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Infectious Agents > Bacterial Cell Structures > Flashcards

Bacterial Cell Structures Flashcards

1
Q

What are the 2 classes of cell envelope?

A
  • gram positive and gram negative
  • classification based on ability of species to retain a crystal violet-iodine stain when treated with organic solvents (alcohol)
  • staining property depends on the morphology and composition of the bacterial envelope
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2
Q

Gram-positive envelope structure

A
  • cell membrane: phospholipid bilayer similar to eukaryotic membranes
  • cell wall: located outside cell membrane, thick peptidoglycan (murein) layer (single molecule that is several layers thick)
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3
Q

Gram-negative envelope structure

A
  • inner membrane: phospholipid bilayer similar to gram-positive cell membrane and eukaryotic membranes
  • cell wall: peptidoglycan (murein) layer that is much thinner than gram-positive cell wall
  • outer membrane: inner leaflet is phospholipid, outer leaflet is unique bacterial structure (lipopolysaccharide)
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4
Q

Which type of bacteria have increased resistance to mechanical, heat, or drying damage?

A

Gram-positive, due to thicker cell wall

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5
Q

Why are gram-negative bacteria more impermeable?

A

Due to presence of outer membrane

  • is especially impermeable to hydrophilic compounds
  • gram-neg bacteria are less susceptible to many toxic compounds (including antibiotics)
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6
Q

Periplasm

A

Extra compartment between inner and outer membrane of gram-neg bacteria

  • is 20-40% of cell volume, contains the cell wall and a gel-like solution of proteins that facilitate nutrition and inactivate toxic compounds
  • periplasmic space allows concentration of detoxifying enzymes (inactivation of antibiotics) –> in gram-positive bacteria, these are secreted into surrounding medium
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7
Q

Cell membrane

A

Osmotic barrier modified by the presence of specific transport systems

  • phosopholipid bilayer that lacks sterols (except for mycoplasma)
  • contains embedded membrane proteins
  • site of action of some disinfectants and detergents
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8
Q

What are the functions of cell membrane proteins?

A
  • specific transport systems for secretion and uptake
  • macromolecular synthesis/secretion proteins, such as cell wall
  • metabolic proteins: electron transport system
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9
Q

Where is the cell membrane located in gram-negative bacteria?

A

Inner membrane

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10
Q

Peptidoglycan

A

Aka: murein, mucopeptide, or R layer

  • imparts cell shape and allows bacteria to survive in hypotonic solution
  • is the basic layer just outside the cell membrane in all eubacteria (except mycoplasma)
  • composed of a complex polymer (giant interlinked molecules) forming concentric sheets
  • thicker in gram-pos than in gram-neg
  • is the site of action for lysozyme and lysostaphin
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11
Q

What binds to peptidoglycan>

A

TLRs

- peptidoglycan is a foreign substance that initiates an innate reaction

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12
Q

Teichoic/lipoteichoic acids

A

Special components of gram-positive cell walls

  • attached to peptidoglycan or anchored in the cytoplasmic membrane, can account for up to half of cell wall mass
  • species-specific structure composed of sugar backbone with side groups
  • highly antigenic and useful as taxonomic markers
  • *unique to gram-positives only!!**
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13
Q

What are the 3 sheets of peptidoglycan?

A
  • backbone: alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) monomers
  • side chain: tetrapeptides attached to NAM, D-alanin, L-alanine, D-glutamic acid, and L-lysine
  • crosslinkages: linked side chains from peptides from different backbone units, the site of penicillin action
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14
Q

What is the difference in gram-neg and gram-pos side chains?

A
  • gram-negative side chains are directly linked by interpeptide bonds
  • in gram-positive bacteria side chains are crosslinked by an interpeptide bridge of amino acids
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15
Q

What gives the structural integrity of the cell wall?

A

Cross links

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16
Q

What type of antibiotics attacks the crosslinks?

A

Beta-lactin

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17
Q

Outer membrane

A

Gram-negative only!!

  • lipoproteins: attach cell wall to outer membrane
  • outer membrane proteins, includes porins to allow nutrients to pass thru and many proteins involved in host-pathogen interaction
  • inner leaflet: phospholipids
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18
Q

Outer leaflet

A

Lipopolysaccharide (LPS)

  • highly antigenic!
  • induces toxic response in mammals when released from lysed cells (endotoxin)
  • unique bacterial structure that is species-specific
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19
Q

3 components of LPS

A
  • lipid A: hydrophobic portion that interacts with phospholipids inner leaflet (the toxic portion)
  • core oligosaccharide: short carbohydrate chain attached to lipid A
  • O polysaccharide: long hydrophilic carbohydrate chain attached to core oligosaccharide that covers bacterial surface (the antigenic and serotype-specific structure of LPS)
20
Q

Capsule or slime layer

A

Located outside the cell wall (gram-positive) or outer membrane (gram-negative)

  • amorphous carb polymers that vary from species in sugar composition and structure
  • immunogenic (K antigens) and is useful in serotyping and as vaccine antigens
  • some are nonimmunogenic and antiphagocytic
  • could serve as adhesins, allowing adherence to epithelial surfaces
  • colonies appear as moist, slimy
  • capsule is sometimes similar to O polysaccharide (O antigen capsule)
21
Q

What is the difference between a capsule and slime layer?

A
  • capsule: outer polymer is tightly adhered to the bacterial surface
  • slime layer: polymer is loosely associated and continuously shed
22
Q

How do capsules provide an extra layer of protection?

A

Prevents lysosomal enzyme hydrolysis or complement-mediated lysis

23
Q

Acid-fast bacterial cell wall in Mycobacterium

A
  • gram-pos cell envelop structure, but cell wall contains mycolic acid and a large amount of lipids (complex, long-chain hydrocarbons substituted with sugars)
  • serves as waxy, protective cover making them impervious to harsh chemicals
  • prevents penetration of gram stain reagents unless bacteria are pretreated with heat or a detergent –> once dye penetrates cells it cannot be removed = acid-fast
  • slow growing bacteria, due to waxy coat causing slow uptake of nutrients
24
Q

S layer

A

Present on some fish pathogens located outside the cell wall (gram-pos) or outer membrane (gram-neg)

  • outer layer of protein subunits arranged in crystalline array, usually a single kind of protein molecule capable of self-assembly
  • resistant to proteolytic enzymes and protein-denaturing agents
  • helps protect against phagocytosis and may participate in adherence to surfaces
25
Q

Is the S layer a protein or a polysaccharide?

A

Protein!

26
Q

Chlamydia spp.

A

Chlamydial cell envelopes resemble the gram-negative envelope except they have no peptidoglycan layer
- instead of peptide crosslinkages in the peptidoglycan layer, a network of disulfide bonds among envelope proteins imparts rigidity to the chlamydial cell envelope

27
Q

Mycoplasma

A

Lack peptidoglycan cell wall

- susceptible to osmotic lysis

28
Q

Do bacteria have organelles?

A

No true organelles
- cyoplasm consists of a basic-staining, densely packed granular region surrounding an amorphous, less dense nuclear region (nucleoid)

29
Q

Chromosome

A

Naked DNA, usually a circular, covalently closed double-stranded molecule

  • E. coli chromosome: 4.6 million base pairs, 4288 protein coding genes
  • chromosome is highly supercoiled to allow it to fit inside the bacteria cell
30
Q

Plasmids

A

Smaller, accessory DNA molecules that replicate independently of the chromosome

  • can sometimes be transferred between bacterial cells (conjugation)
  • often carry genes that confer a selective advantage under certain conditions (ex: antibiotic resistance genes)
31
Q

Ribosomes

A

Protein translation machinery, granular appearance of cytosol is due to dense packing of ribosomes

  • 3 kinds of RNA, and over 50 different proteins
  • prokaryotic ribosomes have 70S structure, can be dissociated into 50S and 30S subunits
  • prokaryotic ribosomes are not separated from chromosomal DNA by a nuclear membrane = transcription and translation occur simultaneously!!!
32
Q

What ribosomal molecule is used for classification?

A

16S used for taxonomy

- highly conserved structure among eubacteria

33
Q

Polysome

A

More than one ribosome can attach to a single mRNA strand

- allows for bacteria to adapt to changes in the environment quickly

34
Q

Why are ribosomes the target of many antibiotic classes?

A

Structural differences between prokaryotic and eukaryotic ribosomes

35
Q

Flagellum

A

Organ of bacterial locomotion

  • helical filament that is driven by a motor at its base, rotates relative to bacterial surface to propel bacteria thru medium
  • impart motion by rotation only!! not by bending like eukaryotic flagella
36
Q

Monotrichous

A

One flagella

- ex: vibrio cholerae

37
Q

Peritrichous

A

Multiple flagella located around their surface

- ex: E. coli

38
Q

3 parts of flagellum

A
  • filament: long, helical outermost structure extending into medium, composed of several thousand copies of flagellin
  • -> flagellins are highly antigenic and have immunologic specificity (H antigens)
  • hook: connects filament to basal body (separate protein from flagellin, is closer to the surface so we do not generate an immune response)
  • basal body: anchors in membrane, flagellar motor driven by protonmotive force (gives the spinning motion)
39
Q

Are H antigens a protein or a polysaccharide?

A

Protein

40
Q

Are O antigens a protein or a polysaccharide?

A

Polysaccharide

41
Q

Are K antigens a protein or a polysaccharide?

A

Polysaccharide

42
Q

Attachement pili (fimbriae)

A

Organelles of attachement to surfaces

  • hair like projections extending from cell membrane
  • typical E. coli has 100-300
  • composed of structural proteins (pillins) and adhesin proteins at tip
  • adhesion proteins mediate attachment and exhibit remarkable specificity (important virulence factor mediating attachement)
  • fimbriae of different bacteria are antigenically distinct
43
Q

Adhesins bind to a _____ receptor on the host surface

A

Specific

44
Q

Conjugative pili

A

Specialized organelles for exchange of plasmid DNA during conjugation (bacterial mating)

  • completely different structurally from fimbriae (attachement pili)
  • hollow tube connecting two bacteria through which DNA is transferred
  • longer than flagella
45
Q

Endospores

A

Resting stage with high resistance to heat, may survive boiling for 1 or more hours

  • most resistant form of life known
  • dehydrated cells composed of 5-15% calcium dipicolinate covered by keratin-like protein
  • special differential staining procedures are required to observe
  • extremely low metabolic activity, can survive for years in this state
  • Bacillus and Clostridium spp
46
Q

L forms

A

Laboratory forms

  • occurs spontaneously (bacterial induced)
  • lack cell wall, may or may not revert
  • may occur in vivo and be associated with antibiotic resistance
  • special media (high osmolarity) can induce formation of L forms
  • antibiotics that target cell wall can induce the L form (temporary state)

Infectious Agents (44 decks)

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