Bacterial Structure 2 – Flashcards
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Gram Negative Cell Envelope |
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LPS Outer membrane -Oligosaccharide O-side chain (O antigen) -Loss by pathogens reduces virulence -Core provides resistance to hydrophobic compounds -Basis for resistance to bile acids by Enteric bacteria -Outer membrane can exclude large compounds (example, the drug vancomycin is too large to pass through the pores in the outer membrane) -Divalent Calcium and Magnesium strengthen LPS intermolecule interactions (can be weakened by EDTA) -Lipid A portion is toxic to humans -Lipid A of LPS contains only saturated fatty acids (adds rigidity) -Extensive external size increases cross interactions with other molecules (reduces interactions of external compounds to lipid bilayer) |
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Lipopolysaccharide |
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-Activates B cells -Stimulates macrophages/dendritic cells to produce inflammatory cytokines such as IL-1, IL-6, and TNFa -Causes fever within 60-90 minutes -Causes early leukopenia -Fatal in doses of ~10-100 µg (~1,000 x more potent than PG) -Enhances glycolysis; can lead to hypoglycemia |
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Septic Shock/Endotoxin Shock |
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-LPS alone can induce shock, but other bacterial components can cause same symptoms -Flu-like symptoms: fever, chills, malaise -confusion -Increased heart and respiratory rates -Extreme hypotension induction of IL-1, IL-6, and TNFa produce vasodilation via NO and other mediators -Multiorgan failure due to hypoperfusion -Activation of coagulation pathway by cytokines leads to disseminated intravascular coagulation (DIC) |
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Bacteria With Different Cell Envelopes |
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-Mycobacteria: -Have an outer layer of mycolic acid, a lipid-like,waxy substance -Related to gram-positive bacteria, but do not stain Gram-positive due to mycolic acid layer -Mycolic acid provides resistance to detergents, many commonly used antibiotic used to treat gram-negative and gram-positive bacteria (i.e., vancomycin), and other antibacterial agents -Mycoplasmas -No cell wall, one membrane; membrane contains sterols, which impart rigidity to the membrane -Related to gram positive bacteria |
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Mycobacterial Cell Wall
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-Lipid-rich cell wall results in: -Acid fastness -Slow growth -Resistance to detergents -Resistance to penicillin and vancomycin -Clumping |
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Cholesterol in Bacterial Membranes |
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-Membrane cholesterol is ALMOST exclusive to animals -Bacteria that can have cholesterol in their membranes include: -Helicobacter -Mycobacteria -Mycoplasma -Ehrlichia -Chlamydia -Borrelia -ProbablyTreponema -These organisms must acquire cholesterol from the host -Bacteria often modify cholesterol following uptake |
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Outer membrane vesicles (OMVs): Express delivery of virulence factors |
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-OMVs are membrane “blebs” originating from Gram negative bacteria -All G- organisms produce OMVs -Quantities of OMVs vary depending on the bacterial species and growth conditions -OMVs can contain adhesins, toxins, degradative enzymes, hemolysins, etc. |
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Formation of OVM's |
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-OMVs are derived from the outer membrane -Contents of the vesicle are NOT random- OMVs are heavily enriched for virulence factors |
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; ; ; Function of OVM's |
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-OMVs can diffuse and enter spaces too small for whole bacteria -OMVs can increase toxin delivery to host tissue -Vesicles containing β-lactamase can increase antibiotic resistance and protect neighboring susceptible organisms -DNA can be transferred via OMVs -OMVsmight fuse with host cells |
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Serum sensitivity/resistance of Gram positive and Gram negative bacteria |
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-The complement system has the ability to kill many Gram-negative bacteria by cell lysis, using the membrane attack complex -Some pathogenic bacteria have the ability to resist complement lysis Ex: Neisseriagonorrhoeae utilizes an outer membrane protein that binds to factor H, increasing the conversion of C3b to iC3b -Gram-positive bacteria are naturallyresistant to lysis by complement due to the thick peptidoglycan cell wall of these bacteria -However, Gram-positive bacteria still are opsonized by complement. |
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Capsules |
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-Also called slime layer or glycocalyx -Usually composed of polysaccharides (polypeptide in B. anthracis) -Important virulence factor -Antiphagocytic- capsule-specific antibody required -Poorly immunogenic -May influence adherence -Partially inhibit formation of MAC for complement-mediated lysis |
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Vaccinations with capsules effective sometimes |
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-Works for H. influenzae type b and S. pneumoniae -Antibodies cannot be generated against hyaluronic acid (S. pyogenes) or sialic acid (some N. meningitidis strains) |
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Flagella |
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-Most common means of bacterial motility -Number and location of flagella vary by species -Composed of protein subunits -Highly antigenic (Called H antigens) -Can be important virulence factor -Attached to cell by complex basal body structure -Movement is caused by PMF-powered flagellar motor |
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Pili or Fimbriae |
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-Composed of protein subunits -Smaller and simpler in structure than flagella -Usually cover entire surface of cell -Specialized sex pili transfer plasmids during conjugation -Promote adherence to host or other bacteria -Specialized protein at end of pilus may bind specific molecule on host cell -Antigenic |
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Endospores |
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-Differentiated structures formed by some Gram + bacteria only -Dormant form of bacteria -Contain minimal material necessary for reproduction of a bacterial cell upon germination -Provides a mechanism for long-term survival of bacterium under conditions unfavorable to the bacterium (low nutrients, etc.) -Resistant to heat, drying, radiation, acids, alcohols, many chemical disinfectants and antibiotics. None of the antimicrobial handwashing agents are sporicidal. -Sensitive to steam, glutaraldehyde, hypochlorite, peracetic acid -Nuisance in disease (e.g. relapsing infections caused by Clostridia)
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Endospore Formation |
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Core: genome with protein synthesis machinery, energy and AA production components -Spore wall: peptidoglycan -Cortex: concentric layers of spore-specific peptidoglycan (thickest layer of spore) -Spore coat: keratin-like protein coat with ample disulfide bonds that add strength and chemical resiliency -Exosporum: remnant of mother cell membranes |
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Endospore Germination |
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-Activation -Stimulated by fresh medium, change in temperature, pH or age of spore -Requires water and triggering nutrient such as alanine -Germination -Cortex swells as it is rehydrated -Excretes calcium and dipicolinic acid -Loses temperature resistance -Outgrowth -Core enlarges, mRNA synthesis begins, spore coat ruptures, spore wall expanded to form new peptidoglycan |
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Biofilms |
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-Composed of polysaccharides -May be composed of multiple species -Form on: -solid substrates in contact with moisture -soft tissue surfaces in living organisms -Catheters and implanted devices -at liquid air interfaces -Cause dental caries -Can protect host by blocking pathogens -Protect bacteria from antibiotics, chemicals, and from being washed away |
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Internal Structures |
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-Nucleoid (bacterial chromosome) -Almost always circular; attached to inner membrane -Plasmids (extrachromosomal genetic elements) -Almost always circular; often contain antibiotic resistance and virulence genes -Storage granules -Carbon storage polymers (poly-beta-hydroxybutyric acid –PHB) -Polyphosphate -Sulfur granules -Inclusion bodies-aggregated proteins (visible by microscopy) -Ribosomes |
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Plasmids |
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-Most currently-used antibiotics are produced by bacteria or fungi resistance genes already exist -Widespread antibiotic use increases prevalence of plasmids containing resistance genes -Many bacteria are naturally competent -Plasmids can be transferred among bacteria of different species -Some bacterial species are only virulent if they contain a virulence plasmid. |
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Bacterial Ribosomes |
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Large subunit (31 proteins) chloramphenicol clindamycin macrolides (erythromycin, azithromycin, etc.)
Small subunit (21 proteins) tetracycline streptomycin |
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Oxidase Test |
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-Detects presence of cytochrome C oxidase, an enzyme in the electron transport chain -Differentiates Pseudomonads from Enterobactereciae -Presence or absence of cytochromeoxidase does not impact antibiotic choice or other treatment decisions |
