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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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;
;
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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

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Bacterial Structure 2 – Flashcards

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