Bacterial Shapes and Structures

Bacterial Structure

• Bacteria are categorized by shape into cocci (round), bacilli (rod-shaped), and spirochetes (spiral-shaped).
• Some bacteria can change shape, described as pleomorphic.
• Cocci can form clusters, chains, pairs, or pairs with pointed ends.
• Rods can have square, rounded, club, fusiform, or comma shapes.
• Spirochetes can be relaxed or tightly coiled.

Learning Objectives

• Students should be able to define typical bacteria.
• Bacteria possess morphology with typical appearances.
• Cell wall structure, including peptidoglycans, is a key component.
• Gram-positive and gram-negative bacteria have distinct cell wall structures.
• Cell membranes, mesosomes, ribosomes, and genetic material are components of bacteria.
• Specialized structures (capsule, pili, flagella, glycocalyx) are outside the cell wall.

Structure of Bacterial Cells

• Peptidoglycan is a rigid structural component of bacterial cell walls.
• Peptidoglycan is composed of glycan chains and peptide bonds.
• It provides shape and prevents lysis from osmotic pressure.
• In gram-positive bacteria, peptidoglycan forms a thick layer.
• In gram-negative bacteria, peptidoglycan forms a thinner layer.

Bacterial Morphology

• Gram-positive cocci come in clusters (e.g., Staphylococcus) and chains (e.g., Streptococcus).
• Gram-positive rods include Bacillus and Salmonella.
• Gram-negative rods include Vibrio and Fusobacterium.
• Spirochetes, such as Borrelia, have relaxed or tightly coiled shapes.

Bacterial Cell Wall

• Cell walls are the outermost component of most bacteria, with exceptions like Mycoplasma.
• They are a rigid structure made up of peptidoglycans.
• Gram-positive cell walls have a thicker peptidoglycan layer compared to gram-negative.
• The cell wall is composed of an inner layer of peptidoglycan and an outer membrane, with varying thickness and composition depending on the bacterial type.

Peptidoglycan

• Also known as murein or mucopeptide, it is found only in bacterial cell walls.
• It is crucial in maintaining the bacteria's shape.
• Peptidoglycan is constructed of glycan chains bound together by peptide bonds.
• N-acetylmuramic acid (NAM) and N-acetylglucosamine (NAG) form the repeating sugars, connected by a beta 1,4 glycoside bond.
• Lysozyme, an enzyme found in human secretions, targets these bonds.

Cell Wall Differences in Gram-Positive and Gram-Negative Bacteria

• Gram-positive walls are thicker, stronger, contain teichoic acids, and have a high peptidoglycan content.
• Gram-negative walls are thinner, contain lipopolysaccharide (LPS), and have a lower peptidoglycan content.

Cytoplasmic Membrane

• The cytoplasmic membrane is composed of a phospholipid bilayer.
• It contains hopanoids, similar to eukaryotic sterols, but not all bacteria.
• It's important for functions like transporting molecules into and out of the cell, generating energy, synthesizing cell wall precursors, and secreting enzymes and toxins.

Mesosome

• Invaginations of the cytoplasmic membrane.
• Important during bacterial cell division.
• They help divide DNA and form cross-walls.

Cytoplasm

• Divided into an amorphous matrix and a nucleoid region.
• The matrix contains ribosomes, nutrients, metabolites, and plasmids.
• The nucleoid region houses the bacterial DNA.

Ribosomes

• Sites of protein synthesis.
• Differ from eukaryotic ribosomes in size with 70S bacterial ribosomes compared to 80S in eukaryotes.
• Contain 50S and 30S subunits.
• Antibiotics that target bacterial ribosomes can inhibit protein synthesis.

Granules

• Storage areas within the cytoplasm.
• Contain nutrients and stain with various dyes.

Nucleoid

• Area of cytoplasm where bacterial DNA is located.
• DNA is a single circular molecule with approximately 2,000 genes.
• Unlike eukaryotic DNA, lacks a nuclear membrane, nucleolus, mitotic spindle, and histones.
• Bacterial DNA has no introns.

Plasmids

• Extra-chromosomal double stranded circular DNA.
• Replicate independently of the bacterial chromosome.
• Some plasmids can integrate into the bacterial chromosomes.
• Common in both gram-positive and gram-negative bacteria.
• Types include transmissible plasmids for conjugation, and non-transmissible plasmids.

Transmissible Plasmids

• Can be transferred between bacteria via conjugation.
• Relatively large with several genes encoding sex pilus and transfer mechanisms.
• Typically present in a few copies per cell.

Non-transmissible Plasmids

• Small and lack conjugation transfer genes.
• Usually present in multiple copies (10–60) per cell.
• Often carry genes for antibiotic resistance, heavy metal resistance, toxins, and other beneficial traits for the bacteria.

Transposons (Jumping Genes)

• Pieces of DNA that move easily within or between bacterial genomes, plasmids, and bacteriophages.
• Often carry genes coding for drug resistance, toxins, and other metabolic enzymes.
• Not capable of independent replication.

Specialized Structures Outside the Cell Wall

• Capsule: Gel-like outer layer, aiding in bacterial attachment and masking antigens from host immune systems.
• Flagella: Whip-like appendages, permitting chemotaxis which is movement towards or away from stimuli.
• Pili/Fimbriae: Hair-like structures which facilitate attachment to surfaces.
• Glycocalyx/Slime Layer: Polysaccharide layer covering surfaces enabling adhesion.

Spores

• Highly resistant structures formed by Bacillus and Clostridium in response to adverse conditions.
• Contain the bacterial DNA, cytoplasm, and a protective outer layer.
• Formed due to exhaustion of nutrients.
• Resistant to harsh environmental factors.

Endospores

• Dormant, resistant forms of some bacteria.
• Location (Central, Terminal, Subterminal)
• High resistance to heat, chemicals, and radiation.

Endo/Exotoxins

• Endotoxins are part of the gram-negative outer membrane and induce toxic responses.
• Exotoxins are proteins secreted by bacteria and cause a variety of effects.

Lipopolysaccharide (LPS)

• Component of Gram-negative outer membranes.
• Known as endotoxin due to its link to toxic responses in the body.
• Has three distinct parts: lipid A (toxic portion), core polysaccharide, and O-antigen (variable portion).

Porin Proteins

• Protein channels in the outer membrane of Gram-negative bacteria.
• Allow passage of various molecules.

Acid-Fast and Associated Cell Walls

• Mycobacteria have thick cell walls rich in lipids (Waxy substances) containing mycolic acid.
• Cell walls resist gram staining and decolorization, leading to the term "acid-fast".

L-form Bacteria

• Lack the rigid structure of the cell wall.
• Formed when exposed to antibiotics or lysozyme.

Teichoic Acid

• Found in gram-positive cell walls in the peptidoglycan layer.
• Helps with cell wall maintenance.
• Plays a role in septic shock, bacterial attachment, and virulence mechanisms.

Important Note

• Differences exist between the ribosomes of bacteria and humans which is the basis for antibiotics that target protein synthesis in bacteria but not in humans.

Cell Structure summary

• The diagram of the cell wall shows the layers are different for Gram (+) and Gram (-) bacteria.
• Essential components are the components that a bacteria ALWAYS use.
• Non-essential components are the components that one type of bacteria have, but not the other type.

Test your knowledge on the basic shapes of bacteria and their structural components. This quiz covers topics such as cocci morphology, rigidity, gram-positive vs gram-negative differentiation, and bacterial locomotion. Perfect for students of microbiology or anyone interested in bacterial characteristics.