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Medical Microbiology. Prof. Dr. Jie YAN ( 严杰 ) Department of Medical Microbiology and Parasitology E-mail: email@example.com School of Medicine Zhejiang University. Introduction to medical microbiology. Microbes / Microorganisms.
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Medical Microbiology Prof. Dr. Jie YAN (严杰) Department of Medical Microbiology and Parasitology E-mail: firstname.lastname@example.org School of Medicine Zhejiang University
Microbes / Microorganisms • The word “microbe” comes from the Greek words mikros, meaning small life. Somicrobes / microorganisms are small living things that are too small to be seen by naked eye. • Microorganisms were probably the first organisms to appear on the earth. • However, these organisms were not seen until about 3 centuries ago when lenses powerful enough to make them visible were made. • Viruses, bacteria, fungi, protozoa and some algae are all in this category.
Distribution • The distribution of microorganisms is universal in nature including air, soil, water, animals and human body.
Relationship with human beings • There is a close relationship between microorganisms and human beings. • Beneficial activities: Most microbes are benefit to human beings, some are necessary (nitrogen and arbon cycles). • Harmful activities: Only a small portion of microbes cause human diseases, which called pathogenic microbes. Medical Microbiology • Medical microbiology is a branch of Microbilogy to study biological character, pathogenicity and immunoty, laboratory diagnosis,and prevention and control of pathogenic microbes.
Prokaryotes / Eukaryotes • The prokaryotic cell, in contrast to the eukaryotic cell, has no nuclear membranes, mitochondria, endoplasmic reticulum, Golgi body, phagosomes and lysosomes. • Prokaryotes generally possess only a single circular chromosome, which is bound to a specific site on the cell membrane - the mesosome. • Prokaryotic ribosomes are 70S (30S and 50S subunits) in size, whereas eukaryotic ribosomes are larger (80S, 40S and 60S subunits).
Classification of microbes • According to organizational structure, microbes can be divided into three types: • Prokaryotes (Eubacteria and Archaebacteria) • Eukaryotes (fungi, Protozoa, algae) • Acellular entities (viruses) • Eubacteria include Bacteria, Chlamydiae, Mycoplasmas, Richettiae, Spirochetes, and Actinomycetes. Some of them cause human diseases.
Viruses • Viruses are very small particlesand have no basic cell structure. A simplest virus consists of one core and oneprotein coat (capsid). The core composed with a nucleic acid molecule, either DNA or RNA. • Virusesare obligate parasites totally dependent on their host cells for replication.
Fungi • Fungi is a kind of eukaryotic cells. So they have various organelles, for examples, nuclear membranes, mitochondria, endoplasmic reticulum, Golgi body, phagosomes and lysosomes.
New challenge in medical microbiology • The numerous emergingand re-emerginginfectious diseases such as AIDS,SARS, avian influenza, tuberculosis, viral hepatitis and so on.
Bacteriology Morphology and Structure of Bacteria
Unit for measurement of bacteria ismicrometer (μm) On the average, bacteria are 2-8 μm in length and 0.2-2.0 μm in diameter. Exceptions include some spiral shaped bacteria that can reach 4- 500 μm. Size of bacteria 1000
Spherical (Cocci, sing.Coccus ) • Rods (Bacilli, sing. Bacillus) • Spiral (Spiral bacteria) • vibrio spirillum helicobacterium Shape of bacteria
Diplococci: Pair of cells divide in one plane Streptococci: Chain of cells formed by dividing in one plane several times Tetrad: Divide in two planes Sarcinae: Divide in three planes Staphylococci: Divide in many planes and remain together as a cluster Spherical bacteria Different arrangements depending on the plane of division
Rod-shaped bacteria • Considerable variation in length and diameter: 0.5-1 μm in width and 2-5 μm in length. • Most of rod-shaped bacteria are single arrangement. Diplobacilli: Bacilli that remain in pairs after they divide. Streptobacilli: Bacilli that remain in chains after they divide. Coccobacilli: A short Bacilli that nearly looks like a cocci.
Divided into: Vibrio: comma shaped Spirillum: helical Spiral-shaped bacteria
bacterial structuresmay be defined: • Cell envelopePlasmids Flagella • Pili • Capsules • Spores
Important bacterial structures Cell envelopePlasmids Flagella Pili Capsules Sspores
Cell envelope • Bacterial envelope is divided into cell membrane and cell wall (Gram positive) plus an outer membrane (Gram-negative). Gram-positive cocci Gram-negative bacilli (Gram-staining method)
Cell wall: general component-peptidoglycan • Cell wall consists of peptidoglycan layer and attached structures. Gram-positive Gram-negative
Peptidoglycan • glycan backbone:N-acetyl muramic acid and N-acetyl glucosamine are alternatively linked by -1,4 linkage. • 4-peptide side chain: links to N-acetyl muramic acid. • peptide bridge: links side chains (gram-negative bacteria have no peptide bridges). • Penicillin can block the linkage between peptide side chain and bridges to kill gram-positive bacteria.
Cell wall: characteristits of gram-positive bacteria • Peptidoglycan layer is thick (15-50 layers). • There are some special components such as teichoic acids, the major superficial antigen of gram-positive bacteria. Cell wall: characteristits of gram-negative bacteria • Peptidoglycan layer is thin (1-2 layers). • There is outer membrane located in outside of peptidoglycan layer but no any teichoic acids.
Outer membrane • Outer membrane of a gram-negative bacterium is composed of phospholipids, membrane proteins andlipopolysaccharide (LPS)
Lipopolysaccharide (LPS) • LPS is also called endotoxin (poisonous to mammal cells). • LPS has 3 regions: an external O antigen, a middlecore, and an inner lipid A. • O antigen is a polysaccharide to act as the somatic antigen of gram-negative bacteria. • Core polysaccharide links O antigen with lipid A. • lipid A decides toxicity.
Maintaining bacterial shape. Resistance to osmotic pressure Providing a platform for surface appendages such as flagella and pili. Providing a pathogenic function to adhere host cells (For gram-positive bacteria, the major adhesin is teichoic acids. For Gram-negative bacteria, the major adhesin is pili and some of outer mambrane proteins). Playing an essential role in bacterial division Participating bacterial material exchange Containing major antigens. Cell wall: function
When bacteria are treated with 1) enzymes (e.g. lysozyme) with cell wall hydrolytic activity or 2) antibiotics inhibiting peptidoglycan synthesis, wall-less bacteria are generated which is called L-forms of bacteria. L-forms of bacteria can cause chronic infections. L-forms of bacteriaare difficult to cultivate and usually require a medium with a right osmotic strength. It is resistant to antibiotics (e,g. penicillin) and difficulty to detect (e.g. absence of O antigen). Wall-less forms of bacteria
Electron micrograph of Staphylococcus A: L-form; B: wild type
Important bacterial structures Cell envelopePlasmids Flagella Pili Capsules Spores
Plasmids • Plasmids are small, circular / line, extra-chromosomal double-stranded DNA. • Usually present in multiple copies and are capable of self-replication. • Often code for pathogenic factors and antibiotic resistant factors. Are not essential for bacterial survival.
Important bacterial structures Cell envelopePlasmids Flagella PiliCapsules Spores)
Flagellum is composed of flagellin and provide motility. • It extends from cell envelope and projects as a long strand. • Flagellum is slender that can not be seen by light microscopy unless a special stain is applied. Flagella: general description
Flagella: structure • Basal body: • a structure to insert into cell envelope. • Flagellin is an antigen (H antigens).
Flagella: function • Motility of bacteria: move towards foodstuffs or away from toxic materials. • Identification of bacteria:According to the mobility and antigenicity of flagellin (H antigen). • Possible pathogencity: chemotaxis to the suitable sites in hosts for colonization.
Important bacterial structures Cell envelopePlasmids FlagellaPiliCapsules Spores
Pili • Pili are hair-like strands of bacteria. • They are shorter and thinner than flagella, only visible under electron microscope.
Pili • Pilus is composed of special protein called pilin. • Two types can be distinguished: • Ordinary pili • Shorter, thinner, numerous for a bacterium • Relative to bacterial adhesion (adhering to host cells) • Contribute to virulence of some pathogenic bacteria • Sex pili • Longer, coarser, only 1-4 for a bacterium • Relative to bacterial conjugation (a pattern of bacterial genetic material exchanges) • The recent data revealed the sex pili of some bacteria has the ability to adhere host cells.
Ordinary pili Sex pili Recipient Donor bacterium Electron graph of pili
Important bacterial structures Cell envelopePlasmids Flagella PiliCapsules Spores
Capsules and slime layers • Capsule is a structure surrounding outside of cell envelope. • Usually, slime layer is thinner than capsule. • They are usually demonstrated by the negative staining or “capsule stain” which gives color to the background.
Capsules and slime layers • They are usually composed of polysaccharide. However, in some certain bacilli, they are composed of polypeptide. • They are not essential to bacterial viability. • Some strains within a bacetrial species can produce a capsule, whereas the others can not. • Capsules are often lost during in vitro culture. • The capsules contribute to invasiveness (virulence) of bacteria by protecting them from phagocytosis by phagocytes.
Important bacterial structures Cell envelopePlasmids FlagellaPiliCapsules Spores
Spores • Under adverse conditions, such as nutrient / water depletion, some bacteria form a thick wall inside the cytoplasmic membrane leading to a resting stage known as spores. • Spores contribute to bacterial resistance.
Spores • One spore-forming bacterium can only produce one spore which has no propagation ability. • One spore germinates into one vegetative bacterial cell which can propagate / multiplication. • Spore can be seen after staining with dyes. Sometimes, it can also be seen as a colorless area by using conventional bacterial staining methods. • Spores are commonly found gram-positive bacilli. • Different sizes, shapes and positions of spores will help us to identify spore-forming bacteria.
Structure of spores Corespore wall /core CortexCoatExosporium
strain type species genus family O157:H7 Coli Escherichia Enterobacteriaceae Classification of bacteria • Taxonomic terms: • Family: a group of related genera. • Genus: a group of related species. • Species: a group of related strains. • Type: sets of strains within a species (e.g. biotypes, serotypes). • Strain: one line or a single isolate of a particular species. • The basic taxonomic group is species.
Staphylococcusaureus S. aureus Genus species 金黄色葡萄球菌 species genus Classification of bacteria
Summary Structure of bacteria include essential structures ofcell wall,cell membrane,cytoplasm, and nuclear material (nucleoid). Some bacteria also have one or more of the particular structures of capsule, flagella, pili, endospores. Structure of cell wall, cell wall structural differences between Gram-positive and Gram-negative bacteria, concept ofplasmid, and functions of bacterial particular structures are the most important contents, because of their close association with bacterial pathogenesis.