Lecture 2 - Bacteria I

Bacterial and archaea structure and function (recap)

- Prokaryotes differ from eukaryotes in the size and simplicity (most prokaryotes lack a internal membrane systems, including nucleus)

Prokaryotes are divided into two taxa

Bacteria and Archaea

Shape, Arrangement, and Size

SHAPE

- Cocci and rods most common

- various others

ARRANGEMENT

- determined by plane of division

- determined by separation (after division) or not

SIZE

- varies

Shape - Cocci (s., coccus)

Spheres

Shape: Cocci - diplococci (s., diplococcus)

Pairs

Shape: Cocci - Streptococci

Chains

Shape: Cocci - Staphylocci

Grape-like clusters

Shape: Cocci - Tetrads

4 cocci in a square gemoetry

Shape: Cocci - Sarcinae

Cubic configuration of 8 cocci

Shape & Arrangement: Bacilli (s., bacillus)

Rods

Shape & Arrangement: Coccobacilli

Very short rods

Shape & Arrangement: Vibrios

Resemble rods, comma shaped

Shape & Arrangement: Spirilla (s., spirillum)

rigid helices

Shape & Arrangement: Spireochetes

Flexible helices

Size

- Smallest - 0.3 micrometers (Myocoplasma)

- Average rod: 1.1 - 1.5 x 2-6 micrometers (E.coli)

- Very large: 200-700 micrometers

Size: Oscillatoria Red blood cell

7000 nanometers

Size: E. Coli

1300 x 4000 nanometers

Size: Strepococcus

800-1000 nanometers

Size: poxvirus

230 x 320 nanometers

Size: Influenza virus

85 nanometers

Size: T2 E.coli bacteriophage

65 x 95 nanometers

Size: Tobacco mosaic virus

15 x 300 nanometers

Size: Poliomyelitis virus

27 nanometers

1 micrometer = ? nanometers

1000 nanometers

Cell organization in Prokaryotes

External structures

Cell envelope

Cytoplasm

Components of a prokaryotic cells

- Nucleoid

- Ribosome

- Cytoplasmic Membrane

- Cell wall

- Capsule

- Flagella

- Fimbriae

Prokaryotes: Bacterial cell envelope

Plasma membrane, Cell wall, Layers outside the cell wall (Sometimes)

Prokaryotes: Bacterial Plasma membrane

absolute requirements for all living organisms

Plasma Membrane Functions

- Encompasses the cytoplasm

- Selectively permeable barrier

Interacts with external environment:

- Receptors for detection of and response to chemicals in surroundings

- transport systems

- Metabolic processes (ATP)

Fluid Mosaic Model of Membrane structure

Lipid bilayers with floating proteins:

- Amphipathic lipids

= Polar ends (hydrophilic - interact with water)

= Non-polar tails (hydrophobic - insoluble in water)

Membrane proteins

Fluid Mosaic Model

Structural model of the plasma membrane where molecules are free to move sideways within a lipid bilayer.

Membrane Proteins: Perpheral

- loosely connected to membrane

- easily removed

Membrane proteins: Integral

- Amphipathic = embedded within membrane

- Carry out important functions

Membrane Steroids

Cholesterol (a steroid) is found in eucaryotes

Membrane Hopanoids

A bacteriohopanetetrol (a hopanoid) is found in bacteria

Bacterial Lipids

- Bacterial saturation levels of membrane lipids reflect the environmental conditions such as temperature

- Membranes lack sterols but do contain sterol-like molecules, instead contain hopanoids (Stabilize membrane)

Bacterial cell wall

peptidoglycan (only bacteria) - rigid structure hat lies just outside the cell membrane

Bacterial Cell Wall - gram positive

- Stain purple; THICK peptidoglycan

Bacterial Cell Wall - gram negative

- Stain pink or red; THIN peptidoglycan and outer membrane

Cell Wall Functions

- Maintains the shape of the bacterium (almost all bacteria have one)

- Helps protect cells from osmotic lysis

- Helps protect from toxic materials

- May contribute to pathogenicity (ability to cause disease)

peptidoglycan structure

- Meshlike polymer of identical subunits forming long strands

Peptidoglycan alternating sugars

2 ALTERNATING SUGARS:

- N-acetylglucosamine (NAG)

- N-acetylmuramic acid

Alternating D- (only bacteria) and L- (other protein structures) amino acids

4 most common amino acids

= L-alanine

= D-glutamic acid

= Meso-diaminopimelic acid

= D-alanine

Strands are corossed link

- Peptidoglycan strands have a helical shape

- Peptidoglycan chains are crosslinked by peptides for strength

- Nature of cross linking also provides flexibility to an extent

Chemical structure of cross-linking peptidoglycan components of cell wall

- Peptide chain

- N-acetyl glucosamine

- Pentapeptide bridge

- N-acetyl muramic acid

Gram-positive cell walls

- Composed primarily of peptidoglycan (lots)

- May also contain large amounts of TEICHOIC ACIDS (negatively charged)

- Some gram-positive bacteria have layer of proteins on surface of peptidoglycan

Teichnoic Acid

- Help maintain cell envelop

- Protect from environmental substances

- May bind to host cells

Gram-negative cell walls (has outer membrane)

- More complex than gram-positive

- Consists of a thin layer of peptidoglycan surrounded by an outer membrane

- Outer membrane composed of lipids, lipoproteins, and lipopolysaccharide (LPS)

- No teichoic acid

Gram-negative outer membrane

- Outer membrane lies outside the thin peptidoglycan layer

- Braun's lipoproteins connect outer membrane to peptidoglycan

- Other adhesion sites reported

Gram-negative cell walls

- Peptidoglycan is ~5-10% of cell wall weight

- Have a periplasmic space between the outer membrane and the inner membrane

Periplasmic Space

The space between the inner and outer cell membranes in Gram-negative bacteria.

- May constitute 20-40% of cell volume

- Many enzymes present in periplasm

= hydrolytic enzymes, transports proteins, and other proteins

Structural Features of Gram-positive and gram-negative cell walls

Lipopolysaccharides (LPSs) in gram (-) negative outer membranes

CONSISTS OF THREE PARTS:

- Lipid A (embedded in the outer membrane)

- Core polysaccharide (extends out from the cell)

- O side chain or O antigen (extends out from the cell) - immunogenic

Gram negative bacterial cell wall

Importance of LPS

- Contributes to negative charge on cell surface

- Helps stabilize outer membrane structure

- May contribute to attachment to surfaces and biofilm formation

- Creates a permeability barrier

- Protection from host defenses (O antigen)

- Can act as an endotoxin (lipid A)

LPS: can act as a endotoxin (lipid A)

"endotoxin" response elicited by gram (-) bacteria only...dont confuse with exotoxin shock or the broader term septic shock

Gram-negative outer membrane permeability

more permeable than plasma membrane due to presence of porin proteins and transporter proteins

- Porin proteins form channels through which small molecules

- (600-700 daltons) can pass

Osmosis

Movement of solvent (such as water) through a semipermeable membrane) such as the plasma membrane in a living cell) into a solution of higher solute (such as sugar or salt) concentration that tends to equalize the concentrations of the solute on the two sides of that semipermeable membrane.

osmosis example

Osmotic Protection - Hypotonic Environments

Solute concentration outside the cell is less than the inside the cell.

Water moves into cell and cell swells - can burst

Osmotic Protection - Hypertonic Environment

Solute concentration outside the cell is higher than inside, water leaves the cell.

Plasmolysis occurs - cell shrinks

Plasmolysis

Cell shrinks

Evidence of protection nature of the cell wall - Lysozyme

breaks down the bond between N-acetyl glucosamine and N-acetylmuramic acid

Evidence of protection nature of the cell wall - Penicillin

Inhibits peptidoglycan synthesis

Cell wall: Lysozyme & Penicillin

If cells are treated with either of the above they will lyse if they are in a hypotonic solution

Cells may survive loss of cell wall ONLY in isotonic conditions

- Protoplasts

- Spheroplasts

These forms are "man-made" by manipulation of bacteria using cell wall-damaging agents (lysozyme or penicillin)

Protoplasts

May survive loss of cell wall in only an isotonic environment:

Protoplasts (cell wall-less form derived from gram+ bacteria)

Spheroplasts

May survive loss of cell wall in only an isotonic environment:

Spheroplasts (cell wall-less form derived from gram- bacteria)

Cells survive loss of cell wall only isotonic environments

- Another name- Lawless (L) forms (derived from bacterial when treated with penicillin and are susceptible to lysis in hypotonic solutions; can revert back to normal form)

Mycoplasma (exception)

- Does not produce a cell wall EVER

- Plasma membrane more resistant to osmotic pressure and can survive in hypotonic environment - born this way

Lysis of a protoplast

Components outside of the cell wall

Extracellular material secreted by bacteria in form of:

- Slime layer

or

- Capsule

Slime layer

- Are diffuse, unorganized and easily removed

- Made of exopolysaccharides, glycoproteins, and glycolipids

- Slime may protect the bacteria from harsh environments and aid in adherence

Capsule

- Capsule is an amorphous gelatinous layer surrounding the entire bacterium

- Composed of polysaccharide or sometimes protein (e.g. Bacillus anthracis)

- The sugar component of the capsule varies in different bacterial species and also determines serological type within a species

Bacterial and archaeal cytoplasmic structures

- Cytoskeleton

- Intracytoplasmic membranes

- Inclusions

- Ribosomes

- Nucleoid and plasmids

Protoplast and cytoplasm

- Protoplast is plasma membrane and everything within

- Cytoplasm = material bounded by the plasmid membrane

Inclusions

- Granules of organic or inorganic material that are stockpiled by the cell for future use

- Some are enclosed by a single-layered membrane

= membranes vary in composition

= some made of proteins; others contain lipids

= may be referred to as microcompartments

Storage inclusions

- storage of nutrients, metabolic end products, energy, building blocks

- glycogen storage

- carbon storage (poly-B-hydroxybutyrate (PHB))

- phosphate - polyphosphate (volutin)

- amino acids - cyanophycin granules

Inclusions: Gas Vacuoles

- found in aquatic, photosynthetic bacteria and archaea

- provides buoyancy in gas vesicles

Inclusions: Magnetosomes

- found in aquatic bacteria

- magnetic particles for orientation in earth's magnetic field

- cytoskeletal protein MamK (helps form magnetosome chain)

Ribosomes

COMPLEX STRUCTURES:

- Consisting of protein and RNA

- sites of protein synthesis

ENTIRE RIBOSOME:

- bacterial and archaea ribosome = 70S

- eukaryotic (80S)

- S = svedburg unit

The nucleoid

- Irregular shaped region in bacteria and archaea

- Usually not membrane bound

- Location of chromosome and associated proteins

- Usually 1 (a closed circular, double-stranded DNA molecule

- Supercoiling and nucleoid associating proteins (NAP) probably aid in folding (different proteins differ from histones

Plasmids (extra)

- Extrachromosomal DNA

= found in bacteria, archaea, some fungi

= usually small, closed circular DNA molecules

- Exist and replicate independently of chromosome

= episomes - may integrate into the chromosome

- Contain few non-essential genes (non-core function)

= confer a selective advantage-additive features to host (e.g., drug resistance) - are helpful but NOT required for core survival under normal circumstances

Plasmids (continued)

- May exist in many copies in cell

- Inherited stably during cell division

- curing is the loss of a plasmid

- classification of plasmids based on mode of existence, spread, and function

External structures

- Extend beyond the cell envelop in bacteria and archaea

FUNCTION:

- protection, attachment to surfaces, horizontal gene transfer, cell movement

- pili, fimbriae, and flagella

Fimbriae

- short, thin, hairlike, proteinaceous appendages (up to 1000/cell)

- mediate attachment to surfaces

- some (type IV fimbriae) required for motility or DNA uptake

Sex Pili

- similar to fimbriae except longer, thicker, and less numerous (1-10/cell)

- genes for formation found on plasmids

- required for conjugation

Flagella

- threadlike, locomotor appendages extending outward from the plasma membrane and cell wall

FUNCTIONS:

- motility and swarming behaviour

- attachment to surfaces (may function as a virulence factor)

Bacteria Flagella

- thin, rigid protein structures that cannot be observed with bright-field microscope unless specially stained

- ultrastructure composed of three parts

- pattern of flagellation varies

Patterns of flagella distribution

- monotrichous

- polar flagellum

- amphitrichous

- lophotrichous

- peritrichous

Monotrichous

one flagellum

Polar flagellum

flagellum at one end of the cell

amphitrichous

one flagellum at each end of the cell

lophotrichous

cluster of flagella at one or both ends

Peritrichous

spread over entire surface of cell

three parts of a flagella

- Filament

- Hook

- Basal body

Flagella - Filament

- extends from cell surface to the tip

- hollow, rigid cylinder

- composed of the protein flagellin

- some bacteria have a sheath around filament

Flagella - Hook

- links filament to basal body

Flagella - Basal Body

- series of rings that drive flagellar motor

Endospore Structure (concentrated cell)

- Spore surrounded by a thin covering called exosporium

- Thick layers of protein form the spore coat

- Cortex, beneath the coat, thick peptidoglycan

- Core has nucleoid and ribosomes