Question 1

Which of the following best explains why Gram-negative bacteria are generally more resistant to antibiotics like penicillin than Gram-positive bacteria?

Accepted Answer

Gram-negative bacteria possess an outer membrane containing lipopolysaccharides (LPS) that acts as a selective barrier, limiting antibiotic penetration.

Question 2

A patient is infected with a strain of Streptococcus pneumoniae that is highly virulent. Which structural feature most likely contributes to its ability to evade the host immune system?

Accepted Answer

A capsule composed of polysaccharides that prevents phagocytosis and hinders opsonization

Question 3

During Gram staining, why do Gram-positive bacteria retain the crystal violet-iodine complex after the decolorization step?

Accepted Answer

Their thick peptidoglycan layer becomes dehydrated and traps the crystal violet-iodine complex.

Question 4

Which of the following best describes the molecular structure of peptidoglycan in bacterial cell walls?

Accepted Answer

Alternating units of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) cross-linked by short peptide chains

Question 5

Penicillin exerts its bactericidal effect by interfering with which specific step in bacterial cell wall synthesis?

Accepted Answer

Blocking the transpeptidation reaction catalyzed by penicillin-binding proteins (PBPs), preventing peptide cross-linking

Question 6

A Gram-negative bacterium is exposed to a new antibiotic that cannot pass through porins. What is the most likely outcome?

Accepted Answer

The antibiotic will be unable to reach its target, and the bacterium will survive.

Question 7

Which of the following is a function of teichoic acids in Gram-positive bacteria?

Accepted Answer

They reinforce the peptidoglycan layer, impart a negative charge, and bind cations such as Mg$$^{2+}$$ and Ca$$^{2+}.$$

Question 8

A mutation causes a Gram-negative bacterium to overexpress efflux pumps. What is the most likely clinical consequence?

Accepted Answer

The bacterium will actively expel antibiotics, increasing drug resistance.

Question 9

Why is the release of lipid A from Gram-negative bacteria during antibiotic treatment potentially dangerous for patients?

Accepted Answer

Lipid A is a component of LPS that acts as an endotoxin, triggering excessive cytokine release and potentially leading to septic shock.

Question 10

Which of the following best explains the rapid growth rate of many bacterial species?

Accepted Answer

Bacteria initiate new rounds of DNA replication at the origin (oriC) before previous rounds are complete, resulting in overlapping replication cycles.

Question 11

Ciprofloxacin is effective against bacteria because it:

Accepted Answer

Stabilizes the DNA–enzyme cleavage complex, preventing re-ligation and causing double-stranded DNA breaks during replication.

Question 12

A laboratory technician forgets to add the decolorizer during Gram staining. What will be the appearance of Gram-negative bacteria under the microscope?

Accepted Answer

They will appear purple, similar to Gram-positive bacteria.

Question 13

Which of the following best describes the role of the bacterial capsule in pathogenesis?

Accepted Answer

It acts as an anti-phagocytic shield, blocks complement activation, and mimics host molecules to evade immune detection.

Question 14

Which equation best represents the osmotic pressure ($$\Pi) $$that the bacterial cell wall must withstand, given by van't Hoff's law?

Accepted Answer

$$\Pi = iCRT$$

Question 15

A Gram-positive bacterium is placed in a hypotonic solution after its cell wall is degraded by lysozyme. What is the most likely outcome?

Accepted Answer

The bacterium will undergo osmotic lysis as water rushes in.

Question 16

Which of the following is a direct consequence of the activation of the bacterial SOS response after ciprofloxacin treatment?

Accepted Answer

Error-prone DNA repair mechanisms that may introduce additional mutations

Question 17

Which of the following statements about porins in Gram-negative bacteria is correct?

Accepted Answer

Porins are protein channels in the outer membrane that permit diffusion of small hydrophilic molecules into the periplasm.

Question 18

Which of the following best explains why teichoic acids are important for Gram-positive bacteria?

Accepted Answer

They help maintain cell wall structure, regulate autolytic enzymes, and bind essential cations.

Question 19

A clinical isolate of Neisseria meningitidis is found to lack a capsule. Which of the following is the most likely consequence?

Accepted Answer

The bacterium will be less virulent due to increased susceptibility to immune clearance.

lec 05:Bacterial Cell Structure and Function: Cell Envelope, Wall, Capsule, and DNA Replication

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