Bacterial Morphology and Cell Sizes

Bacterial Morphology

• Bacteria come in various shapes: spherical (cocci), rod-shaped (bacilli), comma-shaped (vibrios), spiral (spirilla), and pleiomorphic (variable shapes).
• Morphology is not always a reliable indicator of a bacterium's physiology, ecology, or phylogeny.
• Morphology can be influenced by selective forces, such as nutrient uptake efficiency (surface-to-volume ratio) and motility.
• Some bacteria form multicellular organizations like hyphae (branching filaments), mycelia (tufts of hyphae), and trichomes (smooth, unbranched chains).

Cell Sizes

• Prokaryotes typically range from 0.2 μm to over 700 μm in length/diameter.
• Most rod-shaped bacteria are between 0.5 μm and 4.0 μm wide and 1-15 μm long.
• Eukaryotic cells are larger, ranging from 10 μm to over 200 μm.
• Minimum size is limited by the minimum space needed for the genome, proteins, and ribosomes.
• There are exceptions to the general size of bacterial cells, such as Thiomargarita namibiensis (up to 700 μm in diameter) and Epulopiscium fishelsoni (200-700 μm long).

Advantages of Being Small

• A higher surface-to-volume ratio allows for faster nutrient/waste exchange per unit volume.
• This supports a higher metabolic rate and faster growth rate, leading to faster evolution.

Limits on Small Size

• Size reduction is limited by the minimum complement of cellular structures required for survival.
• Diameters below 0.15 μm are unlikely.
• "Very small" cells are common in open marine environments, typically ranging from 0.2 μm to 0.4 μm.

Bacterial Cytoplasm

• The largest area within the cytoplasm is the nucleoid region, which houses the chromosome(s) and DNA replication machinery.
• The nucleoid is not enclosed by a membrane and lacks histone proteins.
• DNA in the nucleoid is compressed by various mechanisms, including the use of cations to shield negative charges, binding of small, positively charged proteins, and DNA supercoiling.
• The remainder of the cytoplasm contains macromolecules like tRNA, rRNA, mRNA, proteins, and ribosomes.
• Inclusion bodies and microcompartments are also present, including:
  • Sulfur globules (sulfur storage for energy)
  • Polyhydroxybutyrate granules (carbon storage)
  • Gas vesicles (buoyancy control)
  • Carboxysomes (location of carbon fixation reactions)
  • Magnetosomes (organelles for direction finding).

Bacterial Cytoskeleton

• Bacterial cytoskeleton is composed of a series of internal proteins that help maintain cellular structure.
• Some cytoskeletal proteins are involved in cell wall synthesis during cell division, such as MreB (homolog of actin) and FtsZ (homolog of tubulin).
• Other cytoskeletal proteins are involved in moving internal components, like plasmids.

Cell Envelope

• Encompasses all layers surrounding the bacterial cytoplasm: cell membrane, cell wall, and outer membrane (if present).

Cytoplasmic Membrane

• All cells have a plasma membrane that separates the cell's interior from the environment.

• It plays crucial roles in:

  • Capturing energy (electron transport chains, proton motive force)
  • Holding sensory systems (detecting environmental changes)
  • Acting as a permeability barrier (but not structural)

• The plasma membrane is typically a phospholipid bilayer with embedded proteins.

• It has a hydrophobic core and hydrophilic surfaces interacting with the external environment or the cytoplasm.

• Variation in fatty acid groups attached to a glycerol backbone creates chemical differences, connected by ester linkages.

• It may contain sterol-like molecules called hopanoids, which contribute to stability across temperature ranges.

Transport Across the Plasma Membrane

• Small molecules like O2 and CO2 can readily diffuse across the membrane.
• Water is helped across by aquaporin protein channels (osmosis).
• Facilitated diffusion and co-transport: protein channels move particles down a concentration gradient, requiring no ATP energy.
• Co-transport is a form of active transport, leveraging energy from a concentration gradient.
• Active transport example (ABC transporters): ATP-Binding Cassette protein transporters move particles against a concentration gradient, requiring ATP energy.

Protein Secretion

• Protein secretion involves shipping proteins outside the cell using ATP energy.
• Examples of secreted proteins include toxins, siderophores, and enzymes for breaking down substrates.

Cell Wall

• Provides shape and protection from osmotic lysis and mechanical forces.
• Composed of a matrix of crosslinked strands of peptidoglycan subunits (NAM, NAG, and a small peptide chain).

Peptidoglycan Structure

• Peptidoglycan is composed of N-acetylmuramic acid (NAM), N-acetylglucosamine (NAG), and a small peptide chain.
• The structure of the peptides and crosslinks varies between species, leading to the distinction between Gram-negative and Gram-positive bacteria.
• Some amino acids found in the peptidoglycan are D-forms, which are stereoisomers (mirror images) of the L-forms usually found in biological proteins.
• Cell wall formation involves transglycosylation and transpeptidation.

Cell Wall Degradation

• Lysozyme and lysostaphin are enzymes that can degrade the cell wall.
• Without a cell wall, bacteria cannot resist osmotic pressure changes.

The Bacterial Cell Surface

• The cell surface includes flagella, pili, capsules, surface arrays, and stalks.

Flagella

• Spiral, hollow, rigid filaments extending from the cell surface, with variations in location and number across species.
• Composed of three main parts:
  • Filament (multiple flagellin proteins, 5-10 μm long)
  • Hook protein portion (connects the filament to the basal body)
  • Basal body (disk-like structure that rotates the filament like a propeller).
• Energy for spinning flagella is derived from the proton motive force (PMF).
• Flagella generate runs and tumbles through spinning, a process that is energetically expensive (requiring ~1000 H+ translocated per rotation).

Chemotaxis

• involves chemoreceptor proteins sensing changes in concentrations of attractants or repellents, leading to directed movement.

Internal Flagella

• Some spirochetes have flagella in the periplasm that rotate the entire cell body like a corkscrew.

Nonflagellar Motility

• Gliding motility: smooth sliding over a surface, not fully understood (e.g., Myxobacteria, Cyanobacteria).
• Twitching motility: slow, jerky process using pili to extend, attach, and pull along a surface (e.g., Pseudomonas aeruginosa).
• Act-dependent motility: polymerization of actin in host cells propels bacteria into adjacent cells (e.g., Shigella dysenteriae, Listeria monocytogenes).

Adherence Molecules

• Allow cells to stick to surfaces.
• Pili (fibers of pilin protein), with additional proteins on their tips for sticking.
• Sex pili are different structures involved in conjugation (transferring plasmids between cells).
• Adhesive pili (fimbriae) also contribute to adherence.

Stalk

• An extension of the cell envelope tipped by a "holdfast" of polysaccharides.
• Provides extra surface area for nutrient absorption (e.g., Caulobacter, Hyphomonas).

Capsules

• Thick layer of polysaccharides surrounding some cells (both G+ and G-), providing adhesion, defense against host immunity, and protection from desiccation.
• Can help form biofilms, enhancing survivability in harsh environments.

Surface Arrays (S-layers)

• Crystalline array of interlocking proteins that can provide protection against predation or bacteriophage infection.
• Found in both Gram-positive and Gram-negative cells, and even in Archaea.

Bacterial Taxonomy

• Most microbial species remain uncultured.