Chapter 3 - Bacteria & Archaea

Distinguishing features of Prokaryotes

1. The way their DNA is packaged: lack of true nucleus & histones, so DNA floats around in the cytoplasm

2. The makeup of their cell wall: peptidoglycan & other unique chemicals

3. Their internal structures: lack of membrane-bound organelles

Structures found in all bacterial cells

• Cytoplasmic membrane (membrane surrounding cytoplasm)

• Cytoplasm (inner most part of cell)

• Ribosomes (on rough ER for protein synthesis)

• Cytoskeleton (network of proteins)

• Genetic material (1 chromosome, aka haploid)

Structures found in most bacterial cells

• Cell wall

• Glycocalyx (surface coating)

Structures found in some bacterial cells

• Flagella, pili, fimbriae (hair-life appendages on surface of cell wall)

• Outer membrane (part of cell wall)

• Nanowires/nanotubes

• Plasmids (additional DNA; can carry genes for anti-bacterial resistance)

• Inclusions

• Endospores (can’t be killed easily by disinfectant; hardy structures)

  • Not reproductive; for survival of cell!

• Microcompartments

Most of these are observed in archaea as well

Bacterial shapes & arrangements

• Bacteria mainly function independently as single-celled, unicellular organisms

  • Some act as a group in colonies or biofilms

• Shapes: cocci, bacilli, vibrio, spirillum, spirochete (cocci & bacilli = most common)

• Bacteria have average size of 1 micron

• Pleomorphism: variations in cell wall structure caused by slight genetic or nutritional difference

  • So they adopt multiple shapes (shape shifters) → very rare

Cocci

• Spherical/ball shaped

• Cocci can be perfect spheres, but can also exist as oval & bean-shaped

• Cocci = plural; Coccus = singular

Bacilli

• Cylindrical/rod shaped

• Rods are varied in form

  • Coccobacillus = when it’s short & plump

  • Vibrio = when it’s gently curved

• Bacilli = plural; Bacillus = singular

Spirillum

• Bacterium with slightly curled/spiral-shaped body

• Rigid, short, few twists (resembles a corkscrew)

Spirochete

• Bacterium with a spiral cell

• Flexible, long, many twists (resembles a spring)

Filaments

• Multiple branches produced off of a basic rod structure

• Very rare

Arrangements of cocci

1. Single

2. Diplococci (pairs)

3. Tetrads (groups of 4)

4. Sarcina (cubical packet of 8, 16, or more cells → up to 64)

5. Streptococci (chains)

6. Staphylococci (irregular clusters)

Arrangements of bacilli

1. Single

2. Diplobacilli (pair of cells w/ ends attached)

3. Streptobacilli (chain of several cells)

4. Palisades (cells of a chain remain partially attached by a small hinge region at the ends)

Usually just single, not chains:

5. Spirilla (only sometimes in short chains)

6. Spriochetes: rarely remain attached after cell division

External structures

1. Flagella

2. Fimbriae

3. Pili

4. Nanotubes/nanowires

5. S layer

• Single layer of a protein linked together like tiny chain link fences

• **Only produced when bacteria are in a hostile environment**

6. Glycocalyx

• Coating of repeating polysaccharide or glycoprotein units

• Slime layer: loose, protects against loss of water & nutrients (hydrates)

• Capsule: more tightly bound, denser, & thicker

1. Flagella - external structures/appendages

• Primary function = motility (movement)

• Structure (3 distinct parts):

  • 1. Filament (outermost)

  • 2. Hook (sheath)

  • 3. Basal body

• Immune system makes antibodies against flagella to inhibit movement

1. Flagella - arrangement

• Polar arrangement: flagella attached at one of both ends of the cell

  • 1. Monotrichous: single flagellum

  • 2. Lophotrichous: small bunches/tufts of flagella emerging from same site

  • 3. Amphitrichous: flagella at both poles of the cell

• Peritrichous arrangement: flagella are dispersed randomly over surface of cell

1. Flagella - function

• Primary function: motility (movement)

• Chemotaxis: movement of bacteria in response to chemical signals

  • Positive vs. Negative chemotaxis

  • Run: rotation of flagellum counterclockwise, resulting in a smooth linear direction

  • Tumble: reversal of the direction of the flagellum, causing the cell to stop & change course

2. Fimbriae

• Primary function: adhesion (attachment)

• Small, bristle-like fibers sprouting off the surface of many bacterial cells

  • Short compared to flagella

• Allow tight adhesion b/w fimbriae & epithelial cells, allowing bacteria to colonize & infect host tissues

3. Pili

• Used in conjugation b/w bacterial cells

• Well characterized in gram-negative bacteria

• Type IV pilus can transfer genetic material, act like fimbriae & assist in attachment, & act like flagella & make a bacterium motile

4. Nanotubes (nanowires)

• Very thin, long, tubular extensions of the cytoplasmic membrane

• Used as channels to transfer amino acids or to harvest energy by shuttling electrons to iron-rich substances

5. S layer

• Outside cell wall

• Single layers of thousands of copies of a single protein linked together like tiny chain link fences

• Only produced when bacteria are in a hostile environment

6. Glycocalyx

• Coating of repeating polysaccharide or glycoprotein

• Slime layer: loose, protects against loss of water & nutrients (also aids in attachment) → can appear as plaque

• Capsule: more tightly bound, denser, & thicker; produces a sticky (mucoid) character to colonies on agar

Capsules

• Formed by many pathogenic bacteria

• Have greater disease-causing abilities (increased virulence)

• Protects against host white blood cells called phagocytes

• Biofilms: capsules can be responsible for biofilm formation

  • 2 types of places that biofilm forms:

    • 1. Plaque on teeth → protects bacteria from being dislodged

    • 2. Colonization of plastic catheters, IUDs, metal pacemakers, & other implanted medical devices

The Cell Envelope

• Lies outside the cytoplasm

• Composed of 2 or 3 basic layers that each perform a distinct function, but together act as a single protective unit:

  • 1. Cell wall

  • 2. Outer membrane (part of cell wall in Gram-negative bacteria)

  • 3. Cytoplasmic membrane

1. Cell wall - function

1. Helps determine the shape of a bacterium

2. Provides strong structural support to keep the bacterium from bursting or collapsing because of changes in osmotic pressure

• Certain drugs target the cell wall (particularly its peptidoglycan), disrupting its integrity & causing cell lysis (disintegration or rupture) of the cell

3. Gains its relative rigidity from peptidoglycan (the target of antibiotics)

1. Cell wall - Peptidoglycan

• Compound composed of a repeating framework of long glycan (sugar) chains cross-linked by short peptide (protein) fragments

• Provides a strong but flexible support framework

Gram-Positive cell wall

• Thick, homogenous sheet of peptidoglycan

• Contains teichoic acid & lipoteichoic acid

  • Function in cell wall maintenance & enlargement

  • Contribute to acidic charge on cell surface

Gram-Negative cell wall

• Single, thin sheet of peptidoglycan

• Thinness gives gram-negative cells more susceptibility to lysis

• Contains outer membrane over peptidoglycan

2. Outer membrane - composition

• Similar in composition to most membrane, except it contains specialized polysaccharides & proteins

• 1. Lipopolysaccharide (LPS):

  • Signaling molecules & receptors

  • Endotoxin (toxic to body when disassociated from bacteria; leads to inflammation & septic shock)

• 2. Porin proteins:

  • Special membrane channels in outer membrane that allow certain chemical to penetrate

  • Affects entry of antibiotics & other chemicals

2. Outer membrane - functions & characteristics

1. Contributes an extra barrier

• Resistant to certain antimicrobial chemicals

• **Makes gram-negative bacteria more difficult to kill than gram-positive**

2. Alcohol-based compounds dissolve lipids in outer membrane & therefore damage the cells

• Alcohol swabs used to cleanse skin before certain medical procedures

3. Treatment of infections caused by gram-negative bacteria requires drugs that can cross the outer membrane

Gram Stain

• Helps determine if cell wall is Gram-positive or Gram-negative

• Purple = positive

• Pink = negative

Non-typical cell walls

• Acid-Fast bacteria

• Archaea

Acid-Fast bacteria

• Not gram-positive or gram-negative

• Mycobacterium & Norcardia: contain peptidoglycan & stain gram-positive, but bulk of cell wall is composed of unique lipids, so not considered gram-positive

• Myolic acid:

  • Very long chain fatty acid

  • Found in cell walls of acid-fast bacteria

  • Contributes to the pathogenicity of the bacteria b/c it protects bacteria from getting destroyed

  • Makes bacteria highly resistant to certain chemicals & dyes

Archaea

• Exhibit unusual & chemically distinct cell walls

• Some have cell walls composed entirely of polysaccharides

• Others have cell wall made of pure protein

• All lack true peptidoglycan structure

• Some lack a cell wall entirely

Cell-wall-deficient bacterias

• Mycoplasmas

• L-forms

Mycoplasmas

• Naturally lack a cell wall

• Sterols in the cell membranes stabilize the cell against lysis

• Mycoplasma pneumoniae: “walking pneumonia”

L forms

• Bacteria that naturally have a cell wall but lose it during part of their life cycle

• Role in persistent infections

• Resistant to antibiotics

3. Cytoplasmic membrane

• A lipid bilayer w/ protein embedded

• Functions:

  • Energy reaction (ATP production → happens in mitochondria in eukaryotic cells)

  • Nutrient processing

  • Synthesis

  • Regulates transport of nutrients & waste

  • Selectively permeable: special carrier mechanisms for passage of most molecules

Internal structures (in the cytoplasm)

• Cytoplasm: 70-80% water plus complex mixture of sugars, amino acids, & salt

• Bacterial chromosomes & plasmids (in cytoplasm):

  • Hereditary material of most bacteria exists in the bacterial chromosome

  • DNA is aggregated in the nucleoid (nucleus holds DNA in eukaryotic cells)

  • Plasmids: nonessential pieces of DNA (so bacteria can survive without) → confer protective traits such as drug resistance & toxin & enzyme production

Ribosomes

• All bacteria have this

• In the cytoplasm

• Site of protein synthesis (same in eukaryotic cells)

• Composed of rRNA (60%) & protein (40%)

• Consist of a large & small subunit:

  • Large & small subunits together: 70S

  • Eukaryotic ribosome: 80S

Inclusion bodies

• In the cytoplasm

• Used for food storage

• Pack gas into vesicles for buoyancy

• Store crystals of iron oxide w/ magnetic properties

Cytoskeleton

• In cytoplasm

• Made of long polymers of protein

  • Contribute to cell shape

  • Unique to prokaryotic cells → may be a potential target for antibiotic development

Endospores

• Vegetative cell: metabolically active

  • When this bacterial cell dies, endospores are released, which are dormant bodies: metabolically inactive

• Produced by Bacillus, Clostridium, & Sporosarcina

• Sporulation: induced by environmental conditions → creation of endospore

  • To survive environmental stress

• Endospores resist extremes of heat, drying, freezing, radiation & chemicals that would kill vegetative cells

• Not meant for reproduction →