Prokaryotes: Bacteria and Disease

• Prokaryotes are microorganisms lacking a nucleus and other membrane-bound organelles.

Lecture Outline

• The lectures include the topics of bacteria and disease, prokaryotic cell anatomy, structures and functionality, Gram staining, and diversity in prokaryotes.
• Assumed prior knowledge for the lectures includes major metabolic, replication, and biosynthetic pathways.

Bacteria and Disease

• Bacteria can cause disease and death in humans.
• Bacterial infections are treatable with antibiotics.
• Bacterial infections are commonly seen with other diseases and treatments such as AIDS, immunosuppressants, and cancer chemotherapy.
• Hospitals may be a source for bacterial infections such as MRSA.
• An emerging issue is multidrug-resistant bacteria.
• Few bacteria actually cause disease in humans.
• It is important to distinguish between commensal vs pathogenic bacteria.

Pathogenic Bacteria

• To cause disease, bacteria should be able to encounter and gain access to the host.
• Pathogenic bacteria must be able to evade defense mechanisms.
• Pathogens need to colonize and maintain colony.
• Damage or toxicity as a result within the host
• Spread of infection is a key aspect of pathogenicity
• Zoonotic infections are not always the case, such as COVID originating from an animal population.

Prokaryotic Cell Structure

• Key components include the pilus, capsule, cytoplasm, ribosomes, peptidoglycan cell wall, a plasma membrane, nucleoid containing DNA, and a plasmid.

Key Features of a Prokaryote Cell

• Prokaryotic cells are structurally simpler and smaller than eukaryotic cells.
• They lack membrane-bound organelles (nucleus, endoplasmic reticulum, Golgi, mitochondria, etc).
• Possess a single circular DNA genome.
• Plasmids are free in the cytoplasm.
• Enzymes and ribosomes are also free in the cytoplasm.
• Exhibit a rigid, complex cell wall with appendages.
• Highly adaptive to extreme environments (pH, enzymes, viscosity, etc).
• Rigid cell walls helps prevent cell rupture.

Flagella, Fimbriae and Pili

• Flagella are used for motility and also as a sensory organelle (chemicals, temperature).
• Fimbriae and pili are used for cell adhesion.
• Sex Pili are important for movement of DNA between cells by conjugation.
• Tremendous diversity exists in these structures.

Capsule

• Capsules are known as a slime layer, surrounds certain Gram-negative and Gram-positive bacteria.
• It consists of polysaccharides, especially with Bacillus anthracis utilizing D-glutamic acid.
• Capsules aid in cell adherence through a "Slime layer."
• The capsule is anti-phagocytic due to its smooth surface and negative charge.
• It provides protection from neutrophils by being impermeable to lysosome contents.
• It is essential for bacterial success.
• Capsules, not currently targeted by treatments, often uses polysaccharides similar to those in human cells, causing cross-reactivity issues.

Cell Wall

• The cell wall is located outside the cell membrane and is a common antibacterial target.
• It is not unique to bacteria, also occurring in plants, fungi, algae, and archaea.
• It contains peptidoglycans, which are disaccharides + amino acids.
• Functions of the it include: giving structure to the cell and maintaining positive internal osmotic pressure.
• Variation in structure exists, with common differences between Gram +ve and Gram –ve bacteria.

Peptidoglycan

• Peptidoglycan, not found in eukaryotes, is made of NAG and NAM and consists of a carbohydrate backbone.
• The carbohydrate backbones are cross-linked by peptide bridges with tetrapeptide sidechains.

Gram +ve Peptidoglycan

• If stain is positive in gram stain, the peptidoglycan will form amino acid cross bridges between lysine and d-alanine.

Gram –ve Peptidoglycan

• Have peptidoglycan much narrower
• Gram-negative's, do not have the glycine cross bridge.

Gram +ve Cell Wall

• Gram-positive cell walls contains surface proteins, teichoic acid, and lipoteichoic acid.

Gram –ve Cell Wall

• Gram-negative cell wall includes an outer membrane, lipoprotein, peptidoglycan, a cytoplasmic membrane, and porin proteins.

Periplasm

• The periplasm is an aqueous gel-like region between two membranes of Gram-negative bacteria.
• The region contains enzymes associated with nutrient acquisition and transport.
• It contains defense enzymes used to resist antibiotics, such as β-lactamase.

Outer Membrane

• Porins allow passive diffusion of molecules into periplasmic space.
• Braun's lipoprotein anchors the outer membrane..
• Lipopolysaccharides or (LPS) gives a negative charge.
• LPS is very antigenic and induces autoimmune reactions.

Lipopolysaccharide LPS

• Lipid A is embedded in membrane.
• The Core polysaccharide contains KDO.
• Lipopolysaccharide is antigenic varies between bacteria.
• It has a net negative charge over cell surface of Gram -ve bacteria.
• It may allow Gram -ve bacteria to evade immune response by triggers.

Classification - The Gram Stain

• Gram +ve and Gram -ve bacteria will both take in the stain.
• When iodine is added, insoluble crystals will form.
• The gram -ve stain will escape the now transparent stain, which will be stainined with safranin.
• The use of safranin stain will stain both types of bacteria.

Plasma (Cytoplasmic) Membrane

• This constitutes a phospholipid bilayer with embedded proteins.
• The membrane contains cytoplasm and regulates transport.
• Three types of active transport in bacteria include: PMF-driven transport, ABC transport (ATP-Binding Cassette), and Group translocation e.g. glucose 6-P.
• It is the site of assembly of cell wall components.

Prokaryotic DNA - Nucleoid

• Prokaryotic DNA is circular, supercoiled, and double stranded.
• Generally, there is only one chromosome in the DNA.
• It is not associated with histone proteins.
• The E. coli chromosome contains 4.6 x 106 base pairs.

Prokaryote DNA - Plasmids

• Plasmids are small loops of extrachromosomal DNA.
• They are not essential for survival a priori.
• Plasmids may contain genes that are expressed in response to certain situations.
• Plasmids may be used to transfer genes between cells, e.g. resistance to antibiotics.
• Plasmids can be incorporated into the chromosome .
• A single DNA strand is transferred between cells by conjugation using ‘rolling circle' replication.

Ribosomes and Translation

• Ribosomes are free in cytoplasm and composed of two large subunits.
• A small 30S subunit binds to mRNA.
• A large 50S subunit binds, completing the 70S ribosome.

Representative Bacteria

• Streptococcus pneumoniae are Gram +ve bacterium and encapsulated coccus. It causes respiratory infections.
• Escherichia coli are Gram -ve with flagella and fimbriae. It causes enteric and urinary and nosocomal infections.
• Escherichia coli 0157:H7 are Gram -ve rods that are anaerobic and a causal agent of haemorragic disease due to toxin secretion.
• Clostridium difficile are Gram +ve rods that are anaerobic. It forms part of the normal gut microflora. Causes inflammation (colitis) and abdominal disease.
• Helicobacter pylori are Gram -ve spirals that moves using flagella and is associated with chronic superficial gastritis.
• Mycobacterium tuberculosis are Gram +ve rods able to reside inside tissues due to a hydrophobic cell wall.
• Staphylococcus aureus are Gram +ve cocci, often MRSA, causes a variety of infections and diseases.

Summary

• Bacteria are major causes of infectious disease in humans.
• Bacterial structure contains both similarities and differences (exploitable) to eukaryotic cellular structure.
• Significant differences exists between Gram +ve and Gram -ve bacterial cell walls.