Bacterial Cell Structure and Walls

Bacterial Cell Structure

• Bacterial cells are smaller and simpler than eukaryotic cells, lacking recognizable organelles.
• All cellular activities happen in bacteria, though not in specialized structures.
• Bacteria have broad metabolic capabilities and can grow and divide rapidly in various environments.
• Anton van Leeuwenhoek first observed bacteria in 1676.

Bacterial Cell Walls

• Function: The cell wall maintains the cell shape, protecting against physical and chemical stresses. Metabolic activities are often cell wall-dependent.
• Gram-positive cell walls:
  • Primarily composed of a thick peptidoglycan layer (about 50-90%).
  • Contain teichoic acid layers.
• Gram-negative cell walls:
  • Composed of a thin peptidoglycan layer (only 5-20%).
  • Have an outer membrane, outside the peptidoglycan layer, composed of lipopolysaccharides (LPS), phospholipids and proteins.

Peptidoglycan

• Peptidoglycan is the main component of bacterial cell walls.
• Its structure: alternating sugars (N-acetylglucosamine (NAG) and N-acetyl muramic acid (NAM)) linked by peptide bridges.
• Synthesis is inhibited by some antibiotics like penicillins and cephalosporins. It can be broken down by lysozyme.

Teichoic Acid

• Located in Gram-positive cell walls.
• It's a major surface antigen in Gram-positive bacteria.
• Induces the production of inflammatory mediators such as TNF-α and IL-1.

Outer Membrane (Gram-negative)

• Found only in Gram-negative bacterial cell walls.
• Composed of lipopolysaccharides (LPS), phospholipids, and proteins.
• LPS includes a lipid A region (toxic), a core polysaccharide, and an outer polysaccharide (somatic or O antigen).

Lipoprotein Molecules

• Found in Gram-negative bacteria.
• Link the peptidoglycan layer and the LPS layer in the cell wall.
• Plays roles in host tissue adherence, bacterial virulence factors, and antibiotic resistance.

Periplasmic Space

• Present only in Gram-negative bacteria.
• A gel-like matrix between the inner cytoplasmic membrane and outer membrane.
• Site of enzymes (e.g. B-lactamases) that degrade certain antibiotics (like penicillin).

Cell Wall Functions

• Maintains cell shape.
• Protects the cell membrane from lysis in hypotonic environments.
• Plays a role in cell division.
• Influences the response to Gram staining.

Cell Wall Deficient Bacteria

• Protoplasts, spheroplasts, and L-forms are examples.
• These lack cell walls and can be produced by treating cells with lysozyme or penicillin.
• L-forms can revert back to the parental form if the cell wall inhibitor is removed.

Mycoplasma

• A type of bacteria that naturally lacks a cell wall.
• They are pleomorphic and resistant to penicillin.

Gram-Positive vs. Gram-Negative Comparison

• Note the differences in cell wall thickness, peptidoglycan content, and the presence of teichoic acid, outer membrane, and lipopolysaccharides (LPS) .
• This differences impact antibiotic effectiveness.

External Structures

• Capsule: Gelatinous layer (glycocalyx), which can be slime or capsule, protecting bacteria from phagocytosis, and can help bacteria adhere to surfaces. It has medically important uses.
• Flagella: Long thread-like appendages used for bacterial motility (movement).
• Pili/Fimbriae: Thin hair-like structures helping in bacterial adhesion to surfaces, and in some cases, genetic transfer. Includes sex pili for DNA transfer.

Cytoplasmic Membrane

• Semipermeable lipid bilayer (phospholipid) around the cytoplasm.
• Found inside the cell wall.
• Key function in transport, energy production, metabolism, and bacterial replication.

Cytoplasm

• Gel-like region within the cytoplasmic membrane containing ribosomes, chromosome, plasmids, and essential cellular components for metabolism and growth.

Nucleoid

• Region of the cytoplasm where the bacterial chromosome is located. Unlike eukaryotic cells, bacteria do not have a true nucleus.

Plasmids

• Extra-chromosomal DNA found in bacteria.
• Contain genes for antibiotic resistance or virulence.

Ribosomes

• Cellular structures composed of RNA and protein.
• Synthesize proteins in the cytoplasm.
• Bacterial ribosomes are smaller than eukaryotic ones (sedimentation rate of 70S).

Mesosome

• Membrane structures that invaginate into the cell for particular functions, including DNA replication and sporulation, and function similarly to eukaryotic cell's respiratory enzymes.

Bacterial Spores

• Highly resistant structures formed under unfavorable conditions (low nutrients, temperature changes).
• Have a protective outer layer.
• Medical importance as indicators for sterilization processes or potential biological weapons.

Summary of Key Bacterial Structures

• Cell wall (thick or thin, Gram+),
• Capsule,
• Flagella,
• Pili,
• Cytoplasmic membrane,
• Cytoplasm,
• Nucleoid,
• Plasmids,
• Ribosomes,
• Mesosomes,
• Spores.