Question 1

A patient develops pneumonia after inhaling a small number of Streptococcus pneumoniae. Which portal of entry did the pathogen most likely use, and what virulence factor helps it evade the host immune system?

Accepted Answer

Respiratory tract; capsule formation

Question 2

Which of the following best describes the function of siderophores in bacterial pathogenicity?

Accepted Answer

They bind iron more tightly than host proteins, allowing bacteria to acquire essential iron.

Question 3

A researcher is comparing two bacterial toxins. Toxin A has an $LD_{50}$ of $0.03\ \mathrm{ng/kg}$, while Toxin B has an $LD_{50}$ of $12\ \mathrm{mg/kg}$. Which toxin is more potent, and why?

Accepted Answer

Toxin A, because a lower $LD_{50}$ means less is needed to be lethal.

Question 4

Which of the following scenarios best illustrates antigenic variation as a microbial strategy to evade the immune system?

Accepted Answer

A virus changes its surface proteins so that previously formed antibodies are ineffective.

Question 5

A patient receives a sterile cardiac catheterization but develops fever, chills, and hypotension. The washwater used was sterile but contained endotoxins. Which test should have been performed to detect the presence of endotoxins?

Accepted Answer

LAL (Limulus amebocyte lysate) assay

Question 6

Which of the following is a key difference between exotoxins and endotoxins?

Accepted Answer

Exotoxins are proteins secreted by bacteria, often highly specific and potent, while endotoxins are lipid A components of LPS in gram-negative bacteria.

Question 7

A strain of E. coli acquires a plasmid encoding for a toxin gene. What is the most likely consequence of this genetic change?

Accepted Answer

The bacterium gains the ability to produce a new toxin, increasing its pathogenicity.

Question 8

Which of the following best explains why the influenza vaccine must be updated annually, while vaccines for diseases like measles provide long-term protection?

Accepted Answer

Influenza virus undergoes antigenic variation, frequently changing its surface antigens.

Question 9

Which of the following enzymes allows bacteria to break down host connective tissue and spread through the body?

Accepted Answer

Collagenase

Question 10

A patient presents with symptoms of paralytic shellfish poisoning after eating contaminated shellfish. Which type of microorganism and toxin is most likely responsible?

Accepted Answer

Alga; saxitoxin

Question 11

Which of the following is a mechanism by which biofilms contribute to bacterial pathogenicity?

Accepted Answer

Biofilms shield bacteria from phagocytosis and increase resistance to antibiotics.

Question 12

Which of the following best describes the action of A-B exotoxins?

Accepted Answer

The B component binds to the host cell receptor, allowing the A component to enter and disrupt cellular function.

Question 13

A patient is infected with Mycobacterium tuberculosis. Which cell wall component is primarily responsible for its resistance to digestion by phagocytes?

Accepted Answer

Mycolic acid (waxy lipid)

Question 14

Which of the following is an example of a cytopathic effect (CPE) caused by viral infection?

Accepted Answer

Formation of a syncytium (multinucleated cell)

Question 15

Which of the following best explains why unbroken skin is generally impenetrable to most microorganisms?

Accepted Answer

The keratinized surface and tight junctions prevent microbial entry.

Question 16

Which of the following is a correct statement about the portals of exit for pathogens?

Accepted Answer

The respiratory and gastrointestinal tracts are the most common portals of exit.

Question 17

A patient is infected with Trypanosoma brucei gambiense, which frequently changes its surface glycoproteins. What is the main advantage of this strategy?

Accepted Answer

It enables the parasite to evade the host's immune response by antigenic variation.

Question 18

Which of the following best describes the role of coagulase in bacterial pathogenicity?

Accepted Answer

It coagulates fibrinogen, forming a clot that protects bacteria from phagocytosis.

Question 19

Which of the following is a real-life example of a disease caused by a membrane-disrupting exotoxin?

Accepted Answer

Streptococcal pharyngitis caused by Streptococcus pyogenes producing streptolysin

Question 20

Which of the following statements about the Limulus amebocyte lysate (LAL) assay is correct?

Accepted Answer

It uses horseshoe crab blood cells that clot in the presence of endotoxins.

Microbial Mechanisms of Pathogenicity: Entry, Dose, and Adherence

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