Bacterial Cell Structure Lecture 6 PDF

LECTURE 6 Bacterial Cell Structure Ms. Peters  Bacterial Cell Structure Structure is small and simple with no recognizable organelles Cells appear featureless and two-dimensional when viewed under a microscope Small size and structure allows rapid growth and division...

LECTURE 6 Bacterial Cell Structure Ms. Peters  Bacterial Cell Structure Structure is small and simple with no recognizable organelles Cells appear featureless and two-dimensional when viewed under a microscope Small size and structure allows rapid growth and division Can be organized into three categories: – Internal Structures – Cell Envelope – External Structures  Internal Structure Cytoplasm – Dense, gelatinous matrix composed of water, enzymes, nutrients and waste – Contains discrete cell masses such as chromatin body, plasmid, ribosomes, mesosomes and granules  Internal structure Cytoplasm -Chromosomes and Plasmids Hereditary material in the form of a single circular strand is designated as the chromosome Since no nucleus is present, genetic material is aggregated in a dense area of the cytoplasm called the nucleoid Chromosomes are the minimum genetic requirements for survival Some bacteria contain extra pieces of nonessential genetic material called plasmids  Internal structure Cytoplasm -Ribosomes Ribosomes are the protein synthesis machinery of the cell found abundantly in the cytoplasm in chains They translate the genetic code to amino acid (the building block of proteins) Bacterial ribosomes are never bound to other organelles. Sometimes in eukaryotes, they are bound to the endoplasmic reticulum (rough) in but are usually free-floating  Internal structure Cytoplasm - Inclusion Bodies and Granules Inclusions are nonliving components of the cell that have no metabolic activity and are not bounded by membranes Most common inclusions are glycogen, lipids, pigments Granules are densely compacted substances where nutrients and reserves may be stored as glycogen and phosphate which supplies energy for metabolic processes. These structures are important for the storage of nutrients E.g., sulphur bacteria which has reserve sulphur granules used up during starvation  Internal structure Cytoplasm -Mesosomes Mesosome is an extension of the cell membrane present in cytoplasm as infoldings and serves to increase surface area in photosynthetic prokaryotes Carry enzymes for aerobic respiration, help in cell wall formation, participate in DNA replication and distribution to daughter cells They also help in the respiration process, secretion process, to increase the surface area of the plasma membrane and enzymatic content. Mesosomes are found in gram-positive bacteria.  Cell Envelope Plasma (Cell) Membrane Composed of two lipid layers (the lipids linked to proteins and polysaccharides). It is located under the cell wall. Gram-negative bacteria have inner and outer membranes whereas gram-positive bacteria have only inner cell membrane. Outer cell membrane of gram-negative bacteria is composed of lipopolysaccharide (LPS) and lipoproteins  Cell Envelope Plasma Membrane Cont. Selectively permeable to some ions and molecules Control the flow of metabolites through the cell by active transport of molecules Generates energy by oxidative phosphorylation. Secretion of enzymes and toxin. Synthesis of cell wall precursors (have an important role in cell wall synthesis).  Cell Envelope Plasma Membrane Cont. https://www.alamy.com/stock-photo/plasma-membrane.html?sortBy=relevant  Cell Envelope Periplasmic Space The space between the plasma membrane and the outer membrane in the gram-negative bacteria. The small space between the plasma membrane and the cell wall in the gram-positive bacteria Contains a gel like matrix of mono and oligosaccharides, amino acids, peptides, soluble biosynthetic precursors of the peptidoglycan and degrading enzymes Proteins in the periplasm are classed into three functional groups: Enzymes (catabolic and degrading enzymes for protection against penetrated harmful substances. Enzymes necessary for the biosynthesis of cell wall components)  Cell Envelope Periplasmic Space Chaperones (proteins required for correct folding of the newly synthesized proteins ) High affinity binding proteins for vitally important substrates such as amino acids  Cell Envelope – Cell Wall Rigid wall surrounding the bacteria The main structural component is peptidoglycan (murein) (polysaccharides and proteins). It is found only in bacterial cell wall The cell wall consists of polymer of two sugar derivatives N- acetylglucosamine (NAG) and N- acetylmuramic (NAM) acid cross linked by short chains of amino acids (inter peptide bridges) Function of cell wall: – Protection the internal structures. – It maintains the shape of bacterial cell. – Contain components which can be toxic to host cell. – It plays a role in cell division  Cell Envelope Cell Wall Cont. In gram-positive bacteria, the cell wall is composed of a thick peptidoglycan layer constituting 50-80 nm of the cell wall Responsible for the rigidity of cell walls and retention of crystal violet dyes during gram staining. The large amounts of PG make gram-positive bacteria more susceptible to antibiotics (penicillin) that inhibit cell wall synthesis. Also comprises teichoic acid and a thin layer of lipid  Cell Envelope Cell Wall Cont. In gram-negative cell walls there is an inner layer of peptidoglycan This layer is thin constituting 5-10 nm of cell wall which does not retain the crystal violet stain well Has an outer layer of lipopolysaccharides (LPS) containing lipid A (endotoxin) and polysaccharide The periplasmic space is found between the inner and outer layers No teichoic acid  Cell Envelope Cell Wall Cont. Bacterial cell wall completely removed – protoplast Bacterial cell wall partially removed – spheroplast Enzymes such as lysozyme in human tears can digests bacterial cell walls thus providing a defense against eye infections Some antibiotics act by inhibiting cell wall formation thus destabilizing the bacteria  Cell Envelope https://www.researchgate.net/figure/Differences-between-Gram-negative-and-Gram-p ositive-bacteria_fig2_357901900  Cell Envelope Outer Membrane Less permeable than the plasma membrane Has tiny openings called porins that prevent the entry of harmful substances and antibiotics (making gram-negative bacteria infection more difficult to treat) Composed of lipopolysaccharides (LPS) LPS considered an endotoxin and is responsible for the toxicity of gram-negative bacteria  External Structures – Flagella – Pilli/Fimbriae – Capsule/ Slime Layer  External Structures Flagella Long, thin, filamentous, helical structure composed of flagellin protein The body of flagella is divided into three parts: – Basal body – Hook – Filament Most rod-shaped bacteria have flagella, while most cocci are non-motile Motility is an important feature used in laboratory identification of pathogens  External Structures Flagella Cont. Functions of the flagellum include: – They help an organism in movement towards nutrients and away from harmful chemicals – Act as sensory organs to detect temperature and pH changes. – Used to increase reproduction rates. Flagella vary in number and arrangement: – Bacterial cells with no flagellum - Atrichous – Bacterial cells may carry a single polar flagellum - Monotrichous – Bacterial cells with a single tuft of flagella emerging from the same site – Lophotrichous  External Structures Flagella Cont. – Bacterial cells with several flagella present at both ends- Cephalotrichous – Bacterial cells where a single flagellum appears at both ends of the cell – Amphitrichous – Bacterial cells with flagella covering the entire body - Peritrichous  External Structures Pili/Fimbriae Refers to hollow hair like surface appendages that are for attachment rather than locomotion Essential for virulence of some pathogenic bacteria Fimbriae – Small, brush like fibers (2 – 10 nm) diameter on the surface of the bacterial cells – Has an inherent tendency to stick to each other and surfaces forming aggregates – Some pathogen can colonize and invade host tissues using fimbriae e.g., Gonococcus spp.  External Structures Pili/Fimbriae Pili – An elongate rigid, tubular structure that is larger than fimbriae – Structure is made of a special protein called pilin – There are two types of pili: » True pili which has only been found in gram negative bacteria and are involved in conjugation (sex pili) » Attachment/Ordinary Pili which plays a role in attachment of mucous membrane through specific receptors cells lining mucus membrane  External Structures Glycocalyx A coating of macromolecules (Polysaccharides and proteins) surrounding the external surface of the cell Vary in thickness, organization and compositions Usually found in two forms: – Capsule » Rigid structure of repeating units of polysaccharides that is tightly bound to the cell surface » Gives a prominent mucoid character to colonies – Slime layer » Disorganized loose soluble shield around bacteria » Prevents the lost of water and nutrients  External Structures Glycocalyx Cont. Functions include: – Protects the bacteria from dehydration and deleterious agents – Resist phagocytosis and contributes to the virulence of bacteria – Aids in attach to solid and aqueous surfaces – Participates in the formation of biofilms – Used as antigen (K- antigen) in certain vaccines https://www.pharmacy180.com/article/capsules-and-slimes-733/  External Structures Glycocalyx Cont. Differences:  Homework Define Biofilm Define planktonic and sessile Briefly list and describe the five stages of biofilm formation Why are bacterial biofilms notoriously resistant to antibiotics? What other ways does a biofilm offer protection to the bacteria