Bacteriology 3: Bacterial Structure

Gram-positive envelope structure

1. Teichoic acid

2. Peptidoglycan (thicker cell wall)

3. Periplasm

4. Inner membrane (phospholipid bilayers)

5. Proteins

Gram-negative envelope structure

1. Pilus/flagellum

2. LPS (lipid A)

3. Outer membrane

4. Peptidoglycan (thinner cell wall)

5. Periplasm

6. Inner membrane

7. Proteins

Bacterial cell wall

1. Called peptidoglycan or murein layer

2. Prevents bacteria from exploding

3. The synthesis (enzymes) and structure are unique to bacteria → good drug target

4. Understanding ___ structure is key to understanding the action of many antibacterials

Basic cell wall (peptidoglycan) structure

1. Carbohydrate backbone (repeating disaccharide)

2. Backbone appears as strands of NAM-NAG

3. Strands linked together by peptide bridges

4. Structure looks like a screen

Peptide bridges

What are the strands of peptidoglycan linked together with?

NAM-NAG

The backbone of peptidoglycan appears as strands of ___ - ___

Interpeptide (pentaglycine) bridge

Links together between L-lysine and D-alanine on NAM

Parallel, parallel, bottom

Interpeptide bridge linkage on NAM:

On gram +ve: Between ___ stands

On gram -ve: Between ___ strands and to ___ strands

DD-transpeptidase (penicillin-binding protein; D-alanyl-D-alanine-cleaving-peptidase)

Catalyzes the transpeptidation that crosslinks the peptide side chains of peptidoglycan strands

Penicillin

Structurally similar to D-alanine-alanine, and irreversibly binds to and inhibits the activity of the DD-transpeptidase enzyme by forming a highly stable penicilloyl-enzyme intermediate

Peptidoglycan cell wall

Unique feature of prokaryotes

Lipopolysaccharide (LPS)

A major component of the Gram-negative outer membrane

O-antigen (also O-polysaccharides), core region, lipid A (endotoxin)

Three components that LPS is composed of

O-antigen

The carbohydrate that confers smoothness and reduces phagocytosis

1. Smooth = has O-antigen

2. Rough = stops at core region

3. Reduces surface hydrophobicity

4. Highly antigenic

5. Can be used for serotyping

Lipid A (endotoxin)

1. Toxic component of LPS

2. Composed of saturated, long-chain fatty acids attached to a phosphorylated glucosamine dimer

3. Released following cell lysis; potent stimulator of cytokine production

4. Dysregulates (disrupts) the balance between proinflammatory and anti-inflammatory response; septic shock

5. Common physiologic effects include capillary leakage and hypotension; can result in multi-organ failure

Teichoic acids

1. Found only in Gram-positives

2. Polymers of glycerol or ribitol

3. Linked by phosphodiester bonds to the cell wall NAM (ribitol) or to the membrane phospholipids (glycerol)

Protective

Antibodies against teichoic acids are not ___.

Endotoxin

The biological activities of teichoic acids are similar to ___ in Gram-negative organisms (proinflammatory).

Peptidoglycan, bacteriophages

Teichoic acids serve as attachment sites to link ___ to the cytoplasmic membrane and as receptors for certain ___.

Autolysins

Teichoic acids regulate ___ during cell-cell separation after division.

Periplasmic space

1. A concentrated gel-like matrix in the space between the inner cytoplasmic membrane and the bacterial outer membrane in Gram-negative

2. Cryo-electron microscopy has shown that a much smaller ___ is present in Gram-positive

3. Contains hydrolytic enzymes, toxic enzymes, exotoxins for secretion

Flagella

1. Long protein structure involved in locomotion

2. Various arrangements are possible

Monotrichous, amphitrichous, lophotrichous, peritrichous

Types of flagella

Basal body, hook, filament

What is the flagellum composed of?

Filaments

Polymers of flagellin

Cell motility

What are flagella responsible for?

Random motion

What does clockwise movement of flagella mean?

Directional movement

What does counterclockwise movement of flagella mean?

Chemotaxis

Function of flagella

Response to environmental cues

Pili (fimbriae)

Composed of pilin subunits assembled into a filamentous structure

Adherence, colonization

Attachment pili (=fimbriae) are involved in ___ and ___.

F pili (fimbriae, sex pili)

Involved in bacterial conjugation and transfer of genes from one cell to another

Capsules

Usually simple polymeric carbohydrate structures

Slimy/mucoid

Capsules confer a ___ appearance to a colony.

Antigenic

Capsules cover/hide more highly ___ cellular components.

Immune response/phagocytosis, harsh environment (drying)

What does the capsule protect from?

Host mimicry

The capsule may be involved in ___.

Porins

1. Found in the outer membrane of Gram-negative bacteria

2. Multimers form pores

3. Small molecules can pass through pores

4. Diffusion occurs more readily across the inner membrane

Endospores

1. Formed under adverse growth conditions (nutrient limitation, desiccation, oxygen tension)

2. Highly resistant to insult

3. Metabolically dormant

4. Environmentally stable

5. Core contains Ca++ complex to dipicolinic acid

6. Bacillus and Clostridium

Endotoxins (LPS), exotoxins (secreted toxins), enterotoxins

Bacterial toxins

Enterotoxins

Protein toxins released by a microorganism that targets the intestines

1. Pyrogenicity (capacity to induce fever)

2. Leukopenia followed by leukocytosis

3. Hypotension

   1. “Gram-negative shock”

   2. Life-threatening complication of septicemia (disseminated intravascular coagulation (DIC): widespread activation of the clotting cascade that results in the formation of blood clots in the small blood vessels throughout the body)

   3. Many of the effects of endotoxin are mediated by tumor necrosis factor (TNF)

4. Attempts at therapy using anti-endotoxin or anti-TNF antibodies

Actions of endotoxins

Exotoxins

Extracellular bacterial products that have direct effects on eukaryotic cells leading to pathological events

1. Enzymatic activity (e.g., peptidase, ADP ribosyltransferase (cholera toxin, diphtheria toxin)

2. Interfere with signal transduction or directly inhibit cellular functions (e.g., anthrax toxin)

3. Detergent/membrane damaging activity forming pores– hemolysins

4. Blocking nerve function (tetanus and botulinum toxins)

5. Many are A/B toxins (multicomponent toxins)

Exotoxin actions

Membrane-damaging exotoxins

1. Many bacterial toxins form pores in eukaryotic cell membranes, producing oligomeric rings, e.g.,

   1. Streptolysin O of Streptococcus pyogenes

   2. Listeriolysin of Listeria monocytogenes

   3. Alpha-toxin of Staphylococcus aureus

2. Other toxins, such as phospholipases, degrade components of the membrane

   1. e.g., Clostridium perfringens alpha toxin

A-B toxins

1. Consist of at least one active (A) and one binding (B) subunit

2. The B-subunit binds to the target cell surface and delivers the A (toxic) subunit inside the cell often by receptor-mediated endocytosis

Iron acquisition

1. The need for iron by microbes

2. Host iron sources; most is bound

   1. Hemoglobin

   2. Ferritin

   3. Lactoferrin

   4. Transferrin

3. Siderophores

4. Other microbial iron-acquiring strategies (ferric iron reductase: reduce insoluble Fe3+ to Fe2+)

Hemoglobin, ferritin, lactoferrin, transferrin

Host iron sources