Microbial World and Bacterial Cell Structure PDF

Dr/ Ingy Nassar  Microorganisms are classified as: 1. Prokaryotes: Bacteria and blue green algae are prokaryotes. Bacteria are unicellular free-living organisms having both DNA and RNA. They are capable of performing all essential processes of life, e.g., growth, reproduction, and metabo...

Dr/ Ingy Nassar  Microorganisms are classified as: 1. Prokaryotes: Bacteria and blue green algae are prokaryotes. Bacteria are unicellular free-living organisms having both DNA and RNA. They are capable of performing all essential processes of life, e.g., growth, reproduction, and metabolism. 2. Eukaryotes: Fungi, algae other than blue green, protozoa, and slime moulds are eukaryotes. The microorganisms of medical interest include protozoa, fungi, bacteria, and viruses. With the exception of viruses they can be seen with light microscope. Differences between prokaryotes and eukaryotes  BACTERIA They form a large group of unicellular prokaryotic microorganisms, varying in size from 0.1-10μm long. They have a simple cell structure, contain both DNA and RNA. Most medically important bacteria will grow on artificial culture media in the lab. and reproduce by binary fission. Shapes of bacteria The basic shapes of bacteria: Cocci, round or oval cells bacilli, rod or stick shaped vibrios, curved or comma shaped rods Spirilla, non-flexuous spiral forms Spirochetes, slender and flexuous spiral forms Each form is further subdivided by their staining reactions, mainly the Gram and acid-fast stains.  BACTERIAL CELL STRUCTURE A. Cell envelope B. Cytoplasm and its components C. External structures A- Cell envelope 1- Cell Wall One of the structures unique to bacteria and upon which classification is based on is the Peptidoglycan. It is a rigid layer, which surrounds the cytoplasmic membrane of all bacteria (except mycoplasma which have no cell wall at all). 1- Cell Wall Although both Gram-positive and Gram-negative bacteria contain peptidoglycan in their cell wall, the two types of cell wall are different. Gram Positive Cell Wall Gram Negative Cell Wall Peptidoglycan layer is the major constituent (50%-80% of the cell Peptidoglycan layer is thinner (only 5%-10% of the cell wall) wall) Special Components: Special Components (external to peptidoglycan):  Teichoic acid (Major surface antigen) Outer membrane: It is a bilayer structure, composed of an inner and an outer leaflet. - The inner leaflet is similar in composition to any biologic cytoplasmic membrane (phospholipid). - The outer leaflet is replaced by lipopolysaccharide (LPS). The LPS consists of: a-Lipid A, which is responsible for toxicity of Gram-negative bacteria (endotoxin). Structure of Gram-positive cell wall Gram staining Red staining Violet staining Functions of Cell Wall: 1. It is responsible for the shape of the bacterial cell. 2. It can stand high internal pressure. 3. The LPS is the site of O antigen and endotoxin in Gram-negative bacteria. 4. In Gram-positive bacteria, teichoic acid is the major surface antigen. 5. The site of action of many antibiotics. 6. Contains specific receptors for bacterial viruses. 7. Plays a role in cell division. A- Cell envelope 2- Cytoplasmic membrane It is a thin, elastic, semipermeable membrane consisting of a phospholipid bilayer.  BACTERIAL CELL STRUCTURE A. Cell envelope B. Cytoplasm and its components C. External structures  B- Cytoplasm and its components Bacteria lack structures like mitochondria, endoplasmic reticulum and the Golgi apparatus. On the other hand, it contains Ribosomes Bacterial nucleoid Plasmid Inclusion granules  B- Cytoplasm and its components 1. Ribosomes: Ribonucleoprotein granules, visible by electron microscope and are sites of protein synthesis, they are composed of two subunits one of 50 S and the other of 30 S. Ribosomes appear to be the points of attack of certain antibiotics. 2. Bacterial nucleoid: It consists of a single long double helix of DNA-devoid of nuclear membrane and of nucleolus in the form of a closed circular thread about 1 mm long. 3. Plasmid: Small circular DNA present as an extra-chromosomal element encodes variable number of genes. 4. Inclusion granules: They represent excess metabolites stored as nutrient reserve, e.g. lipid, glycogen, starch, sulphur or polyphosphates.  BACTERIAL CELL STRUCTURE A. Cell envelope B. Cytoplasm and its components C. External structures  C. External structures 1. Capsule: Capsule is a mucogelatinous layer surrounding the cell wall. It is not a constant structure and its presence is not linked with the viability of bacteria. The capsular substance is usually antigenic and is made of polysaccharides, though in some species it is a protein in nature. In pathogenic bacteria, the capsule plays an important part in: 1. Virulence 2. Protecting them against phagocytosis 3. Protecting the cell wall against attack by antibacterial agents.  C. External structures 2. Flagella: Long filamentous, wavy appendages, existing only in certain bacterial species, can be seen by electron microscope. The flagella are either displayed around the entire cell (peritrichous arrangement) or may be found at one or both poles (polar arrangement). Flagella are organs of locomotion. 3. Fimbriae or Pili: Pili are hair-like projections, found mainly in Gram- negative bacteria. They differ from flagella in being shorter, straight, and thinner.  MCQ 1. Microorganisms of medical interest can be seen by light microscope except: a. Bacteria b. Viruses c. Protozoa d. Fungi 2. --------------- are living organisms that belong to prokaryotes kingdom a. Plants b. Fungi c. Protozoa d. Bacteria 3. Bacteria can exist in different shapes in which vibrios are a. Slender and spiral form b. Rod or stick shaped c. Curved or comma shaped d. Round or oval cells  MCQ 4. Bacteria can exit in different group patterns, in which Staphylococci have arrangement of: a. Clusters b. Chains c. Chinese letter appearance d. Diplococcic 5. ---------- are cytoplasmic content in bacterial cell and serve as energy and nutrient reserves a. Bacterial chromosome b. Ribosomes c. plasmid d. Inclusion granules 6. The organs of movement in all motile bacteria are a. plasmid b. Capsule c. Fimbriae d. Flagella  MCQ 7. ------------------- is bacterial structure which protects them aganist phagocytosis a. Fimbriae b. Capsule c. Mesosomes d. Ribosomes 8. Mycoplasma are naturally occurring bacteria which lack: a. Nuclear material b. Ribosomes c. Cell wall d. Inclusion granules True or False 1. Bacteria are multicellular organisms and can’t be seen by naked eye. (F) 2. Bacteria are heterogeneous group of unicellular organism with 0.01-1 um long. (F) 3. Gram’s stain divides bacteria into gram positive (red stain) or gram negative (violet stain) (F) 4. Plasmids are extra chromosomal DNA that could control antibiotic resistance. (T) 5. Ribosomes are tiny granules involved in protein synthesis. (T) 6. Fungi are larger than bacteria with more complex cell structure. (T) 7. Bacteria are eukaryotes and Fungi are prokaryotes. (F) 8. Teichoic acid is a major surface antigen of gram positive bacteria. (T) 9. Lipopolysaccharide is responsible for toxicity of gram positive bacteria. (F)