Micro - structures, functions and microscopy

What are the different morphologies of bacterial cells?

• Cocci

• Spirochetes

• Rods

• Budding and appendaged bacteria

• Spirillum

• Filamentous bacteria

What is the general structure of the cytoplasmic membrane?

• Phospholipid bilayer - polar outer and hydrophobic interior

• Main cytoplasmic membrane component is phosphatidylethanolamine

• Strengthened by hopanoids and cardiolipin

What are the general differences between bacterial and archaeal cell walls?

• Bacteria and Eukarya have ester groups

• Archaea have ether groups

What is the structure of Archaeal membranes?

• Contains phytanyl structures rather than a usual lipid

• Phytanyl structures can combine to form a biphytanyl structure

• Form a monolayer of a membrane

• ‘Lipid divide’ - differences in membrane structures causing the divide of bacteria/archaea and eukarya on the tree of life

• Some bacteria encode genes to synthesise archaeal lipids

How are bacterial membrane proteins specialised for their function?

• Membranes contain very specific proteins

• Fit into the membrane due to their chemical structure

• Proteins containing specific sequences on the 5’ (N) end allows combination with certain proteins that are then passed through the periplasm to be inserted into the membrane

What is the structure of the bacterial cell wall?

• Peptidoglycan (murein) containing

• Main component is the disaccharide, bonded using a B(1,4) bond

• Uses D amino acids rather than L amino acids (D-alanine)

• Uses peptide bonds to combine with other amino acids from peptidoglycan

How are gram -ve cell walls made?

• Peptidoglycan polymerisation and crosslinking is exclusively driven by PBP3 and PBP2

• PBP: Penicillin Binding Protein

• PBP3 - septum-divisome

• PBP2 - elongation-elongasome

• Most efficient way to kill a bacterium using antibiotics - rupture the cell wall

What is the cell wall in gram -ve bacteria?

• A combination of peptidoglycan cell wall and the outer membrane

• Lipoproteins allow the outer membrane and the peptidoglycan layer to be joined together

• Lipids and polysaccharides are within the outer membrane

What is the structure and function of porein proteins?

• Unique structure of entirely B sheets - form B barrels

• Selective channels that allow molecules to cross the outer membrane to reach the periplasm

• Specific transporters bring amino acids, sugars and ions into the cytoplasm

• Build a semi-permeable barrier

What is the structure and function of the lipid bilayers in the bacterial cell wall?

• Second lipid bilayer in gram -ve bacteria

• Not just phospholipids as in the cytoplasmic membrane

• Large polysaccharide component - lipopolysaccharide

• Only half the outer membrane of gram -ve bacteria is phospholipids and lipopolysaccharides, other half is poreins

• Allows chemicals and nutrients to be transported into the cell

• A ‘smooth’ membrane will contain all sections

• A ‘rough’ membrane will only contain the lipid and core polysaccharide

How can bacterial units in a culture be counted?

• Cells can be counted by microscopy but also by culture

• Serially dilute the cultures, plate and view the numbers

• Able to calculate CFUs from the plated cultures at different dilutions

How does a gram stain help in distinguishing different types of bacteria?

• Exploits the differences in bacterial structure

• Distinguish between gram +ve and gram -ve bacteria

How is a gram stain completed?

• Thin film of cells from a culture is applied to a slide and allowed to air dry

• Flame is used to fix the cells to the glass

• Flood with crystal violet for one minute

• Rinse, then flood with iodine for 3 minutes

• Slide is rinsed with water and flooded with ethanol for 20 seconds to extract the crystal violet complex from gram -ve cells

• Add safranin for 1-2 minutes, then rinse with water

What colour does each type of bacteria become during a gram stain?

• Gram +ve are stained purple

• Gram -ve are stained pink

What are different cell surface structures on a bacterium?

• Capsule

• Fimbriae & pili

• Flagella

What role do capsules have in a bacterium?

• Can be polysaccharide, protein or both

• Play a role in pathogenesis and biofilm formation

What role do fimbriae and pili have in a bacterium?

• Proteinaceous but can be glycosylated

• Play a role in pathogenesis, biofilms and conjugation

• Involved in swimming or ‘twitching’ motility

What role do flagella have in a bacterium?

• Multimeric protein complex traversing both the inner and outer membranes

• Filaments are helical in shape

• Molecular machines that allow bacteria to swim in liquid environments

• Nearly 5% of the E. coli genome encodes motility-related proteins