opposite poles. - **Amphitrichous**: Single flagellum found at each of two opposite poles. - **Peritrichous**: Multiple flagella found at several locations about the cell. ### Endospores - Certain genera of Gram-positive bacteria such as *Bacillus*, *Clostridium*, *Sporohalobacter*, *Anaero bacter*, and *Helibacterium* can form highly resistant dormant structures called endospores. - In almost all cases one endospore is formed and this is not a reproductive process. - Endospores show no detectable metabolism and can survive extreme physical and chemical stresses such as high levels of UV light, gamma radiation, detergents, heat, freezing, pressure and desiccation. - The spore is sometimes surrounded by a thin covering known as the exosporium, which overlies the spore coat. - The cortex lies beneath the spore coat and consists of peptidoglycan. - The core contains the spore chromosomal DNA, which is encased in chromatin-like proteins known as SASPs (small acid-soluble spore proteins). - These proteins tightly bind and condense the DNA and are in part responsible for resistance to UV light and DNA-damaging chemicals. - Up to 20% of the dry weight of the endospore consists of calcium dipicolinate within the core, which is thought to stabilize the DNA. ### Bacterial Reproduction: Asexual - Cell elongates and DNA is replicated. - Cell wall and plasma membrane begin to divide. - Cross-wall forms completely around divided DNA. - Cells separate. ### Staining of Bacteria - We most often use basic stains to examine bacteria. - Basic stains due to their positive charge will bind electrostatically to negatively charged molecules, such as many polysaccharides, proteins and nucleic acids. - Some commonly encountered basic stains are crystal violet, safranin (a red dye), and methylene blue. - **Respiration in bacteria**. - **Bacteria and temperature**. - **Bacteria and energy sources**.