Bacterial Cell Structures

Question 1

What are the 2 classes of cell envelope?

Answer 1

• 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

Question 2

Gram-positive envelope structure

Answer 2

• 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)

Question 3

Gram-negative envelope structure

Answer 3

• 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)

Question 4

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

Answer 4

Gram-positive, due to thicker cell wall

Question 5

Why are gram-negative bacteria more impermeable?

Answer 5

Due to presence of outer membrane

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

Question 6

Periplasm

Answer 6

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

Question 7

Cell membrane

Answer 7

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

Question 8

What are the functions of cell membrane proteins?

Answer 8

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

Question 9

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

Answer 9

Inner membrane

Question 10

Peptidoglycan

Answer 10

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

Question 11

What binds to peptidoglycan>

Answer 11

TLRs

- peptidoglycan is a foreign substance that initiates an innate reaction

Question 12

Teichoic/lipoteichoic acids

Answer 12

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!!**

Question 13

What are the 3 sheets of peptidoglycan?

Answer 13

• 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

Question 14

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

Answer 14

• 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

Question 15

What gives the structural integrity of the cell wall?

Answer 15

Cross links

Question 16

What type of antibiotics attacks the crosslinks?

Answer 16

Beta-lactin

Question 17

Outer membrane

Answer 17

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

Question 18

Outer leaflet

Answer 18

Lipopolysaccharide (LPS)

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

Question 19

3 components of LPS

Answer 19

• 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)

Question 20

Capsule or slime layer

Answer 20

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)

Question 21

What is the difference between a capsule and slime layer?

Answer 21

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

Question 22

How do capsules provide an extra layer of protection?

Answer 22

Prevents lysosomal enzyme hydrolysis or complement-mediated lysis

Question 23

Acid-fast bacterial cell wall in Mycobacterium

Answer 23

• 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

Question 24

S layer

Answer 24

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

Question 25

Is the S layer a protein or a polysaccharide?

Answer 25

Protein!

Question 26

Chlamydia spp.

Answer 26

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

Question 27

Mycoplasma

Answer 27

Lack peptidoglycan cell wall | - susceptible to osmotic lysis

Question 28

Do bacteria have organelles?

Answer 28

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

Question 29

Chromosome

Answer 29

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

Question 30

Plasmids

Answer 30

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)

Question 31

Ribosomes

Answer 31

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!!!

Question 32

What ribosomal molecule is used for classification?

Answer 32

16S used for taxonomy | - highly conserved structure among eubacteria

Question 33

Polysome

Answer 33

More than one ribosome can attach to a single mRNA strand | - allows for bacteria to adapt to changes in the environment quickly

Question 34

Why are ribosomes the target of many antibiotic classes?

Answer 34

Structural differences between prokaryotic and eukaryotic ribosomes

Question 35

Flagellum

Answer 35

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

Question 36

Monotrichous

Answer 36

One flagella | - ex: vibrio cholerae

Question 37

Peritrichous

Answer 37

Multiple flagella located around their surface | - ex: E. coli

Question 38

3 parts of flagellum

Answer 38

- 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)

Question 39

Are H antigens a protein or a polysaccharide?

Answer 39

Protein

Question 40

Are O antigens a protein or a polysaccharide?

Answer 40

Polysaccharide

Question 41

Are K antigens a protein or a polysaccharide?

Answer 41

Polysaccharide

Question 42

Attachement pili (fimbriae)

Answer 42

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

Question 43

Adhesins bind to a _____ receptor on the host surface

Answer 43

Specific

Question 44

Conjugative pili

Answer 44

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

Question 45

Endospores

Answer 45

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

Question 46

L forms

Answer 46

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)