Bacteria - Structure and Function

Bacteria Overview

Unicellular, ubiquitous, mostly beneficial in normal microbiota

Most bacteria are not…

Pathogenic (causing disease)

Are bacterial small or large?

Small (they are smaller than eukaryotic cells)

Bacterial Morphology

• Small

• Shapes and arrangements

Surface Area-to-Volume

Smaller size = higher ratio

Why is higher surface area-to-volume important?

More nutrients and waste can diffuse out of bacteria

What does bacterial shape influence?

• Obtaining nutrients

• Attachment

• Movement

• Invasion

Monomorphic

Bacteria that have one shape

Pleomorphic

Bacteria that have multiple shapes dependent on environment

Cocci

Ball-shape

Bacilli

Rod-shaped

Coccobacillus

Intermediate between cocci and bacilli

Strep

Chain

Staph

Cluster of cocci

Streptobacilli

Strips of bacilli

Streptococci

Strips of cocci

Palisades

Cluster of bacilli

Staphylococci

Clusters of cocci

Vibio

Comma-shaped

Spiralla

Spiral-shaped

Spirochetes

Corkscrew-shaped

Filamentous

Long, thread-like

Internal Bacterial Structures

• Cytoplasm

• Nucleoid

• Ribosomes

• Inclusion bodies

• Cytoskeleton

Which internal structure do bacteria lack?

Membrane-bound organellessuch as a nucleus and mitochondria.

Cytoplasm

Aqueous solution for all cellular functions/structures

Nucleoid

• Region of bacterial genome (1 circular chromosome)

  • No nuclear envelope (nucleus)

Ribosome

• RNA and Protein (70S)

  • vs. 80S in Eukarya

• Site of translation (RNA → Protein)

• Densely dispersed

Inclusion Bodies

• Aggregates for storage of nutrients

• Viral replication

• Insoluble, recombinant proteins

Cytoskeleton

Structure and support body

Plasma Membrane

Boundary separating intra vs. extracellular spaces

Function of Plasma Membrane

• Response to environment (fluid, gas, waste levels)

• ATP generation, electron transport, transport

  • Passive or active diffusion

Structure of Plasma Membrane

• Fluid Mosaic lipid bilayer

  • Allows protein movement trans or surface

  • Selective permeability

Selective Permeability

• Small molecules, gases, and water allowed

• Charged, bound, large molecules require help

How does temperature affect the plasma membrane?

Colder temp = harder to move around

Passive Transport

• No energy

• Concentration gradient: High → Low

  • Osmosis and solute diffusion

Osmosis

• Movement of water to equalize solute

  • Water moves to higher solute concentration

  • Isotonic, hypertonic, hypotonic

Isotonic

Equal solute inside and outside cell = no water gain/loss

Hypertonic

Higher solute outside cell = water out of cell = cell shrinkage

Hypotonic

Higher solute inside cell = water into cell = cell lysis/swelling

Simple Diffusion

Molecules pass freely through membrane (high → low)

Facilitated Diffusion

Molecules require transporter (High → Low)

Active Transport

• Movement against gradient (Low → High)

  • Energy required to open/close protein pump

• Faster

External Bacterial Structures

• Cell wall

• Glycocalyx

• Pili

• Flagella

• Capsule

• Slime Layer

Peptidoglycan

Protein and sugar mesh cross-linked with amino acids

How are bacteria classified based on cell wall?

Gram Stain

Graim Stain

Differentiates bacteria as Gram+, Gram-, Gram Indeterminant

Which bacteria are gram-indeterminant?

Spirochetes, Mycobacteria, Mycoplasmas

Gram-negative Bacteria

• Contain lipid-rich outer-membrane (2nd)

• Thin layer of peptidoglycan

• Periplasmic Space - OM & PG

• Stain pink

Outer Membrane

• Contain LPS (Lipid A/Endotoxin)

  • Causes inflammation if lysed

• Outer-membrane proteins (porins)

Porins

• Gram-negative OMPs from the outer membrane

• Form channels for transport that excludes large/harmful molecules

Which gram-bacteria is more resistant?

Gram negative is more resistant

Gram-positive Bacteria

• Thick PG layer

  • High sensitivity to penicillin and lysozyme

• Contains teichoic acids

• Stains purple

Teichoic Acids

• Found in Gram-positive Bacteria

• Glycopolymers attached to PG or membrane to stabilize, maintain, and divide

Mycoplasma

• Lack a cell wall or PG

  • Resistant to PG-targeting therapy

• Pleomorphic

Mycobacteria

• Thick, mycolic acid-rich CW

  • Waxy, lipid coat

• Identified with acid-fast stain (red)

Glycocalyx

• Carbohydrate mucoid layer

• Adherence, protection

  • Slime or capsule

Capsule

• Organized polysaccharide/protein coat acting as a virulence factor

  • Groups species into serogroups

• Used for vaccines (targeting capsules)

Pili

• Hair-like projections with adhesin tips

• Common in Gram(-)

• Virulence factor for adherence

• Conjugation (F Pili) and twitching motility

Flagella Overview

• Long rotary propellors in bacilli

• Infection and invasion

• Taxis to or away from chemicals, light, osmotic pressure, oxygen, temperature

Extracellular flagella

• Monotricious (1)

• Amphitricious (1 on each end)

• Lophotricious (tuft on 1 end)

• Peritrichous (all over)

Intracellular flagella (endo)

Periplasmic space of spirochetes for corkscrew motion

Endospores

• Made by gram-positive bacilli

• Allow for dormant state for resistance

  • Become vegetative as conditions improve

Steps to Infection

1. Enter

2. Adhere

3. Invade and obtain nutrient

4. Replicate and defend

5. Transmit to new host

Adherence

• Colonization of host tissues and multiply

• Depends on adhesin binding to receptors

  • Surface or pili (or both) adhesins

  • Tropism - specificity for adhesin binding

Which adhesin would attach first?

Pili pull to host, surface create stronger adhesion

Tropism

Preference for a host, cell, or tissue based on host factors

Planktonic

Unicellular, free-floating phase

Sessile

Multicellular, biofilm phase

Biofilms

Microbial communities encased in EPS with multiple protective layers

Which layers of biofilm are most protected?

The inner layers

Exopolymeric Matrix (EPS)

Protective, slimy layer of biofilm allowing for nutrient sharing and protection

Quorum Sensing

Cell-to-cell communication system of biofilm for coordination and gene expression (multicellular-like)

Common Sources of Biofilms

Implanted devices, inert objects, natural sources, etc.

Why are biofilms resistant to immune system and antibiotics?

Phagocytes and antibiotics can only reach top layer and there is no active metabolism

Biofilm and Sequelae

Linked to recurring, chronic infections due to release of planktonic bacteria after phagocytosis

Invasion

Use of invasins to invade, obtain nutrients, and damage host cells

Do pathogens always invade?

No, they may stay on the surface of hosts

Invasins

• Virulence factors assisting invasion locally/immediately

  • Uptake induction

  • Motility

  • Toxin release

  • Protection

  • Break down of tissues (collagenases, etc.)

Iron-binding Proteins

Bind to host proteins to internalize iron inside of host protein

Siderophores

High affinity for iron to take to bacteria

Exoenzymes

• Break down nutrients or spread

  • Lipase

  • Protease

  • Nuclease

  • Hyaluronidase

Cytopathic Effects

• Direct damage or induction of autoimmune byproducts

  • Chronic Inflammation

  • Disruption by taking nutrients, exoenzymes, toxins

What are a major virulence factor?

Exotoxins…

Exotoxins

• Proteins produced by both gram bacteria in low concentrations to aid, disrupt, alter, aid

• Potent

• Immunogenic - anti-toxin antibodies, toxoids

Endotoxins

Lipid A of LPS in Gram-Negative ONLY, released from lysis

Which gram-bacteria produces Lipid A endotoxin?

Gram-negative produce Lipid A

Which toxins are responsible for symptoms?

Exotoxins

Toxoid

Toxin deactivated by heat or chemical for vaccines

Lipid A Endotoxin

• Gram-negative ONLY

• Release via lysis results in endotoxemia, systemic inflammation, sepsis

• Lipid-based, no vaccine, wrong antibody can cause sepsis

Septic Shock

Life-threatening hypotension and multi-organ failure

Which toxin is more toxic?

Exotoxins because they require lower concentrations (potent)

Evasion of defenses

Escape host defenses by either hiding or undermining immunity

Hiding

• Antigen Masking

• Antigen Mimicry

• Antigen Variation

• Intracellular

• Latency

Antigen Masking

Covered in host factors to hide pathogen itself

Antigen Mimicry

Antigen production similar to host so host doesn’t attack ‘itself’

Antigen Variation

Once recognized, bacteria changes its antigens

Intracellular Pathogen

Hiding by living inside host

Latency

Remaining dormant in biofilm or endospore, protects from drugs and immunity

Undermining

• Immune suppression

• Phagocytosis interference

Immune Suppression

• Kill immune cells with cytolysins, exoenzymes

• Break down antibodies with proteases

• Cytokine signal interference