Topic 2: Bacterial Cell Structure and Function

Prokaryotes vs Eukaryotes

• Eukaryotes: cells that contain membrane bound organelles

  • ex. yeast, helminths, protozoa

• Prokaryotes: cells that lack a membrane bound nucleus and other organelles

  • Main type we will study → bacteria

Viruses

• neither eukarotic or prokaryotic

• obligate intracellular pathogens - viruses cannot reproduce or carry out metabolic processes outside a host cell

Eukaryotic DNA

• linear

• double stranded

• housed within the nucleus

  • packages - exists as a supercoil

• diploid - 2 copies of every chromosome ( 1 maternal, 1 paternal)

• mitosis (replication of somatic cells) vs meiosis (replication of gametes)

prokaryotic DNA

• circular - single molecule

• double stranded

• located in the nucleoid region ( a visible mass)

  • nucleoid not seperate or protected in cell - just where DNA happens to localize

• haploid → asexual, binary fission

• plasmid (smaller, circular DNA molecules that exist separately from chromosomal DNA)

  • carry non-essential gene products

  • antibacterial resistance can be plasmid driven

• ribisome not membrane-bound → made of rRNA and protein

• flagella are common in prokaryotes

THE BACTERIAL CELL

• gram positive

  • thicker cell wall; more likely to be environmentally stable

  • ex. Clostridium tetani, Streptococcus, Staphylococcus, Bacillus anthracis

gram negative

• thin cell wall; need host protection

• ex. Yersinia pestis, Treponema pallidum, Helicobacter pylori, Bordatella pertussis

THE CELL WALL

• G⁺ = thick (multiple layers) cell wall; G^- = thin (single layer) cell wall

• cell wall - complex semi-rigid structure

  • provides shape

  • external to the plasma membrane

  • provides protection

  • composition: peptidoglycan (nonhuman product)

    • Repeating disaccharides attached to chains of 4 amino acids

PEPTIDOGLYCAN STRUCTURE

• disaccharide - two monosaccharides that alternate to form the carbohydrate backbone

  • N-acetyl-glucosamine (NAG)

  • N-acetyl-muramic acid (NAM)

    • Tetrapeptide chain (4 amino acids) attached to each NAM

    • Amino acids alternate stereoisomers of each other

    • Parallel tetrapeptide chains are generally linked by peptide cross bridges (1 to 5 amino acids) - hold peptidoglycan together!

*variation in length of carbon backbone → size of bacteria

**known target for medication → to stop bacteria from dividing

• LPS will make humans incredibly sick

• pepdydoglycan in between two membranes in G-

GRAM POSITIVE CELL WALL

• up to 25 layers

  • type and strain variatin

• peptide cross bridges will vary

  • 5 L-Glycine in Staphylococcus

GRAM NEGATIVE CELL WALL

• no characteristic peptide cross bridge

  • usually a peptide bond exists between DAP (#3) and Ala (#4) on different tetrapeptide side chains

PERIPLASMIC SPACE ***only in gram negative bacteria

• space between outer surface of plasma membrane and the inner surface of outer membrane

  • 12-15 nm thick

  • peptidoglycan found here

  • gel like consistency due to proteins

    • hydrolytic enzymes

    • binding proteins

    • chemoreceptors

PLASMA MEMBRANE

• Encloses the cytoplasm

• phospholipids & proteins - fluid membrane (bacteria have no cholesterol)

• 2 regions: Hydrophilic, Hydrophobic

• Selective Barrier

integral proteins span the entire membrane → ex. channels (purple)

peripheral proteins are more associated with one side

SELECTIVE PERMEABILITY OF PLASMA MEMBRANE

• small hydrophobic molecules can diffuse through the membrane freely

• 3 systems of membrane transport for hydrophillin and charged molecules

1. simple transport (w membrane spanning proteins)

   • uniporters, antiporters, symporters

a. uniport b. antiport c. couples transport: uniport ad symport

2. group translocation - substance is chemically altered during the transport process

   • ex. phosphotransferase system - transports glucose

     • system consists of 5 proteins - enzyme II C is the only one that spans the membrane

     • energy for this comes from phosphoenol pyruvate (glycolysis)

3. ABC system (ATP Binding Casette) - only in Gram negative

   • substrate binds to the periplasmic binding protein.

   • This binding triggers a conformational change, allowing the complex to interact with the membrane-spanning transporter.

   • The transporter then changes shape, driven by the energy released from ATP hydrolysis, allowing the substrate to be transferred into the cell.

** similar transport mechanism exists in Gram + bacteria.

• the initial binding proteins are anchored directly in the plasma membrane rather than being located in the periplasmic space.

OUTER MEMBRANE of G-

• consists of bilayer of phospholipids, imbedded proteins, Lipopolysaccharide

  • all contribute to prevention of perplasmic enzymes diffusing away

  • porins are specialized channels that allow passage of small molecules and ions

  • LPS tend to make human very sick

LPS components

• O polysaccharide - repeating unit of 4-5 sugar residues

  • varies between differents strains and species of bacteria

• core polysaccharide - connects O poly to Lipid A

  • includes ketodeoxyoctanate (KDO), a sugar that links to other sugars

• Lipi A - achors the entire structure to the outer membrane

  • “endotoxin” - released when cell is lysed during infection

    • induces fever, blood clotting (decreased BP)

      • decreased BP can cause organ failure, shock, death

**positive feedback loop

CYTOPLASMIC INCLUSIONS

• reserve deposits (prokaryotes do not have vesicles)

• 2 main types

  • PHB (poly-β-hydroxybutryric acid) - Lipid like

    • Carbon and energy storage

  • Polyphosphate granules

    • Inorganic phosphate - DNA/RNA synthesis

The glycocalyx (sugar structure) is a sticky layer composed of polysaccharides, polypeptides, or both. It can take two primary forms: capsules and slime layers.

CAPSULES AND SLIME LAYER

• found outside of the cell wall in some bacteria

• Capsule - typically thick; firmly attached to cell wall

• Slime Layer - typically thin; loosely attached to cell wall

Sugar “shell” protects from immune system detection

• Cells in immune system is looking for proteins – so immune system unable to detect bc these shells cover proteins

what do they do??

• Capsules - contribute to bacterial virulence

  • Protect disease causing bacteria from phagocytosis by leukocytes

• Capsules & Slime Layers - attachment to surfaces

  • S. aureus (capsule) and S. epidermidis (slime layer) - high affinity for titanium

    • Titanium in joint replacements → staff can make its way into the bone cells and live inside the bones → antibiotics cant penetrate solid matrix so must cut the bone to reach

FLAGELLA

• long filamentous appendages

• G+ or G- motile bacteria - propel organism

• 3 distinct arrangements

• completely covered by flagella

polar

• could have one at each end

lophotrichous

• multiple coming from one or both ends

FLAGELLAR STRUCTURE

• Filament - long, outermost region with subunits of flagellin protein

  • Several chains intertwined around a hollow core

• Hook - wider region at base

  • Attaches the filament to the motor

• Motor - a biological motor - central rod through a system of rings (L ring, P ring, MS ring, C ring)

  • Embedded in outer membrane (L ring), peptidoglycan (P ring), plasma membrane (MS ring) & cytoplasm (C ring)

All 4 rings found in G - (bc it has an outer membrane)

** so not all rings in G+ ???

Flagellin is prokaryotic; tubulin is eukaroytic

**Immune response from receptors that detect flagelli → not a natural human protein

EUKARYOTIC FLAGELLA

AXIAL FILAMENTS

• consist of flagella that are located within the periplasmic space

• could run the entire length of the cell

**spirochete allows for corkscrew motion (of the entire bacterium)

BIOLOGICAL MOTOR

• the flagellar motor consists of mot and fli proteins → essential for both the motor function and the switching mechanism that controls direction.

• proton motor force - As protons flow down their concentration gradient through the motor proteins, they provide the energy needed for the motor to function.

  • When protons(+) move through rings (+), they create a repulsive force → leads to the rotation of the motor, which in turn causes the flagellum to spin.

    • CW motion - tumbles (random series of movement)

    • CCW - runs (coordinated movement toward stimulus)

CELL TAXIS

• Cells may move towards or away from a stimulus

  • Favorable stimulus - increased runs

  • Unfavorable stimulus - increased tumbles

• Chemotaxis & Phototaxis

  • Chemo/photoreceptors detect stimuli - interact with other proteins to affect flagellar motion

PILI

• Hair like appendages on some bacterial cells → Shorter and thinner than flagella

• helical structure of protein pilin chains around a central core

• With few exceptions, generally G- only

• functions

  • Attachment - facilitate attachment to surfaces, including host tissues.

  • ex. gonorhea attaches to mucus - changes expression on surface of cell

• Conjugation

  • Certain pili, known as sex pili, are involved in conjugation, a form of horizontal gene transfer.

  • donor bacterium forms a conjugation bridge through its pili to connect with a recipient bacterium, facilitating the transfer of genetic material (such as plasmids). This is essential for the spread of antibiotic resistance and other traits among bacterial populations.

ENDOSPORES (structures formed by certain bacteria)

• found primarily in G+and very few G-

• 2 genera that cause human disease - Clostridium & Bacillus

• Highly durable bodies with thick walls

• Formed when essential nutrients or water is lacking (not ideal circumstances)

• Not reproductive

• Survive extremes and germinate when conditions are favorable (could be an animal cell) → tranforms to the vegetative (active) cell

• can remain dormant for many years

**Anthrax Island

endospores can survive extreme heat, lack of water, toxic chemicals, radiation

**can be elimated with extreme heat under pressure (autoclave), chrorine dioxide gas

KEY COMPONENTS OF ENDOSPORES

• Dipicolinic Acid - characteristic of endospores

  • Found in core of endospores

  • Complexes with Ca2+ to give gel like structure → decreases water content

• Small Acid Soluble Spore Proteins (SASPs) - bind tightly to DNA in core, protect DNA from desiccation, radiation and heat

  • also serves as a Carbon & energy source when endospore germinates and transitions to vegetative cell

ENDOSPORE FORMATION (6-8 hours) (in stressful environment)

• DNA replicated

• Forespore formed

• Plasma membrane engulfs forespore

• Cortex of peptidoglycan formed

• Spore Coat formed

• Endospore released from vegetative cell