Bacterial Cell Structures: Morphology and Functions
Living Preparations
Wet Mount
- Add a drop of the culture, and cover it with a glass slide.
- Benefits:
- It’s fast.
- Cells are alive, and you can see them in their natural shape.
- Can examine their behavior.
- Disadvantages:
- Hard to detect cells/characteristics.
- Covering the glass can damage larger cells, contaminate the observer’s fingers, dry out, and also become anoxic.
Staining
- Increases contrast and sometimes size.
- Before staining, we must let it dry and fix it. That way, the sample does not get washed off.
- Positive staining:
- Sticks to the specimen.
- Gives them color.
- Negative staining:
- Does not stick to the specimen.
- Dark background – everything around is dark and stained, but not the cell.
- Simple stains:
- One single dye.
- Shows their size, shape, or arrangement.
- Can only be positive or negative.
- Differential stains:
- 2 different dyes: primary dye and counterstain.
- Distinguishes between cell types or parts.
- Both positive and negative can be seen at the same time.
Bacterial Shapes, Arrangements, and Sizes
General Shapes
- Coccus: roughly spherical.
- Bacillus: rod-shaped
- Coccobacillus: short and plump.
- Vibrio: gently curved, bent rods.
- Spiral: curviform or spiral-shaped. 2 types:
- Spirillum
- Spirochete
- Pleomorphism: cells of species vary in shape and size.
Arrangement or Grouping
- Cocci: greatest variety in arrangement, can make different organizations. Have different because they can divide.
- Single
- Pairs (diplococci)
- Tetrads: grouping of 4
- Irregular cluster (staphylococci or micrococci)
- Chains (streptococci)
- Cubical packet – sarcina
- Bacillus: split in the middle and grow outwards; that is why they don’t have that many. Less varied
- Single
- Pairs – diplobacilli
- Chain – streptobacilli
- Row of cells oriented side by side – palisades
- Spirilla
- Occasionally found in short chains
Structural Biology: Bacteria and Archaea vs. Eukaryotes
Packaging of DNA
- Bacteria and archaea: packed out of genetic material – free in the cytoplasm. Has peptidoglycan holding it together.
- Eukaryotes – nucleus
Cell Wall Makeup
- Bacteria: peptidoglycan
- Archaea: distinct from bacteria and eukaryotes
- Eukaryotes: varies, if present at all
Internal Structures
- Bacteria and archaea: no membrane-bound organelles
- Eukaryotes: membrane-bound organelles
Structure Common to All Bacterial Cells
- Cell membrane: all bacterial cells and cells have this.
- Cytoplasm
- Ribosomes
- One or a few chromosomes.
Structure Found in *Some* Bacterial Cells
- Flagella
- Pili
- Fimbriae
- Capsules
- Slime layers
- Inclusions
- Actin cytoskeleton
- Endospores
External Structures
- Appendages: things sticking out of cells
- Common but not present on all species
- Can provide motility – move around and see different things
- Attach to surfaces, attach to different things
- Sensing
Flagella
- Can provide motility
- Difficult to visualize
- Do a wet mount, look for cells moving in multiple directions, moving in a controlled manner.
- Vary in both number and arrangement:
- Polar arrangement: flagella attached at one or both ends of the cell.
- Monotrichous: single flagellum
- Lophotrichous: small bunches or tufts of flagella emerging from the same site.
- Amphitrichous: flagella at both poles.
- Peritrichous arrangement: dispersed randomly over the structure of the cell.
- Polar arrangement: flagella attached at one or both ends of the cell.
- Flagella structure:
- Made of large numbers of proteins. Three parts:
- Filament: they are really skinny. Major antibody target. Why? They are long and stick out a long way.
- Hook (sheath): adaptor
- Basal body: motor
- Electric motor:
- Driven by an ion gradient
- More outside the cell than in
- Rotates rapidly and moves either way. Go both ways:
- Rotates 360 degrees
- Reversible.
- Propels cell through the environment:
- Chemotaxis: toward/away from chemicals
- Phototaxis: toward/away from light
- Move by runs and tumbles.
- The way it is moving determines whether it can do tumbles or runs. Depends:
- Spin direction
- Tumbles – clockwise
- Runs – counterclockwise
- Microbes do runs first, and then they stop and do tumbles
- Spin duration:
- Runs last longer when moving in a favorable direction.
- Toward/away from something.
- Would move towards food sources.
- Move away from chemicals or bad things.
- Spin direction
- Made of large numbers of proteins. Three parts:
- Flagella of spirochetes
- Axial filaments:
- Most bacteria have an inner and outer membrane; flagella have its between them. This makes the entire cell spin.
- Periplasmic
- Bundles of many flagella.
- They spin in a corkscrew motion.
- As they rotate – cells twist.
- Drill through viscous media.
- Axial filaments:
Pili and Fimbriae
- Fimbria:
- Fibers of proteins
- Sticking out of the cells, fuzzy barrier outside.
- Cells use them to attach to stuff.
- Attaches cell to surface
- Maybe host
- Attaches to other cells
- Biofilms
- Attaches cell to surface
- Sometimes provide motility
- Pulls cell across a solid surface.
- Pilus
- Tube made from pilin
- Attach two cells together
- Genetic material – tube
- Conjugation
The S-Layer and Glycocalyx
- The cells usually have a coating of repeating protein (S-Layer) or polysaccharide (Glycocalyx) units or both.
- Protects cell
- Help adhere to the environment
- Differ in:
- Thickness
- Organization
- Chemical composition
S-layer
- Protein shield
- May also aid in attachment to surfaces
Slime Layer
- Loose shield
- Promotes surface attachment/community formation
- Protects some bacteria from loss of water and nutrients
- Slimy and gross looking.
- Starts with just carbs, and then protein gets added, and others.
Capsule
- Glycocalyx bound more tightly to the cell.
Functions of the Glycocalyx
- Slime layer
- Formation of biofilms
- Allows attachment to many surfaces
- Mainly protects groups.
- Capsule
- Important for pathogenesis
- Prevents phagocytosis
- Blocks access by drugs
- Mainly protects individuals
The Cell Envelope: The Boundary Layer of Bacteria
- Outside of the cytoplasm
- Separates self from everything else
- Composed of two or three basic layers
- Cell wall
- Not all bacteria have one
- Cell membrane
- In some bacteria, the outer membrane
- Gram (-)
- Cell wall
Differences in Cell Envelope Structure
- One major way of classifying bacteria is whether Gram-positive or Gram-negative
- Differences lie in the cell envelope:
- Gram-positive
- Two layers
- Cell wall and cytoplasmic membrane
- Two layers
- Gram-negative
- Three layers
- Cell wall (thinner/more flexible), and cytoplasmic membrane
- Extra layer – the outer membrane
- Three layers
The Gram-Negative Outer Membrane
- Mixture of lipids, proteins (similar to cell membrane), but with specialized polysaccharides and proteins unique to OM
- Outer leaflet (outside face of membrane): contains lipopolysaccharide
- Inner leaflet (inside face of membrane): phospholipid layer anchored by lipoproteins to cell wall
- Outer membrane acts like a sieve
- Only relatively small molecules can penetrate
- Access provided by special membrane channels formed by porin proteins
- Generally non-selective
Can alter size when stressed – keep out antibiotics
Lipopolysaccharide
- Major component of the outer leaflet of an outer membrane
- Provides:
- Structural integrity
- Protection from some chemicals
- Negative charge
- Provides:
- Three domains
- Lipid A (endotoxin)
- Core polysaccharide
- O Antigen
- Dominant antigen of Gram-negative cells
- Endotoxin (lipid A) especially
- Recognized by the immune system → fever and toxic shock
- Some species constantly shed/change O antigen
- Avoid immune detection
- Immune system is always looking for the wrong thing
- Or, if you think you’re hardcore and competitors aren’t, make the immune system always active
- Avoid immune detection
- Endotoxin (lipid A) especially
The Bacterial Cell Wall
- Helps determine the shape of a bacterium
- Provides strong structural support
- Keeps cell from bursting or collapsing
- Tire to membrane’s inner tube
- Osmotic pressure differences
- Rigidity due to peptidoglycan (PG) content
- PG structure:
- Long chains of repeats of disaccharide (a.k.a. glycan)
- NAG: N-acetyl glucosamine
- NAM: N-acetyl muramic acid
- Peptide chains connect chains of glycan repeats
- Cell held in place by a chain-link fence instead of a pile of ropes
- Many layers of chain link
- Cell held in place by a chain-link fence instead of a pile of ropes
- Long chains of repeats of disaccharide (a.k.a. glycan)
- PG structure:
- Keeps cell from bursting or collapsing
- Target of many antibiotics
Gram Staining
- Most universal diagnostic staining technique for bacteria
- Primary stain: Crystal Violet
- Counterstain: Safranin
- Differentiation of microbes as Gram-positive or Gram-negative
- Main differences based on cell wall thickness/characteristics
The Structure of the Bacterial Cell Wall
- Gram-positive cell wall
- Thick (20 to 80 nm), homogeneous sheath of peptidoglycan
- Contains tightly bound acidic polysaccharides
- Teichoic acid and lipoteichoic acid
- Make cell wall more acidic
- Negatively charged
- Important for cell division
- Target of antimicrobial peptides
- Gram-negative cell wall
- Single, thin (1 to 3 nm) sheet of peptidoglycan
- More flexible
- Not as strong
- Need strength from LPS in outer membrane to replace strength from the rigidity of Gram +’s thicker cell wall
- Periplasmic space surrounds the peptidoglycan
- Single, thin (1 to 3 nm) sheet of peptidoglycan
- Atypical cell walls
- Mycobacteria and Nocardia
- Modified Gram-positive
- Cell wall has special lipids embedded on the outer surface
- Peptidoglycan but also mycolic acid
- Neither crystal violet nor safranin binds great to it
- Can look Gram +, Gram -, or unstained
- Why use acid-fast stain on them
- Cell wall has special lipids embedded on the outer surface
- Modified Gram-positive
- Mycobacteria and Nocardia
- Archaea
- No peptidoglycan
- Some have walls of polysaccharide or protein
- Others → no cell wall
Cell Wall-deficient Bacteria
- Mycoplasmas
- Smallest bacteria
- Pleomorphic
- Lots of fun shapes (cocci, filaments, toroids, etc.)
- No cell walls
- Special sterols to strengthen the wall
- Very resistant to lysis
- Smallest bacteria
- Other bacteria may lose the wall at a certain stage of lifestyle
- Wall-less Archaea often use special fatty acids
- Ether linkage
- Branching for better packing
The Structure of the Cell Membrane
- AKA cytoplasmic membrane
- Very thin (5-10 nm)
- Contains
- Phospholipids (30-40%)
- Proteins (60-70%)
- Exceptions: mycoplasmas and archaea
- Other additional lipids for stability
- Functions
- Provides a site for:
- Energy reactions
- Nutrient processing
- Nutrient synthesis
- Regulates transport
- Selectively permeable
- Secretion
- Provides a site for:
Differences in Cell Envelope have Consequences
- Outer membrane – extra barrier in Gram (-)
- Less sensitive to some antimicrobial chemicals
- Can’t get through porins to block cell wall synthesis
- Generally more difficult to inhibit or kill than gram-positive bacteria
- Unless treated with alcohol to permeabilize/dissolve
- Need different drugs than Gram (+)
- Less sensitive to some antimicrobial chemicals
- Cell envelope can interact with human tissues and cause disease
- Toxins attached to Gram (+) cell walls
- Corynebacterium diphtheriae
- Streptococcus pyogenes
- Toxic cell membrane lipids
- Mycobacterium sp.
- Stimulate the immune system
- Toxic shock
- Autoimmune disorders
- Obesity and Type II diabetes related to endotoxins in some patients?
- Toxins attached to Gram (+) cell walls
Bacterial Internal Structure
- Contents of cell’s cytoplasm
- Gelatinous solution
- 70%-80% water
- So packed with other stuff + so many H-bonds between things that it’s not fluid though
- Organized, not a random sack of stuff
- Site for many metabolic activities
- Catabolic (breaking down food for energy)
- Anabolic (building macromolecules)
- Contains larger, discrete cell areas/structures:
- Nucleoid (chromatin)
- Ribosomes
- Granules
- Actin strands
- Separate structures or regions (some surrounded by membranes) but NONE of them are organelles
- Gelatinous solution
Bacterial Chromosome
- Single circular strand of DNA
- Some have more than one chromosome
- Others can be linear
- Aggregated in a dense nucleoid
- Tightly supercoiled
- Sometimes wrapped around histones into chromatin
- Genetic material also present on plasmids
- What do we know about them?
- Why do we care?
Ribosomes
- Protein-making machinery
- 1000’s/cell
- How many depends on what the cell is doing
- More active cells have more
- Made of RNA and protein
- 3 Ribosomal RNA (rRNA)
- Catalytically active RNA
- Does the actual work of gluing together amino acids
- ~80 proteins between 2 subunits
- Provide structure, shape, regulation
- 3 Ribosomal RNA (rRNA)
- Characterized by S units
- Bacterial ribosome is 70S
- 50S subunit: 23S and 5S rRNA
- 30S subunit: 16S rRNA
- Bacterial ribosome is 70S
Inclusion Bodies
- May or may not be bound by a single membrane
- Even if they have a membrane, not organelles
- Membrane-bound inclusions:
- Storage of supplies for a rainy day
- Glycogen, poly β-hydroxybutyrate (PHB)
- Provide energy
- Glycogen, poly β-hydroxybutyrate (PHB)
- Gas Vesicles
- Allow bacteria to float in water
- Get to the right level of light exposure
- Storage of supplies for a rainy day
- Granules
- Type of inclusion body
- Not enclosed by membranes
- Crystals of inorganic compounds
- Type of inclusion body
- Magnetosomes
- Magnetic iron crystals
- Allow orientation in a magnetic field → magnetotaxis
- Usually allows them to swim down → away from O2
The Cytoskeleton
- Long polymers of actin-like proteins
- Arranged in helical ribbons around the cell just under the cell membrane
- Important for cell division
- Machinery travels along it to find the middle
- Contribute to cell shape
Bacterial Endospores: An Extremely Resistant Stage
- Dormant bodies produced by Bacillus, Clostridium, and Sporosarcina
- All Gram (+)
- A few Gram (-) make them too
- Two-phase life cycle
- Vegetative cell
- Metabolically active and growing
- Can be induced by the environment to undergo spore formation (sporulation)
- Endospore Formation
- Wait out the bad times
- Can wait a very long time