Bacteria Questions and Answers

Q: Which one of the following statements about exotoxins is false?

Correct answer: polysaccharide-lipid complex in natureThat statement is false: exotoxins are proteins. It is the endotoxin that is a lipopolysaccharide complex. The other three are true of exotoxins - their action is usually enzymatic, they have specific tissue affinities (tetanus toxin for nerve, cholera toxin for gut epithelium), and being catalytic they are active in minute amounts. Being protein, they are heat labile and can be converted into toxoids.

Q: For uncoating, viruses use

Correct answer: LysozymeUncoating is the removal of the capsid so that the genome can be expressed. It is done by the host’s own lysosomal enzymes once the particle has been taken into a vesicle - the virus supplies nothing for the purpose. Receptors are used in attachment, and viropexis is the engulfment step that precedes uncoating.

Q: Capsules are demonstrated by

Correct answer: Burri’s techniqueA capsule takes up no dye, so it is shown by negative staining: India ink or nigrosin darkens the background while the cell body is counterstained, and the capsule appears as a clear halo between them. That is Burri’s method.

Q: Blood agar is a

Correct answer: enriched mediaBlood agar is nutrient agar with added blood, supplying the extra growth factors that fastidious organisms such as streptococci need - so it is an enriched medium. It happens to be differential as well, since the haemolysis patterns distinguish alpha, beta and gamma streptococci.

Q: Which one of the following statements about endotoxins is false?

Correct answer: action is often enzymaticThat statement is false. Endotoxin is the lipid A of lipopolysaccharide, and it acts not as an enzyme but by triggering the host’s own inflammatory cascade - the cytokines that produce fever and shock. Exotoxins are the ones with enzymatic action. The other three statements are true: endotoxin is heat stable, it is a lipopolysaccharide-protein complex, and it comes from gram-negative bacteria.

Q: Which of the following is true about the structure of the gram-positive cell wall

Correct answer: All of the aboveAll three are true of a gram-positive wall: a thick peptidoglycan layer of many sheets; no true periplasmic space, since there is no outer membrane to bound one; and substantial teichoic and lipoteichoic acids, which give the wall its negative charge.

Q: Gram-negative microorganisms appear

Correct answer: pink-red colorGram-negative cells lose the crystal violet during decolourisation, so they take up the safranin counterstain and appear pink or red.

Q: Primary infection is

Correct answer: initial infection with organism in hostA primary infection is the first infection of a host by a given organism. A reinfection is a later one by the same organism, and a secondary infection is a new one by a different organism in a patient already weakened by disease.

51.

Which one of the following statements about exotoxins is false?

A.action is often enzymatic
B.have specific tissue affinity
C.are active in very minute doses
D.polysaccharide-lipid complex in nature

Answer: D

Correct answer: polysaccharide-lipid complex in nature

That statement is false: exotoxins are proteins. It is the endotoxin that is a lipopolysaccharide complex. The other three are true of exotoxins - their action is usually enzymatic, they have specific tissue affinities (tetanus toxin for nerve, cholera toxin for gut epithelium), and being catalytic they are active in minute amounts. Being protein, they are heat labile and can be converted into toxoids.

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52.

For uncoating, viruses use

A.Specific receptors
B.Lysozyme
C.The mechanism called viropexia
D.The synthetic machinery of host cell

Answer: B

Correct answer: Lysozyme

Uncoating is the removal of the capsid so that the genome can be expressed. It is done by the host’s own lysosomal enzymes once the particle has been taken into a vesicle - the virus supplies nothing for the purpose. Receptors are used in attachment, and viropexis is the engulfment step that precedes uncoating.

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53.

Capsules are demonstrated by

A.Ozeshki’s staining
B.Burri’s technique
C.Loeffler’s technique
D.Morozov’s staining

Answer: B

Correct answer: Burri’s technique

A capsule takes up no dye, so it is shown by negative staining: India ink or nigrosin darkens the background while the cell body is counterstained, and the capsule appears as a clear halo between them. That is Burri’s method.

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54.

The components for complement fixation reaction

A.an antigen, an antibody, complement, sheep erythrocytes, hemolytic serum
B.an antibody, complement, sheep erythrocytes, hemolytic serum
C.an antigen, complement, sheep erythrocytes, hemolytic serum
D.an antigen, an antibody, complement, hemolytic serum

Answer: A

Correct answer: an antigen, an antibody, complement, sheep erythrocytes, hemolytic serum

Five components in all, in two systems: antigen, antibody and complement in the first; sheep erythrocytes and haemolytic serum in the indicator system that reveals whether the complement survived.

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55.

Blood agar is a

A.differential media
B.enriched media
C.selective media
D.simple media

Answer: B

Correct answer: enriched media

Blood agar is nutrient agar with added blood, supplying the extra growth factors that fastidious organisms such as streptococci need - so it is an enriched medium. It happens to be differential as well, since the haemolysis patterns distinguish alpha, beta and gamma streptococci.

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56.

Transformation is

A.the transfer of genetic information through the agency of free DNA
B.the transfer of a portion of the DNA from one bacterium to another by a bacteriophage
C.the transfer of genes from donor cell to recipient by means of physical contact
D.a random, undirected, heritable variation caused by an alteration in the nucleotide sequence at some point of the DNA of the cell

Answer: A

Correct answer: the transfer of genetic information through the agency of free DNA

Transformation is the uptake of naked DNA from the surroundings by a competent cell - the phenomenon Griffith observed in 1928 and Avery explained in 1944, and the one that identified DNA as the genetic material. The alternatives describe the other two mechanisms of bacterial gene transfer: transduction, by a bacteriophage, and conjugation, by cell-to-cell contact.

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57.

Which one of the following statements about endotoxins is false?

A.they are heat stable
B.protein-polysaccharide-lipid complex in nature
C.action is often enzymatic
D.is produced by Gram-negative bacteria

Answer: C

Correct answer: action is often enzymatic

That statement is false. Endotoxin is the lipid A of lipopolysaccharide, and it acts not as an enzyme but by triggering the host’s own inflammatory cascade - the cytokines that produce fever and shock. Exotoxins are the ones with enzymatic action. The other three statements are true: endotoxin is heat stable, it is a lipopolysaccharide-protein complex, and it comes from gram-negative bacteria.

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58.

Which of the following is true about the structure of the gram-positive cell wall

A.is composed of thick peptidoglycan layer
B.periplasmic space is absent
C.include significant amount of teichoic and lipoteichoic acids
D.All of the above

Answer: D

Correct answer: All of the above

All three are true of a gram-positive wall: a thick peptidoglycan layer of many sheets; no true periplasmic space, since there is no outer membrane to bound one; and substantial teichoic and lipoteichoic acids, which give the wall its negative charge.

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59.

Gram-negative microorganisms appear

A.violet color
B.green color
C.pink-red color
D.black color

Answer: C

Correct answer: pink-red color

Gram-negative cells lose the crystal violet during decolourisation, so they take up the safranin counterstain and appear pink or red.

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60.

Primary infection is

A.initial infection with organism in host
B.subsequent infection by the same organism
C.situation, when patient suffering from a disease and new infection is set up from external source
D.all of the above

Answer: A

Correct answer: initial infection with organism in host

A primary infection is the first infection of a host by a given organism. A reinfection is a later one by the same organism, and a secondary infection is a new one by a different organism in a patient already weakened by disease.

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