In which of the following is the peptidoglycan layer present in large quantity?
Answer: A
Correct answer: Gram-positive bacteria
Gram-positive walls are mostly peptidoglycan - many layers, half or more of the wall’s dry weight. Gram-negative walls have a single thin layer under an outer membrane. Fungal and algal walls contain none: they are built of chitin, glucans or cellulose.
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Peptidoglycan is made up of __________
Answer: D
Correct answer: N-acetylglucosamine, N-acetylmuramic acid and amino acids
Peptidoglycan has two parts. The backbone is an alternating chain of NAG and NAM, and short peptide chains of amino acids hang from each NAM and are cross-linked to those on neighbouring strands. Without the peptides there would be no cross-linking and no rigid mesh - which is exactly what penicillin prevents.
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Teichoic acid present in Gram-positive bacteria can bind to which ion?
Answer: C
Correct answer: Mg ions
Teichoic acids are polymers of glycerol or ribitol phosphate, so the wall carries a strong negative charge. That charge binds divalent cations, magnesium in particular, holding a reserve of them at the cell surface where the membrane enzymes need them.
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Cord factor is a ___________
Answer: C
Correct answer: mycolic acid derivative
Cord factor is trehalose dimycolate - a sugar esterified with two mycolic acids, the long-chain fatty acids peculiar to the mycobacterial wall. It makes M. tuberculosis grow in the serpentine cords that give it its name, and it contributes to virulence by damaging mitochondria and inhibiting the migration of leucocytes.
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The outer membrane of the Gram-negative cell wall is anchored to the underlying peptidoglycan by means of which of the following?
Answer: A
Correct answer: Braun’s Lipoprotein
Braun’s lipoprotein is the tether: its lipid end is embedded in the inner leaflet of the outer membrane and its protein end is covalently bonded to the peptidoglycan. Without it the outer membrane would not be held in place, and the cell would shed blebs of it.
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Which among the following acts as receptors for bacteriophage attachment in Gram-negative bacteria?
Answer: B
Correct answer: O antigens
The O-antigen of lipopolysaccharide is the most exposed structure on a gram-negative cell, and many bacteriophages use it as the receptor they recognise before injecting their DNA. Lipid A is buried in the membrane, and teichoic acid belongs to gram-positive walls - where it serves the same purpose for their phages.
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Porins are special proteins that act as channels in the outer membrane of gram-negative bacteria.
Answer: A
Correct answer: True
Porins are trimeric proteins forming water-filled channels through the outer membrane, admitting small hydrophilic molecules up to about 600 daltons. They are what make the outer membrane a selective barrier - and their size limit is one reason large antibiotics are so often useless against gram-negative bacteria.
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The NAG and NAM of the peptidoglycan layer are linked by _________
Answer: A
Correct answer: beta-(1,4) glycosidic linkage
NAG and NAM alternate along the glycan backbone, joined by β-(1,4) bonds - the linkage that gives cellulose and chitin their strength too, and the one that lysozyme cleaves. An α linkage would give a storage polymer such as starch, not a structural one.
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Gram-negative bacteria are more resistant to antibiotics due to the presence of
Answer: B
Correct answer: Outer lipopolysaccharide layer
The outer membrane, with its lipopolysaccharide leaflet, is an extra permeability barrier that many drugs simply cannot cross - hydrophobic ones are blocked by the LPS, and large hydrophilic ones are too big for the porins. That is why gram-negative bacteria are intrinsically resistant to a whole range of antibiotics that kill gram-positive ones.
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Which of the following are present in teichoic acids?
Answer: D
Correct answer: ribitol or glycerol residues
Teichoic acids come in two kinds, built either on ribitol phosphate or on glycerol phosphate, with the units joined by phosphodiester bonds and often carrying sugars or D-alanine. Both give the wall its negative charge.
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