Lecture BIOD 4: Prokaryotic Cell Structure &Function• Morphology of Bacteria:- Prokaryotic cells are generally much smaller than Eukaryotic cells.

- Although similar in morphology, bacteria remarkably vary in size, shape & arrangement. - Most bacteria come in one of three basic shapes: coccus, rod or bacillus, and spiral.

• Coccus:- The cocci are spherical or oval bacteria having one of several distinct arrangements:

- Diplococcus: pair of cocci

- Streptococcus: chain of cocci

- Tetrad: square of 4 cocci

- Sarcina: cube of 8 cocci

- Staphylococcus: cocci in irregular, often grape-like clusters

• Bacillus: - Bacilli are rod-shaped bacteria. They divide in one plane producing one of the following: • Bacillus: a single bacillus • Coccobacillus: oval and similar to a coccus • Streptobacillus: a chain of bacilli

• Spiral:- A helical or corkscrew-shaped bacterium. Spirals come in 1 of 3 forms:

• Vibrio: curved or comma-shaped rod • Spirillum: thick, rigid spiral • Spirochete: thin, flexible spiral

• Pleomorphic:- Bacteria that are variable in shape & lack a single characteristic form.• Prokaryotic Cell Organization:- Bacteria are prokaryotic, single-celled, microscopic microorganisms;- Generally much smaller than eukaryotic cells;- Very complex despite their small size.- Structurally, a typical bacterium usually consists of: • a cytoplasmic membrane surrounded by a peptidoglycan cell wall & maybe an outer membrane;• a fluid cytoplasm containing a nuclear region (nucleoid) and numerous ribosomes; and• often various external structures like a glycocalyx, flagella, and pili.

• Nucleoid:

- The most striking difference between procaryotes and eucaryotes is the packaging of the genetic material.

- Eucaryotes have 2 or more chromosomes contained within a membrane-delimited organelle, the nucleus.

- Procaryotes chromosome, single circle dsDNA, is located in irregular shaped region called nucleoid (also called nuclear body, chromatin body, nuclear region).

- DNA (deoxyribonucleic acid): the nucleic acid that constitutes the genetic material of all cellular organisms. Polymer of nucleotides connected via a phosphate-deoxyribose sugar backbone.

- Many bacteria contain Plamids in addition to their chromosome.- Plasmids are circular dsDNA molecules, replicate independently of chromosome and may integrated in it; not required for cell growth and reproduction.

- Plasmids carry extra genes that render bacteria drug-resistant, give them new metabolic activities, make them pathogenic!!!!, etc.

• Cytoplasmic Matrix, Ribosome and Inclusion Bodies:

- Cytoplasmic Matrix is the substance lying between the plasma membrane and the nucleoid;

- It is a relatively featureless fluid; about 70% water; packed with Ribosomes and inclusion bodies.

- Inclusion Bodies are granules of organic or inorganic material that are stockpiled for future use (starch, fat, sulphur, gas vacuoles, etc.).

- Ribosomes: structures made of protein and RNA; responsible for synthesis of cellular proteins.

• Mesosomes:

- Invaginations of plasma membrane in the shape of vesicles, tubules and lamellae.

- Play a role in cell wall formation during division;

- Involved in chromosome replication and distribution to daughter cells.

• Plasma Membrane:

- Membranes are absolute requirement for all living organisms; interact in a selective fashion with environment.

- Plasma Membrane is made of a phospholipid bilayer with integral and peripheral proteins embedded.

- Imbedded in the membrane are proteins. Some stick completely through the membrane, while some are planted so that they only protrude through one surface.

- These proteins are held in the membrane by virtue of hydrophobic and hydrophilic regions on the protein.

• Functions of PM:

- Separates the cell from its environment.

- Serves as a selective permeable barrier.

- Transport systems used for nutrient uptake,waste excretion, protein secretion.

- PM is the location of a variety of metabolic processes such as respiration, photosynthesis, lipid synthesis and cell wall constituents synthesis.

- Contains special receptor molecules that may help bacteria detect and respond to environmental changes.

• Cell Wall:

- One of the most important parts of the prokaryotic cell.

- Give shape, protect from osmotic lysis, protect from toxic substances.

- Cell Wall is made of peptidoglycan; polymer of peptides (typically 4 amino acids long, cross-linked to other chains) and glycans (made of alternating amino sugars).

- Sugars found in peptidoglycan:

- N-acetylglucosamine (NAG).

- N-acetylmuramic acid (NAM).

• Gram Negative vs Gram Positive:

- Christian Gram developed the Gram stain (1884); bacteria divided in 2 major groups: Gram(+) & Gram(-).

• Gram Positive:

- Thick homogeneous peptidoglycan layer.

- Large amount of teichoic acids; not in Gram(-).

- Inner plasma membrane.

• Gram Negative:- More complex than Gram positive.

- Thick outer membrane; contains lipids, lipopolysaccharides and lipoproteins.

- Thin peptidoglycan layer.

- Periplasmic space.- Plasma membrane.

• Mechanism of Gram Staining:

- Difference between Gram(+) & Gram(-) is due to the physical nature of their cell walls.

- Gram(+) becomes Gram(-) when cell wall removed.

- Peptidoglycan acts as a permeability barrier preventing loss of crystal violet.

- Ethanol is thought to shrink pores of the thick peptidoglycan dye-iodine complex retained bacteria remain purple.

- Gram(-) have very thin peptidoglycan.

- Ethanol treatment extract enough lipid from wall and make more porous purple crystal violet-iodine complex is more readily removed.

- When counterstained with safranin Gram(-) bacteria turn pink.

• Flagella and Motility:

- Need to be stained to be viewed under bright field microscopy.

- The ultrastructure consists of flagellin, hook and basal body.

- Flagellin: hollow rigid cylinder constructed of a single protein.

- Hook: short curved segment that links flagellin to the basal body.

- Basal body: Series of rings that drive the flagella.

• Flagellar arrangements:

1. Monotrichous: a single flagellum, usually at one pole.

2. Amphitrichous: a single flagellum at both ends of the organism.

3. Lophotrichous: two or more flagella at one or both poles.

4. Peritrichous: flagella over the entire surface.

• Flagellar synthesis:

- Complex process involving at least 20 to 30 genes, and 10 or more genes that encode proteins for hook and basal body.

• Flagellar movement:

- Bacterium moves when the filament (in the shape of rigid helix) rotates.

- Flagella act like propellers on a boat.

- The E. coli motor rotates 270 rpm.

- Vibrio alginolyticus rotates 1100rpm.

• Capsules, Slime Layers & S Layers:

- Layers of material outside the cell wall, vary in thickness and rigidity.

- Protects bacteria from phagocytosis, viral infections, pH fluctuation, osmotic stress, ...

- Help bacteria to attach to surfaces of solid objects in aquatic environment and tissue surfaces in plants and animals.

• Pili & Fimbriae:

- Tubular protein (short hair-like) structures extending from a bacterial surface used for attachment to environmental surfaces or cells.

- Sex pili attach to surface of other bacteria during sexual mating.

- Chemotaxis is a response to a chemical gradient of attractant or a repellent molecules in the bacterium's environment.

- Bacteria are attracted nutrients such as sugars and amino acids.

- Taxis is a motile response to an environmental stimulus.

- Chemotaxis is regulated by chemoreceptors located in the cytoplasmic membrane or periplasm of the bacterium bind chemical attractants or repellents.

• Chemotaxis:

• Bacterial Endospore:

- An endospore is not a reproductive structure but rather a resistant, dormant survival form of the organism, enable bacteria to resist harsh environment.

- Endospores are quite resistant to high temperatures (including boiling), most disinfectants, low energy radiation, drying, etc.

- The endospore can survive possibly thousands of years until a variety of environmental stimuli trigger germination, allowing outgrowth of a single vegetative bacterium.

- Spore formation follows a very complex multistage process.

- Under conditions of starvation, especially the lack of carbon and nitrogen sources, a single endospores form within some of the bacteria. The process is called sporulation.

-Transformation of dormant spores into active vegetative cells is done in 3 stages:

(1) Activation. (2)Germination. (3) Outgrowth.