Bacterial Structures and Composition

• Bacterial structures are covered in course 1.1.

Structure of Bacteria

• Bacteria possess structures like capsules, cell walls, plasma membranes, cytoplasm, ribosomes, nucleoids containing DNA, and plasmids.
• Additional structures may include fimbriae, pili, capsules, inclusions, and flagella.
• Structures present within all bacteria include the cytoplasm, ribosomes, cell wall, plasma membrane, and nucleoid.

Bacterial Shapes

• Bacteria come in various shapes, including cocci, rods, spirilla, spirochetes, budding bacteria, and filamentous bacteria.
• Examples of cocci: streptococci and staphylococci
• Examples of rods: bacillus and salmonella
• Examples of spirillum and spirochetes: Helicobacter, Borrelia, and Treponema
• Leptospirillum are an example of budding bacteria.
• Streptomyces and Mycobacteriae are filamentous bacteria.

Chemical Composition of Bacteria

• The cytoplasm makes up about 2/3 of the volume.
• The nucleoid makes up about 1/6 of the volume.
• The envelope makes up about 1/6 of the volume.
• Bacteria are composed of 70-75% water
• Proteins: 17% of a bacterium's volume
• RNAs: 6% of a bacterium's volume
• DNA: 1% of a bacterium's volume
• Lipids: 1% of a bacterium's volume
• Peptidoglycan: 1% of a bacterium's volume

Role and Function of Bacterial Structures

• Bacterial envelopes and membranes establish a boundary and act as a permeability barrier.
• They regulate and transport the flow of materials in and out and conserve energy (PMF).
• The cytoplasmic membrane contains components like carbohydrates, proteins, and lipids.
• Approximately 25% of the Escherichia coli genome (1053 genes) encodes membrane proteins.
• Examples of membrane proteins include aquaporin Z, Msct Ion channel, and proton-driven ATP synthase.

Cell Walls

• Cell walls resist osmotic pressure and maintain an internal composition different from the external environment.
• Protein surface layers (S-layers) may also contribute to cell wall structure and function.

Gram-Positive vs. Gram-Negative Cells

• Gram-negative cells have both an inner (cytoplasmic or plasma) membrane and an outer membrane, with a thin layer of peptidoglycan in between.
• Gram-positive cells lack outer cell membranes but have much thicker cell walls that contain teichoic acids.

Peptidoglycan

• The peptidoglycan layer is a single, covalently linked macromolecular structure surrounding the cell.
• It comprises polymeric glycan strands cross-linked by flexible pentapeptides.

Bacterial Envelopes

• Gram+ bacteria have a monodermic envelope while Gram- bacteria have a didermic envelope.

Teichoic Acids and Lipoteichoic Acids

• TA and LTA are the second most abundant component in the cell walls of some bacteria.
• TA is associated with the peptidoglycan network, reach the external bacterial surface, and are important antigens.
• LTA is linked by covalent bonds to membrane lipids, crossing peptidoglycan entirely to emerge at the surface.

Structures of Gram-Negative Bacteria

• The outer membrane contains lipopolysaccharides (LPS).
• The inner membrane (cell membrane) is surrounded by peptidoglycan
• Murein lipoprotein is also present.

Lipopolysaccharides (Endotoxins)

• Consists of three parts: an O-polysaccharide string, a core antigen, and Lipid A.
• The O-polysaccharidic string is a highly variable part with repeated units of 3 to 8 sugars, with a number of units varies from 20 to 40.
• The core antigen is common to all members of the same genus but different from one genus to another and contains keto-deoxyoctonate (KDO).
• Lipid A consists of 2 amino sugars linked by a 1-6 phosphorylated bond and linked to fatty acids (6 to 7).

Resistance to Polymyxins

• Resistance to Polymyxins is by modification of Lipid A
• Modification of lipid A is chromosomal/ plasmid mediated; discovered 2011 in China; mcr1-gene.