Microbiology Essentials: Bacteria, Archaea, and Eukarya

Key Differences Between Bacteria, Archaea, and Eukarya

Cellular Structure

Bacteria: Lack a nuclear membrane, rarely have membrane-bounded organelles, cell walls are constructed of peptidoglycan, possess a single RNA polymerase, and contain histone-like proteins.

Archaea: Lack a nuclear membrane, rarely have membrane-bounded organelles, cell walls are constructed of various materials, possess RNA polymerase II, and contain histones.

Eukarya: Contain a nuclear membrane, possess membrane-bound organelles, some have cell walls, possess RNA polymerase 1, 2, and 3, and contain histones.

Koch's Postulates and Bacterial Shapes

Koch: Discovered Bacillus anthracis (anthrax) and Mycobacterium tuberculosis. His postulates are:

1. Microbes are present in all cases of the disease.
2. Microbes can be isolated from the diseased host and grown in pure culture.
3. The isolated microbe can cause the same disease when introduced into a healthy host.
4. The same microbe can be re-isolated from the newly infected host.

Bacterial Shapes:

• Coccus: Spherical
• Bacillus: Rod-shaped
• Vibrio: Comma-shaped
• Hyphae: Irregular branching filaments

Most bacteria are 0.2, 0.5-5 micrometers in size.

Cytoplasm of Bacterial Cells

The cytoplasm of bacterial cells contains:

• Nucleoid
• Chromosome
• Synthesizing enzymes
• Ribosomes
• Plasmids
• Inclusion bodies
• Magnetosomes
• Gas vesicles
• Cytoskeletal structures

ParM directs plasmid movement, MreB provides structure, and FtsZ is involved in cell division.

Cell Wall and Transport Mechanisms

The bacterial cell wall is made of peptidoglycan, a polysaccharide composed of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) connected by glycosidic bonds.

• Facilitated Diffusion: Movement from high to low concentration.
• Co-transporters: Move substances against their concentration gradient; symporters move substances in the same direction, while antiporters move them in opposite directions.
• ABC Transporters: Utilize ATP hydrolysis for transport.

Gram-Positive vs. Gram-Negative Bacteria

Gram-Positive: Thick outer layer of peptidoglycan, very narrow periplasmic space, and teichoic acids (which shrink large pores and lock in the crystal violet stain).

Gram-Negative: Very thin layer of peptidoglycan, an outer membrane composed of lipopolysaccharide (LPS), which can trigger an inflammatory response. The staining process strips away outer membrane lipids, making them more loose.

Motility and Adherence

Flagella Structure: Composed of a filament, hook, and basal body.

Gliding Motility: Non-flagellated bacteria slide smoothly.

Twitching Motility: Surface-dependent movement, relying on pili for a slow, jerky process.

Adherence: Pili are used to allow bacteria to attach to surfaces.

Plasma Membrane Composition

Bacteria: Phospholipid bilayer, no sterols, fatty acid chains, ester linkage, high protein content.

Archaea: Bilayer or monolayer, ether linkage, no sterols, high protein content.

Eukarya: Similar to bacteria but contain sterols and have a lower abundance of proteins.

Fungi: Cell walls made of chitin.

Algae: Cell walls made of cellulose.

Flagellum Structure and Function

Bacteria: Made of flagellin subunits, assembled at the tip, hollow, rigid, not covered by the plasma membrane (PM), energy derived from the proton motive force (PMF).

Eukarya: Made of microtubules (9+2 arrangement), assembled at the tip, flexible, covered by the PM, energy derived from ATP, whip-like motion.

Archaea: Multiple types of subunits, assembled at the base, solid, rigid, not covered by the PM.

Cell Wall Composition

Bacterial Cell Wall: Made of peptidoglycan; NAM and NAG form the polysaccharide backbone, connected by beta-1,4 glycosidic bonds. NAM has a peptide chain with four amino acids, including D-amino acids.

Archaeal Cell Wall: Made of pseudomurein; NAG and N-acetyltalosaminuronic acid (NAT) form the polysaccharide backbone, connected by beta-1,3 glycosidic bonds. NAT has a peptide chain with four amino acids, all of which are L-amino acids.

Endosymbiotic Theory

The endosymbiotic theory proposes that mitochondria and chloroplasts developed from symbiotic bacteria, specifically alpha-proteobacteria and cyanobacteria, respectively.

Examples of Microbes

Saccharomyces cerevisiae: Heterotrophic, with cell walls made of chitin; used in the production of bread, beer, and wine.

Giardia lamblia: A protozoan.

Chlamydomonas: Some algae are single-celled, but many are multicellular. All are photosynthetic with cellulose cell walls. Chlamydomonas has a two-flagella form, making it useful for studying eukaryal flagella biogenesis and function.

Methanogens: Reduce carbon dioxide to methane.

Halophiles: Salt-loving; maintain intracellular potassium (K+) to offset the very high extracellular sodium (Na+).