L1 Microbiology PDF

L1 Micro Introduction to Medical Microbiology Microbiology: It is the science that deals with microorganisms (Micros = small, bios = life, Logos = science). Microorganisms: They are small living organisms that cannot be seen by naked eye except by microscope: Bacteria –viruses -fung...

L1 Micro Introduction to Medical Microbiology Microbiology: It is the science that deals with microorganisms (Micros = small, bios = life, Logos = science). Microorganisms: They are small living organisms that cannot be seen by naked eye except by microscope: Bacteria –viruses -fungi Eukaryotes and prokaryotes 1. Prokaryotes : (Pro = premature or primitive, Karyon = nucleus): Cells with premature nucleus (single naked chromosome without nuclear membrane) e.g. bacteria and rickettsia. 2. Eukaryotes: (eu = true, karyon = nucleus): Cells with true nucleus which contain a nuclear membrane, nucleoli, and multiple chromosomes within the nucleus e.g. Fungi. Structure of bacterial cells o Essential structures: found in all bacteria 1. Cell wall. 2. Cytoplasmic membrane 3. Cytoplasm. 4. Nuclear body. o Some (Not all) bacteria may contain one or more of the following structures: 1. Capsule. 2. Flagella 3. Fimbria (pili). 4. Inclusion granules 1|Page  L1 Micro Cell Wall Definition o The outer covering layer of the bacterial cell (outside the cytoplasmic membrane). It is relatively rigid with some elasticity. It is composed of (1) Basic layer = peptidoglycan = mucopeptide. o Formed of carbohydrate and protein. - Carbohydrate backbone is formed of N – acetyl glucosamine and N– acetyl muramic acid. - The protein part is formed of tetrapeptide side chains connected to N acetyl muramic acid - It is responsible for the rigidity of cell wall. o Forms 80% of Cell wall of gram positive bacteria and 20% of cell wall of gram negative bacteria. (2) Additional layer: Its structure varies according to the type of bacteria: - In Gram +ve bacteria it is formed of teichoic acid. - In Gram -ve bacteria it is formed of lipopolysaccharides ( inner :highly toxic, lipid A , middle: polysaccharide core &outer : polysaccharide , somatic O antigen) and lipoproteins. (3) Periplasmic space: The space between the inner and outer membranes containing the peptidoglycan and is filled with gel containing enzymes such as β-lactamase that degrade βlactam drugs. 2|Page  L1 Micro Function of the cell wall: 1- It maintains the shape of the bacteria. 2- Protection of bacterial cell against high osmotic pressure and outer surroundings. 3- It is antigenic (help in identification and stimulate the production of protective antibodies). Teichoic acid in gram +ve bacteria is major surface Ag. Lipopolysaccharide (LPS) in gram –ve bacteria is also major surface Ag (OAg). 4- Site of action of penicillin, cephalosporins and lysozymes. 5- Lipopolysaccharide layer in Gram – ve bacteria is an endotoxin. 6- Responsible for staining properties of bacteria. Abnormalities of the cell wall: - Protoplasts. - Spheroplasts. - L-forms. - Mycoplasma. Quiz List 4 functions of the cell wall? Cytoplasmic Membrane Semipermeable, double layered membrane, just internal to the cell wall, and surrounding the cytoplasm. Composition: Lipoprotein (70% protein and 30% phospholipids). Functions: 1- Selective permeability = selectively transport nutrients to the cell, and waste products outside the cell. 2- Active transport of ions and molecules to the inside of cells. 3- It contains respiratory enzymes and pigments → oxidative phosphorylation → energy to the cell. 4- Play a role in DNA replication. 5- Play a role in cell wall synthesis. 6- Excretion of pathogenicity proteins e.g. IgA protease. 7- Excretion of hydrolytic exoenzymes which degrade the different nutrients into subunits small enough to penetrate the cytoplasmic membrane. Mesosomes: These are invaginations of cytoplasmic membrane inside the cytoplasm of the cell. - Seen in many (but not all) bacteria. - More common in Gram +ve bacteria. - May be central (septal) or marginal (lateral). 3|Page  L1 Micro -Functions: a- Increase surface area of cytoplasmic membrane. b- May be site of respiratory enzymes. c- Give attachment to the chromosome, so involved in cell division. d- Excretion of enzymes as penicillinases. THE CYTOPLASM It is a homogeneous soft gel mass inside the cell that contains: a) Nuclear body b) Ribosomes. c) Storage granules d) Enzymes. e) Extrachromosomal DNA pieces e.g. plasmids. The cytoplasm of prokaryotes has no mitochondria and no endoplasmic reticulum. Ribosomes: o They are complex minute structures inside the cytoplasm. o Composed of RNA (60%) and proteins (40%). o Bacterial ribosomes (70 S) are composed of 2 subunits a large one (50 S subunit) and a small one (30 S subunit). o A group of ribosomes are called polysomes. o They are the site of translation of mRNA during the process of protein synthesis. o They differ from ribosomes of eukaryotic cells (80 S) and this allows some drugs (e.g. streptomycin) to affect bacterial cell but not human cell (patient). The Nuclear Body o It is a mass of DNA molecule coiled to form single chromosome (double stranded DNA molecule). o There is no nuclear membrane and no nucleolus. o It carries all the genetic information of the cell. o It duplicates before cell division. Plasmid extrachromosomal double stranded circular DNA, replicating independent of the bacterial chromosome. 4|Page  L1 Micro The Capsule It is a gelatinous layer formed by some bacteria and lies outside the cell wall. Composition: Polysaccharides in majority of bacteria e.g. pneumococcus and polypeptide in anthrax bacillus. Demonstration Capsule stain (Muri's stain). - Negative stain as India ink. - Unstained halo around the organism in Gram stain. - Electron microscopy. - Serology: e.g. Quellung reaction for pneumococcus. Function: 1- Virulence factor = antiphagocytic factor protect pathogenic bacteria against phagocytosis. 2- Protect bacterial cells against bacteriophage, complement, lysozymes etc. 3- Antigenic (K Ag): help in identification and typing of bacteria e.g. pneumococci. 4- Vaccine preparation. 5- May help pathogenic bacteria in adherence to human tissues. Flagella They are long, hollow, filamentous appendages used as organ of motility. Structure: - Formed of protein called flagellin. - Arise from basal bodies in the cytoplasm. Function: - Responsible for motility → pathogenesis. - Antigenic (H antigen) → Identification of bacteria. Fimbriae = Pili They are short, hair – like filaments found mainly on surface of Gram – ve organisms. Structure: - Formed of protein called (pilin). - Arise from the cytoplasmic membrane. Function: - Organ of adhesion. - Transfer of genetic material (conjugation). - Antigenic. 5|Page  L1 Miciro SPORES Spores are highly resistant resting forms of some bacteria; formed on exposure to unfavorable conditions e.g. dryness, heat and depletion of nutrients. - Sporulation (sporogenesis): the process of formation of spores from vegetative cells. - Sporulation is triggered by exposure to unfavorable conditions. - Germination: opposite to sporulation i.e. formation of vegetative cells from spores in favorable conditions. Medical importance: a) Resistance to heat and chemicals (bad environmental conditions). b) Help identification of bacteria. e.g. spore of cl. tetani is terminal, rounded, bulging (drum stick appearance). 6|Page  Glycocalyx (Slime Layer) Definition: It is a thin coat ,that covers the surface of bacteria like a film. Composition: Polysaccharide. Function: It facilitates bacterial adhesion and colonization to various structures e.g. skin , heart valves , catheters and teeth (e.g. Strept. mutans that form plaque , the precursor of dental caries). Quiz 1- What is the primary component responsible for the rigidity of the bacterial cell wall? a) Teichoic acid b) Lipopolysaccharides c) Peptidoglycan d) Flagellin 2- How does the membrane contribute to the transport of ions and molecules into the cell? a) Passive diffusion b) Osmosis c) Active transport d) Facilitated diffusion 3-What is the opposite process of sporulation, involving the formation of vegetative cells from spores? a) Differentiation b) Germination c) Replication d) Transformation 7|Page