Chapter 3: Bacterial cell structure

Features of prokaryotic cells

• Unicellular organisms

• Lacks a membrane-bound nucleus (DNA is located in nucleoid region)

• Lack of membrane-bound organelles

  • Intracellular structures

3 domains of life

Bacteria, Archaea, and Eukarya

What affects morphology of the bacteria

• Change in environment

• Life stage

• Nutrients available

• Antimicrobial drugs

List the MAIN differences between a Eukaryotic cell and a Prokaryotic cell

• Eukaryotes have a more complex genetic makeup than prokaryotes

• Possess membrane-bound organelles

Bacterial arrangement

• Diplo → Paired

• Strep→ Chain

• Staph → Cluster

• Palisade→ Clusters of cells (e.g., found in bacilli)

E.g. Diplococci → Two cocci-shaped bacteria together

E.g. Staphcocci → Cluster of cocci

E.g., Streptococci → Chain of cocci

Binary fission

1. DNA is copied

2. Cell grows

3. Chromosomes are drawn to opposite ends of the cell (critical size)

4. The septum (partition) begins at the midpoint

5. Septum walls off, two daughter cells form

Largest bacteria

T. Namibiensis

Bacillus (singular)

Rod-shaped cell

Coccus (singular)

Spherical shaped cell

Spiral

Spiral-shaped cell

Vibrio

Comma-shaped

Stella

Star shaped cell

Coccobacilli (shape)

Oval-shaped bacteria or cell

Spirochete (shape)

Spiral-shaped bacteria that move in a corkscrew motion due to a periplasmic flagellum

Plasma Membrane

Thin, selective barrier with flexible phospholipid bilayer and site for metabolic reactions (e.g. ATP)

• Half of the membrane consists of proteins like

  • Transporters

  • Anchors

  • Receptors

  • Enzymes

What passes plasma membrane’s selective permeability

Gases, water, small, noncharged particles

What DOESNT pass plasma membrane’s selective permeability

Ions, large, and polar particles that need protein transporters to enter/exit the cell

How can proteins coordinate assistance to parts of the plasma membrane that need to certain particles

Lipid bilayer is a fluid-mosaic, so lipids and proteins can move and relocate

KEEP IN MIND: At warm temperatures, saturated fats are best to prevent too much fluidity and vice versa

What are Isoprenoids?

They are the long-branched fatty acids or lipids in archaea that are linked to glycerol by ester bonds. Its isoprene units are saturated hydrocarbon

Prokaryotic VS Archaic bacterial membranes

Prokaryote:

• Ester bond → links linear fatty acids to glycerol

• Lipid bilayer

Archaic:

• Ester bond→ links LONG- BRANCHED fatty acids to glycerol

• Lipid MONOLAYER (has unique lipids called tetraether)

What bacteria is more likely to survive? Small or big?

Small bacteria.

Rationale: Their low surface area-to-volume ratio allows nutrients to reach the core of the bacteria faster, and thus fittest to survive

What is the difference between monomorphic and pleomorphic

Monomorphic → Only one shape

Pleomorphic → Can take on multiple shapes (e.g. Helicobacter Pylori)

Ester VS Ether bonds

Ester → O-C=O

Ether → O-C-O (lacks a double bond)

List 5 roles of a bacterial cell wall

1. Extra layer of protection

2. Gives shape to prokaryotes

3. Prevents osmotic lysis (burst)

4. Important in classification

5. Serves for attachment

What bond binds NAM and NAG together in a mesh-like structure

Short peptide cross-link bridges connect NAM and NAG, glucose-resembling residues

What drugs interfere with the peptidoglycan (cell wall) of bacteria

Penicillin (They do NO HARM to archaea)

Describe the composition of peptidoglycan

Peptidoglycan: Component of bacterial cell wall (archaea uses pseudopeptidoglycan)

Made w/ protein (peptid) + sugar (glycan)

• Glycan chains are built on alternating NAG and NAM

What does gram staining technique allow

To classify cells based on cell wall structure (gram positive or gram negative)

Gram-negative bacteria cell wall

• Thin peptidoglycan layer (high resistance to penicillin)

• Outer membrane → Carries a glycolipid called LPS

  • LPS contains a lipid A that is toxic to animals

• Periplasmatic space → between the plasma membrane and the OUTER plasma membrane where nutrients are obtained, and toxins are neutralized

What family makes up most of the Gram-negative bacteria?

Enterobacteriaceae or enteric bacteria

• Salmonella, E. coli, Proteus, Shigella, etc.

Gram-positive bacteria cell wall

• Thicker peptidoglycan layer (high susceptibility to penicillin)

• NO OUTER MEMBRANE

• Periplasmatic space → region between plasma membrane and peptidoglycan cell wall

• Stains purple in gram staining procedures

Pros of gram positive cell walls

• Best for dry environments (retain moisture longer)

• Provide mechanical stress protection

• Teichoic acid → Stabilizes the cell wall, maintaining its shape, transport of cations, and aids in regulating cell division

  • E.g. Staphylococcus aureus uses teichoic acid to promote adhesion and pathogenicity

What is the significance of the Lipid A endotoxins component of lipopolysaccharides (LPS) in Gram-negative bacteria?

It triggers strong immune responses in humans and can lead to septic shock if released in large amounts during infection.

Outer membrane of a gram-negative cell

Makes the gram-negative cell harder to kill due to a selective barrier guarding for:

• Lysozyme (agent damage)

• Variety of drugs

• Detergents + disinfectants

REMEMBER: It’s NOT as selective as the plasma membrane because it carries porins

What are porins

Nonspecific protein channels that form a pore in the outer membrane, allowing substances like amino acids and vitamins to pass, excluding large and harmful molecules

What does acid-fast technique do? Examples of organisms?

Detects a waxy lipid called mycolic acid in cell walls, causing the cell walls to appear red/pink following the procedure.

Helps identify agents for leprosy, tuberculosis, and skin infections

E.g. Nocardia and Mycobacterium

Why are acid-fast bacteria troublesome

They have a waxy layer that makes it difficult for crystal violet to stain, but most importantly, for nutrients, gases, and drugs like antimicrobials to enter, making them grow slow but hard to remove

What are examples of bacteria that lack a cell wall other than Mycoplasma?

• Ureaplasma species

• Acholeplasma bacteria

• Both have unique vulnerabilities and treatment options.

Mycoplasma VS L-form bacteria

Mycoplasma bacteria:

• Lacks a cell wall

• Has sterol

• Pleomorphic

L-form bacteria:

• Had a cell wall but lost it (prob due to antibiotics targeting the cell wall)

• Resistant to stress (boiling and autoclaving)

What is glycocalyx?

A sugar coat

• A sticky carbohydrate-enriched layer associated with the cell wall

• Helps bacteria stick to host + surfaces

• Protects from desiccation and antibiotics

Glycocalyx capsule VS slime layer?

Capsule:

• Well-defined, organized, and tightly adhered to the cell wall

• Increased pathogenicity

Slime layer:

• Loosely associated, unorganized, often allowing easier movement and dispersal.

Give two examples of encapsulated bacteria.

1. Streptococcus pneumoniae

2. Klebsiella pneumoniae

What is the flagella used for

Motility

How can prokaryotes sense where they swim to

The run-and-tumble system allows the cell to move toward/away from a stimulus, stop, tumble to sense surroundings, and redirect accordingly.

Describe the structure of a bacterial flagellum

Almost half of bacteria have a flagellum consisting of:

• Filament

• Hook

• Basal body

• Rotating like a propeller from a rod-and-ring structure within the basal body

How is the flagellum of gram-positive and gram-negative bacteria different?

• Gram-positive→ 2 rings secure flagellum

• Gram-negative→4 rings secure and anchor flagellum to body

Flagella arrangements

• Peritrichous: Flagella all over the cell

• Monotrichous: Single flagellum

• Lophotrichous: A tuft or clusters of flagella at one pole

• Amphitrichous: Flagella at both poles

Periplasmatic flagella (axial filaments)

A type of flagella that is found within the periplasmic space of some gram-negative bacteria, allowing spirochetes to move in their corkscrew motion

How are bacterial flagella different from a Eukaryotic one

Bacterial flagella are made of flagellin (a protein), but Eukaryotic flagella consist of microtubules

What are axial filaments

Periplasmatic flagella that wrap around the cell body, causing movement by rotation found in spirochetes.

What microbes have axial filaments

Treponema and Borrelia

Fimbriae VS Pili.

Fimbriae:

• Short, hair-like structure

• Adhesive properties help bacteria stick to surfaces

• In gram-negative bacteria

Pili:

• Longer, more rigid, and less numerous than fimbriae

• Motility (gliding or “twitching”)

• Adherence to surfaces

• Transfer of DNA between bacterial cells (conjugation).

How does a bacterial ribosome differ from a eukaryotic one?

Bacterial ribosomes: 70S in size (50S + 30S subunits)

Eukaryotic ribosomes: 80S in size (60S and 40S subunits)

REMEMBER: Prokaryotic ribosomes have less mass and diameter than Eukaryotic ribosomes

What eukaryotic organelles contain ribosomes like that of a bacterium?

Mitochondria and chloroplasts contain ribosomes similar to those of bacteria (70S) and are believed to have evolved from prokaryotic ancestors.

What inclusions are present in the bacterial cell, and their functions

They are storage bodies within a cell that reduce active intracellular volume to levels like those of normal-sized prokaryotes

1. Can be: Insoluble granule or membrane-bound inclusions

2. Store various substances

• Nutrients (glycogen and lipid)

• Gas (gas vesicles)

• Metabolic byproducts

Example of inclusion bodies

Carboxysomes → Carbon-fixing enzymes

Magnetosomes → Accumulate magnetic iron

Metachromatic granules (volutin) → phosphate reserves

Lipid inclusions → Energy reserves

Sulfur granules → Energy reserves

Gas vacuoles → Protein-covered cylinders

What is an endospore?

Metabolically inactive structure that allows cells to enter a dormant state

• Resistant to extreme conditions (starvation, heat, drying, freezing, and radiation)

• Vegetative cells→ Term when spores germinate back during favorable conditions

2 genera of bacteria that form endospores

Bacillus and Clostridium (gram-positive) as well as Clostridioides.

Describe sporulation.

Process by which a bacterial cell forms an endospore:

1. Duplication of DNA

2. Package DNA w/ribosome + enzyme for vegetative restoration into Spore Core

3. Surround the spore core w/heat and chemical-resistant layers

4. Release

What characteristics of endospores allow them to resist high temperatures, radiation, etc.?

• A thick, protective coat with low water content provides resistance to heat

• Dipicolinic acid stabilizes DNA in the spore core, providing heat resistance, desiccation, radiation, and chemical damage.