Chapter 1: Bacterial Taxonomy Flashcards
Common stain to quickly determine bacterial morphology
Gram stain
Gram stain steps
(1) Crystal violet (blue) for 60s
(2) Wash with water and flood with iodine for 60s
(3) Wash with water and decolourise with 95% alcohol
(4) Counterstain with safranin (red) and wash with water
Colour of gram positive organisms? Why this colour?
Blue; absorb crystal violet)
Colour of gram negative organisms? Why this colour?
Red; cannot hold crystal violet as washed off by alcohol but absorb safranin
Do both gram positive and negative bacteria have more than one layer to protect cytoplasm and nucleus?
Yes
Do both gram positive and gram negative bacteria have peptidoglycan layers?
Yes: gram negatives just have a thinner layer
Composition of a single peptidoglycan chain
Repeating disaccharide with 4 amino acids in a side chain extending from each disaccharide.
Composition of the entire peptidoglycan chain
Each components AAs covalently bind AAs of other chains to form a cross-linked structure
Enzyme that catalyses cross-linkages in peptidoglycan
Transpeptidase
Location of transpeptidase?
Inner cytoplasmic membrane
Common inhibitor of transpeptidase
Penicillin binds to and inhibits transpeptidase. Hence transpeptidase is known as penicillin binding protein
Features of the gram positive cell wall with respect to peptidoglycan. What about gram negatives?
Extensive cross linking of amino-acid side chains in gram +ve; gram -ves = thin with simple cross linking
Outer cell wall of gram +ve bacteria composition?
Thick layer of peptidoglycan, teichoic acid, polysaccharides and proteins
Inner cell wall composition of gram +ves?
Phospholipid bilayer with embedded proteins (membrane spanning). Touches the cytoplasm
Do bacterial inner cell membranes contain cholesterol and other sterols?
NO. Only animals
What is the structure of gram negative bacterial cell walls?
(1) Cytoplasmic membrane = phospholipid bilayer + protein then periplasmic space (protein and enzymes)
(2) Thin peptidoglycan layer
(3) Outer cell membrane
Do peptidoglycan layers in Gram negative organisms contain teichoic acid? What do they contain instead?
NO! They have murein lipoprotein (small + helical)
Relationship between murein lipoprotein and outer membrane
Murein lipoprotein extends from peptidoglycan layer and binds to the outer membrane
Composition of the outer membrane of gram negative bacteria
Phospholipid bilayer but the outward facing lipid is lipopolysaccharide (LPS). Hydrophobic tails pointing to the centre still
What is the composition of lipopolysaccharide (LPS)
3 covalently linked components
(1) O-specific side chain/O-specific antigen [OUTER carbohydrate chains composed of 1-50 oligosaccharide units]
(2) Central core polysaccaride (water soluble)
(3) Lipid A (endotoxin)
With respect to LPS, do O-specific side chains/O-specific antigens vary between organisms?
Yes. Differ between organisms and are antigenic determinants
Composition of Lipid A (endotoxin)
Disaccharide with multiple fatty acid chains reaching into membrane
Do Gram-positive organisms have endotoxin
No except Listeria monocytogenes
What are the clinical implications of Gram positive bacteria
Thick peptidoglycan layer allows diffusion of low molecular weight compounds e.g. antibiotics, detergents and dyes)
