O'Donnell Bacterial Structure

Eukaryotic cells

Numerous membrane-bound organelles

Relatively simple plasma membrane structure

Prokaryotic cells

Few, if any, membrane-bound organelles

• Still accomplish most of the same functions as eukaryotic cells

Extremely complex membrane/cell wall structure

Pili

Hair-like structures on bacterial cells that allow them to attach to our cells

Nucleoid

Location of chromosome in a bacterial cell

Not membrane-bound

• Irregularly shaped, no true nucleus

Characteristics of bacterial chromosomes

Often circular, sometimes linear

Usually 1 per cell

• Mutations are expressed immediately

• Occasionally more if reproducing rapidly

Often central in the cell, but attached at points to the plasma membrane

Tightly coiled

How do bacteria access the genes they need?

Nucleoid projections

• DNA is uncoiled and streched (Necessary to access all genes)

• Seen in growing cells especially, probably to gain access to genes that are being transcribed for mRNA

DNA gyrase

Introduces negative supercoils with double-strand nicks → relaxes positive supercoils

Topoisomerase IV

Relaxes supercoils and separates chromosomes during replication

What happens if you inhibit DNA gyrase in a bacteria?

Bacteria can’t access genes it needs to express → kills bacteria

What happens if you inhibit Topoisomerase IV?

Bacteria can’t separate chromosome → Cannot divide

Plasmid

Small, closed, circular DNA molecule in bacteria that exists and replicates independently of chromosome

Not required for growth and reproduction (found on chromosome), may carry genes that confer selective advantage

Can be laterally transferred (Passed to neighboring bacteria)

Characteristics of bacterial cytosol

No mitochondria or ER (membrane-bound organelles)

• Oxidative phosphorluation, DNA transcription, and translation all occur near the plasma membrane

Inclusion bodies

• Storage for organic and inorganic materials

Proteins highly organized

• Cytoskeleton-like organization/function

Granular appearance

How is the location of a bacterial ribosome related to its function?

Ribosomes associated with the plasma membrane export proteins, while ribosomes in the cytosol make cellular proteins

What are ribosomes composed of?

Complexes of protein and rRNA (ribosomal RNA)

Two subunits

• 50S Subunit

  • 5S rRNA + 23S rRNA + proteins

• 30S Subunit

  • 16S rRNA + proteins

Why can bacterial ribosomes effective targets for antibiotics?

Prokaryotic ribosomes differ enough from eukaryotic ribosomes to be targeted

Main biochemical difference between prokaryotic and eukaryotic plasma membranes

Prokaryotic membranes use hopanoids

• Can target these with antibiotics

Eukaryotic membranes use cholesterol

Bacterial Cell Wall

Rigid structure that surrounds the plasma membrane and protects bacteria in many different environments

Consists of peptidoglycan

• Can target this with antibiotics

Functions of the bacterial cell wall

Provides shape

Protects the cell from ossmotic lysis

May contribute to pathogenicity

May protect the bacteria from toxic substances

Peptidoglycan structure

Polysaccharide formed from peptidoglycan subunits

Backbone of alternating sugars N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM)

Muramic residues covalently crosslink with each other

• Crosslinks between peptide side chains provide resistance and rigidity

• Can target these crosslinks with antibiotics

Structural differences in cell wall for gram positive and gram negative bacteria

Gram positive

• Thick peptidoglycan layer

Gram negative

• Thin peptidoglycan layer

• Outer membrane

Composition of gram positive cell wall

Composed primarily of peptidoglycan, contain large amounts of teichoic acids

Some have a layer of proteins on surface of peptidoglycan

Composition of gram negative cell wall

Thin layer of peptidoglycan surrounded by an outer membrane, which includes:

• lipids, lipoproteins, and lipopolysaccharide (LPS)

• No teichoic acids

Contain porins, which allow nutrients and water-soluble antibiotics to pass

Porin

Water-soluble channel

Lipopolysaccharide (LPS)

Bacterial structure embedded in the outer membrane of gram negative bacteria that is involved in resisting the immune response

Three parts:

• Lipid A

• Core polysaccharide

• O side chain (O antigen)

Functions of LPS O Antigen

Protection from host defenses

• Blocks lysozyme activity

  • Only enzyme we have that degrades peptidoglycan

• Inteferes with antibody response

Adheres to host tissues

How does O Antigen interfere with antibody response?

It has lots of variability

• Hard for our immune system to recognize/learn this

Function of LPS Core Polysaccharide

Contributes to negative charge on cell surface

• Harder for immune cells to phagocytose

Functions of LPS Lipid A

Attaches LPS to membrane

Helps stabilize outer membrane structure

Can act as an exotoxin (Toxin that is part of the bacteria’s structure)

Released when bacterial cells die

Source of fram negative infection symptoms

• Flu-like symptoms, sore, fever, etc.

Periplasmic Space

Gap between the plasma membrane and the cell wall in gram positive bacteria or the outer membrane in gram negative bacteria

Filled with periplasm

• Periplasmic enzymes

Functions of periplasmic enzymes

Nutrient acquisition

• Bind sugars and amino acids

Electron transport

Peptidoglycan synthesis

Modification of toxic compounds

Inactivation of antibiotics

How does the periplasmic space help bacteria inactivate antibiotics?

Bacteria release enzymes that degrade antibiotics and collect them in the periplasmic space

Cocci bacteria

Spheres

Diplococci bacteria

Pairs of spheres

Streptococci bacteria

Chains of spheres

Staphylococci bacteria

Grape-like clusters of spheres

Bacilli bacteria

Rods

Coccobacilli bacteria

Very short rods

Vibrios bacteria

Curved rods

Shape of cholera

Vibrios

Mycelium

Network of long, multinucleate filaments

Spirilla bacteria

Rigid helices

Spirochetes bacteria

Flexible helices

Pleomorphic bacteria

Organisms that are variable in shape

Goals of differential stains

Divide microorganisms into groups based on their staining properties

See the shape/morphology of the microorganisms

Most common microorganism staining method

Gram stain

What is the acid-fast stain used for?

Tuberculosis

How does gram staining divide bacteria?

Gram positive or gram negative based on differences in cell wall structure

Process of gram staining

Primary stain

• Uses crystal violet, which is a very small molecule

  • Enters ALL cells in the sample, making them purple

Mordant

• Uses iodine

  • Makes big chunks of crystal violet to set the stain in gram positive bacteria

Decolorization

• Wash with 95% ethanol or acetone

  • Dehydrates and disrupts the gram negative cell wall, washing the crystal violet away

Counterstain

• Uses safranin

  • Anything with no color will pick up pink

What color are gram positive bacteria after a gram stain?

Purple

What color are gram negative bacteria after a gram stain?

Pink

How do the results of gram stains and genetic sequencing differ?

DNA sequencing can give you a specific bacteria and strain, while gram stains and other biochemical tests only narrow down the possibilities

How can genetic sequencing identify a bacteria?

Can sequence the gene for the 16S ribosomal RNA, part of the 30S ribosome

• Contains hypervariable regions, which are regions of RNA sequences that vary between strains

• Allows for precise determination of the species of bacteria

Sequencing of the whole DNA genome is also possible and is becoming increasingly affordable