HC13: Bacteria

Complexity

The complexity of an organism is based on its amount of base pairs:

1. Prion: 10³base pairs
   • A single protein
2. Virus: 10^4-5base pairs
   • A set of proteins and nucleic acid
   • Uses living cells
   • 0,03-0,3 mm
   • Made visible with an electron microscope
3. Bacterium: 10⁶base pairs
   • A single cell prokaryote
   • 0,1-10 mm
   • Made visible with a light microscope
4. Protozoa: 10⁷base pairs
   • A single cell eukaryote
   • 4-10 mm
   • Made visible with a light microscope
5. Fungi: 10⁸base pairs
   • A multi-cellular eukaryote
   • 4-10 mm
   • Made visible with a light microscope
6. Helminths: 10⁹base pairs
   • A multi-cellular eukaryote
   • Approximately 40 mm
   • Visible with the unaided human eye

Bacteria are the oldest form of life on earth. Bacteria are called prokaryotes because they don't have a nucleus. Millions can fit into the eye of a needle. Most live by themselves, but some in symbiosis. A bacterium doesn't have a nucleus or cell organelles. Bacteria can adapt to their surroundings very well.

Composition

Eukaryotes:

A eukaryotic cell has a:

• Membrane
• Cytoplasm with organelles
• Nucleus with DNA

Eukaryotic pathogens are protozoa, fungi and helminths.

Prokaryotes:

A prokaryotic cell, like a bacterium lacks a nucleus and cells organelles, but does consist of:

• DNA and RNA in the form of a ring
• Plasma membrane
• Mainly ribosomes are present in the cytoplasm
• Most (but not all) contain a cell wall
  • Plays a part in the immune response → the target of antibiotics
    • Protects the bacterium from its environment
  • Contains:
    • Capsule
    • Flagella → for movement
    • Pili → for attachment

The cell wall is very important for bacteria:

• Protects against the environment
• Has antigenic properties → targeted by the immune response
• Has a role in pathogenesis
• Is relevant in diagnostics

Classification of bacteria

Classification based on cell wall composition:

Gram stains can be used to look at the cell walls of bacteria. A bacterium can be gram positive or gram negative:

• No cell wall → mycoplasma
  • Gram staining cannot be used
• Gram positive → streptococcus pyogenes
  • Keep their purple staining
• Gram negative → escherichia coli
  • Lose the purple staining, are counterstained and become pink
• Acid-fast → mycobacterium tuberculosis
  • The cell wall is so tight it cannot be colored with gram stains

Most bacteria are either gram-positive or negative. Whether a bacterium is gram-positive or negative is based on fundamental differences in their cell wall:

• Gram-positive bacteria
  • Peptidoglycan on the outside
    • This forms the actual cell wall
  • Plasma membrane on the inside
    • A lipid bilayer
  • Lipoteichoic and teichoic acids sticking out of the cell wall
  • More resistant to environmental conditions
  • More difficult to kill with detergents
• Gram-negative bacteria
  • Cell wall
    • An outer membrane
      • • Many proteins can do this
    • Periplasmic space: between the outer membrane and the peptidoglycan
    • Peptidoglycan
  • A plasma membrane on the inside
    • A lipid bilayer
  • LPS
  • More susceptible to environmental conditions
  • Easier to kill with lipophilic detergentia
  • Sensitive to:
    • Ethanol or propanol 70-95%
    • Chlorhexidine 0,05-0,5%
    • Ammonium, phenol
    • Chlorine and iodine

Peptidoglycan:

Peptidoglycan is part of the cell-wall of gram-positive bacteria. It also is present in gram-negative bacteria, but as a very thin layer. It is composed of long chains of polysaccharides cross-linked by peptides. Peptidoglycans ensure that the purple staining remains in the bacteria. It is a stimulator of immune responses:

1. TLRs (toll-like receptors) recognize peptidoglycan
2. Granulocytes and macrophages start to release interleukin and many other cytokines
   • IL-1
   • IL-6
   • TNF
3. Inflammation, fever and shock follow

Many antibiotics for gram-positive bacteria work by disrupting the peptidoglycan layer.

Lipo-polysaccharides:

Lipo-polysaccharide (LPS) is present in gram-negative bacteria. They are the gram-negative counterparts of the lipoteichoic and teichoic acids in gram-positive bacteria. It also is a stimulator of immune responses:

1. LPS interacts with Toll-like receptors
2. Granulocytes and macrophages start to release interleukin and many other cytokines
   • IL-1
   • IL-6
   • TNF
3. Inflammation, fever and shock follow

Classification based on shape:

A bacterium can roughly have 2 different shapes:

• Cocci (round)
• Rod (elongated)

The shape can be distinguished further in:

• Curved
• Spirochete

The most common classification system of bacteria is based on shape and gram negative/positive:

• Gram negative
  • Curved → vibrio campylobacter
  • Rods → escherichia
    • For example salmonella
  • Cocci → neisseria
    • Spherical
• Gram-positive
  • Rods
    • Spore forming → clostridium and bacilus
    • Non spore forming
  • Cocci
    • Groups → staphylococci
      • Multiply in all directions
    • Chains → streptococci
      • Multiply in only 1 direction

Classification based on growth:

Bacteria grow by either following an aerobic or an anaerobic route:

• Aerobic: in the presence of oxygen
• Anaerobic route: deep in the ocean or in the human bowel

Some bacteria can survive in both conditions. If anaerobic bacteria are found in a blood sample, this indicates that there's an infection somewhere in the body where there isn't any oxygen.

Aspects can be shown on a culture plate:

• a-hemolytic: partial hemolytic
  • Greenish
• b-hemolytic: fully hemolytic
  • Yellowish
• Non-hemolytic

Hemolytic bacteria can cause lysis in erythrocytes.

Extracellular versus intracellular:

Bacteria can be extracellular or intracellular:

• Extracellular bacteria are phagocytosed
  • Site of infection:
    • Interstitial spaces
    • Blood
    • Lymph
    • Epithelial surfaces
• Intracellular bacteria can survive inside the cell
  • Site of infection:
    • Cytoplasmic
    • Vesicular
  • For example TBC or salmonella

Bacterial spores

Some gram-positive bacteria are capable of sporogenesis, for example the clostridium species. Spores can infect easily and are hard to eradicate. A bacterium becomes a spore when it's exposed to a warm, humid environment. A bacterial spore is a dehydrated structure with a thick wall, in which the bacterium is in a dormant state. It can survive extreme environmental conditions → they are hard to kill. To get rid of a spore, disinfectants and pasteurization won't work.

Bacterial capsules

A bacterial capsule is a polysaccharide surrounding the cell wall, which may be present in both gram positive and gram-negative bacteria. It prevents complement-dependent phagocytosis. Vaccines for capsules induce capsule-specific antibodies. Currently, vaccines are available for:

• Streptococcus pneumoniae
• Neisseria meningitidis
• Haemophilus influenzae B

These diseases can all cause meningitis.

Replication and genetics

A bacterium has 1, usually circular, chromosome. Division takes place by binary fission → the cell is divided in 2:

1. The bacterium replicates its DNA
2. The cell wall is cut
3. There are 2 bacteria

 This is a form of asexual reproduction and takes only 20 minutes.

Plasmids:

Plasmids are extra pieces of bacterial circular DNA. Plasmids are relevant because bacteria can share plasmids. They replicate independently from chromosomal DNA and convey additional properties:

• Antibiotic resistance
• Toxin production
• Metabolism of additional substrates

Once plasmids have integrated into the chromosome, they stay there permanently. If this hasn't happened yet, the plasmid is still in a circle form and it can be exchanged between bacteria.

Change:

The DNA properties of bacteria can change. Because bacteria replicate so fast, errors are made easily:

• Mutation
  • This occurs sporadically
• Exchange of DNA
  • Transformation: uptake of free DNA and integration into a chromosome
  • Transduction: a bacteriophage (bacterial virus-DNA) is absorbed by bacteria-DNA → the bacteria changes
    • A bacterial virus may carry resistant or toxin-producing genes
  • Conjugation: transfer of the plasmid DNA via pili
• Selection pressure
  • Determines the relevancy for humans
  • If there wouldn't be any selection pressure, bacteria would lose their resistant genes
  • Selection pressure is the cause of resistance to antibiotics

Effects

Bacteria have negative and positive effects:

• Negative aspects
  • Infectious diseases
  • Intoxications
  • Unpleasant smells
  • Et cetera
• Positive aspects
  • Food production
  • Probiotics
  • Medication
    • Antibiotics
    • Vaccines
    • Protein drugs
  • Water purification
  • Fuel production
    • Methanol
    • Ethanol
  • Nitrogen fixation
    • N2removal from the environment and conversion to ammonia for use by plants
  • Oxygen production
    • 50% of oxygen is produced by the cyanobacteria
  • Breakdown of dead organic matter