Deck 23: Pathogens and Infection

Question 1

Indicate true (T) and false (F) statements below regarding bacterial, viral, and eukaryotic pathogens. Your answer would be a four-letter string composed of letters T and F only, e.g. TTTT.
( ) Compared to bacteria and viruses, eukaryotic parasites have simpler life cycles.
( ) Most important pathogenic fungi show dimorphism, growing as either yeast or mold.
( ) Protozoan parasites often require more than one host to complete their life cycle.
( ) Plasmodium falciparum can invade human liver and red blood cells, as well as cells lining the gut in female Anopheles mosquitoes.

Answer 1

F
T
T
T

Question 2

Indicate whether each of the following examples better applies to commensalism (C), mutualism (M), or parasitism (P) in the interaction between a host and a microbe. Your answer would be a four-letter string composed of letters C, M, and P only, e.g. CCPP.
( ) Due to poor public sanitation, a child catches cholera.
( ) A mouse is infected with a virus but shows no noticeable health defect whatsoever.
( ) Biotin and other vitamins produced by intestinal microbiota are regularly absorbed in the human intestine.
( ) Bacteria on the skin of cattle produce antifungal compounds.

Answer 2

P
C
M
M

Question 3

Sort the following events to reflect the order in which they typically occur in viral replication after a virus enters a cell. Your answer would be a four-letter string composed of letters A to D only, e.g. BACD.
(A) Replication of the viral genome and transcription of viral genes
(B) Virus particle assembly
(C) Progeny virion release
(D) Virus particle disassembly

Answer 3

DABC

Question 4

Indicate whether each of the following descriptions better applies to enveloped (E) or nonenveloped (N) viruses. Your answer would be a four-letter string composed of letters E and N only, e.g. NNNN.
( ) They include retroviruses.
( ) They include adenoviruses.
( ) To leave the cell, they normally lyse it.
( ) They are more sensitive to treatment with detergents, heat, or drying.

Question 5

The type III secretion system of pathogenic bacteria …

A) is a general secretion system found only in Gram-positive bacteria.
B) is used to inject effector proteins directly into the cytoplasm of host cells.
C) is similar to the eukaryotic secretion system.
D) is similar to the bacterial conjugation apparatus.
E) All of the above

Question 6

Mycobacterium tuberculosis can cause tuberculosis, a life-threatening lung infection, but can also infect an individual asymptomatically for years. It is not considered to be part of the normal flora, and can infect healthy individuals upon exposure. It can only replicate in the host and thus mainly spreads by direct human contact. Mycobacterium tuberculosis is therefore …

A) a facultative primary pathogen.
B) a facultative opportunistic pathogen.
C) an obligate primary pathogen.
D) an obligate opportunistic pathogen.

Question 7

How has pathogenicity in pandemic strains of Vibrio cholerae been acquired?

A) By transformation
B) By transduction
C) By conjugation
D) By vertical gene transfer

Question 8

You infect human epithelial cells in culture with either nonpathogenic bacteria or Vibrio cholerae, each without any further treatment, in the presence of MDC (an inhibitor of clathrin-dependent endocytosis) or in the presence of filipin (an inhibitor of a clathrin-independent endocytic pathway). You then measure the intracellular concentration of cyclic AMP (in picomoles per milligram of total cell protein) and summarize the results in the following table. Which row (1 or 2) do you think corresponds to infection with V. cholerae? From these results, does cholera toxin enter the cell in clathrin-coated vesicles?

A) Row 1; yes
B) Row 1; no
C) Row 2; yes
D) Row 2; no

Question 9

Which of the following is a primary pathogen?

A) Staphylococcus aureus
B) Streptococcus pneumoniae
C) Yersinia pestis
D) Papillomavirus
E) MRSA (methicillin-resistant S. aureus)

Question 10

A point mutation in the gene encoding the ? subunit of hemoglobin can lead to sickle-cell anemia. Due to formation of mutant hemoglobin aggregates, red blood cells in affected individuals can adopt a sickle-like shape that interferes with their normal function. Interestingly, individuals carrying this mutation are less vulnerable to malaria infection by Plasmodium falciparum, which replicates inside red blood cells. Accordingly, would you expect the frequency of the mutated hemoglobin allele to be higher in West Africa (A) or in Europe (E)? Write down A or E as your answer.

Question 11

Sort the following events to reflect the order in which they take place during the infection of intestinal epithelium by enteropathogenicE. coli. Your answer would be a five-letter string composed of letters A to E only, e.g. BCDAE.
(A) Actin polymerization and pedestal formation
(B) Binding of intimin to Tir
(C) Folding of Tir in the plasma membrane
(D) Insertion of Tir into the epithelial cell
(E) Assembly of type III secretion system

Question 12

Indicate whether the epithelium in each of the following organs is (Y) or is not (N) kept almost sterile in a healthy human. Your answer would be a four-letter string composed of letters Y and N only, e.g. YYYY.
( ) Bladder
( ) Cervix
( ) Lower lung
( ) Colon

Question 13

In the following simplified diagram, three mechanisms for the horizontal transfer of virulence genes to an avirulent bacterium are depicted. Indicate which mechanism (A to C) corresponds to conjugation, transduction, and transformation, respectively. Your answer would be a three-letter string composed of letters A to C only, e.g. CAB.

Question 14

Both Vibrio cholerae and Bacillus anthracis …

A) secrete a lethal factor and an edema factor.
B) secrete toxins that enter the cell in endosomes and eventually enter the cytosol after reaching the endoplasmic reticulum.
C) raise cyclic AMP levels in the cytosol of their target cells.
D) secrete toxins that have enzymatic adenylyl cyclase activity.
E) All of the above.

Question 15

In the bacterium Yersinia pestis, which causes bubonic plague, loss of an operon that is normally responsible for iron storage diminishes the ability of the cells to form biofilms and colonize inside the iron-rich gut of infected fleas. Would you expect the efficiency of transmission of plague to increase (I), decrease (D), or remain unchanged (U) as a result of this mutation? Write down I, D, or U as your answer.

Question 16

In Gram staining, crystal violet (a violet dye) is used to specifically stain Gram-positive bacteria. After performing Gram staining on a bacterial sample taken from an infected animal tissue, you observe the results with a microscope. You find two major types of bacteria in the sample, as shown in the schematic drawing below. According to these results, indicate whether each of the following statements is correct (C) or incorrect (I). Your answer would be a three-letter string composed of letters C and I only, e.g. CCC.

( ) The Gram-positive bacteria in this sample are bacilli.
( ) Crystal violet stains lipopolysaccharide (LPS).
( ) The bacteria stained violet in this sample have a thicker layer of peptidoglycan compared to the other bacteria.

Question 17

How does Helicobacter pylori persist in the hostile environment of the stomach?

A) By expressing adhesin proteins that bind to molecules on the surface of gastric epithelial cells
B) By producing the enzyme urease, which locally neutralizes the gastric acid
C) By chemotaxis, using flagella, toward gastric epithelial cells
D) By producing toxins that target both epithelial cells and immune cells
E) All of the above

Question 18

Indicate true (T) and false (F) statements below regarding the human microbiota. Your answer would be a four-letter string composed of letters T and F only, e.g. TFTT.
( ) The number of human cells in our body is greater than the number of bacterial, fungal, and protozoan cells of our normal flora.
( ) There are far more genes in our microbiome than in our own genome.
( ) Infectious diseases currently cause more human deaths than cardiovascular diseases and cancers combined.
( ) All of the microorganisms that constitute the normal flora are nonpathogens.

Question 19

Which of the following illnesses is NOT associated with infection with the pathogen indicated?

A) Cervical cancer associated with human papillomavirus infection
B) Cancer in chickens associated with Rous sarcoma virus infection
C) Colon cancer associated with Vibrio cholerae infection
D) Stomach cancer associated with Helicobacter pylori infection
E) Atherosclerosis associated with Chlamydia pneumoniae

Question 20

Indicate whether each of the following descriptions better applies to bacterial (B), fungal (F), protozoan (P), or viral (V) pathogens. Your answer would be a four-letter string composed of letters B, F, P, and V only, e.g. FFFB.
( ) They cannot be all grouped neatly into a single phylogenetic tree.
( ) They have the most complex life cycles.
( ) They are essentially fragments of nucleic acid wrapped in a protective shell of proteins and (in some cases) an outer membrane.
( ) They are responsible for diseases such as polio and smallpox.

Question 21

Which of the following conditions is NOT expected to provide protection against HIV-1 infection of helper T cells?

A) A loss-of-function mutation in the CCR5 gene
B) Treatment with enfuvirtide, a membrane-fusion inhibitor
C) Treatment with concanamycin A, an inhibitor of endosomal proton pumps
D) Treatment with AMD-070, a CXCR4 antagonist
E) Treatment with PRO-542, a soluble decoy protein containing CD4 sequences

Question 22

Indicate true (T) and false (F) statements below regarding viral cell entry. Your answer would be a four-letter string composed of letters T and F only, e.g. FFTF.
( ) Endocytosis inhibitors can block HIV entry but not influenza virus entry.
( ) Adenovirus infection is blocked by membrane-fusion inhibitors.
( ) Individuals with a defective CCR5 gene are more susceptible to HIV infection.
( ) Most viruses enter the host cell by phagocytosis.

Question 23

Toxoplasma gondii is an intracellular eukaryotic parasite that can cause serious human infections. Which of the following is NOT true regarding this parasite?

A) It lyses its host cell to leave.
B) The energy for its invasion into the host cell comes from actin polymerization in the host cell.
C) Once inside the cell, it is surrounded by a host-cell membrane that is deprived of many host transmembrane proteins.
D) Once inside the cell, it absorbs nutrients and small metabolites from the host cytosol.
E) Its invasion mechanism is similar to that of malaria parasites.

Question 24

An intracellular pathogen uses one of three major strategies to survive and replicate once inside the host cell, as shown in the schematic drawing below. Indicate which strategy (1, 2, or 3) each of the following pathogens employs for this purpose. Your answer would be a three-digit number composed of digits 1 to 3 only, e.g. 332.

( ) Listeria monocytogenes
( ) Legionella pneumophila
( ) Shigella flexneri

Question 25

Consider the movement in the host cell of two intracellular pathogens, the herpes simplex virus (H) and the bacterium Listeria monocytogenes (L). Indicate whether each of the following descriptions better applies to the virus (H) or the bacterium (L). Your answer would be a four-letter string composed of letters L and H only, e.g. HLLL.
( ) It moves by a "pushing" mechanism (as opposed to a "pulling" mechanism).
( ) It can collide with the plasma membrane and create long, thin protrusions from the cell that can then be engulfed by a neighboring cell.
( ) Its movement is powered by ATP hydrolysis by kinesin and dynein motors.
( ) Its movement relies on dynamic cytoskeletal filaments.

Question 26

Indicate true (T) and false (F) statements below regarding bacterial and viral evolution. Your answer would be a four-letter string composed of letters T and F only, e.g. FFFT.
( ) Bacteria that reside in natural soil that has not been deliberately exposed to antibiotics are normally sensitive to any antibiotic.
( ) Whereas bacterial and eukaryotic pathogens evade the immune response mainly by genetic recombination, antigenic variation in most viruses occurs by error-prone replication mechanisms.
( ) If an antibiotic targets a vital bacterial protein that cannot be altered in any way to become antibiotic-resistant and functional at the same time, the bacteria would never develop resistance to the antibiotic.
( ) It is impossible to develop effective vaccines against viral infections, since viruses evolve very rapidly.

Question 27

To enter the host cell, intracellular bacterial pathogens can induce phagocytosis in cells that are normally nonphagocytic. This is done by two major mechanisms depicted in the following schematic diagrams (A and B). Indicate whether each of the following descriptions better applies to mechanism A or B. Your answer would be a four-letter string composed of letters A and B only, e.g. ABAA.

( ) It is called the zipper mechanism.
( ) It depends on invasin proteins on the surface of the bacterium that bind to their receptors on the surface of the host cell.
( ) It depends on the injection of effector proteins into the host cell by a bacterial secretion system.
( ) It resembles the process of macropinocytosis.

Question 28

After gaining entry into the host cell by the zipper mechanism, the bacterium Listeria monocytogenes escapes phagosomes by secreting listeriolysin O. This protein …

A) is active at neutral pH and cannot be degraded by the proteasome.
B) is active at basic pH and cannot be degraded by the proteasome.
B) is active at neutral pH and is rapidly degraded by the proteasome.
C) is active at acidic pH and cannot be degraded by the proteasome.
D) is active at acidic pH and is rapidly degraded by the proteasome.

Question 29

Modifications of membrane traffic in host cells by three bacterial pathogens are shown in the following schematic drawing. Indicate whether each of the following bacteria is represented by A, B, or C in the drawing. Your answer would be a three-letter string composed of letters A to C only, e.g. BCA.

( ) It is a common cause of food poisoning in humans.
( ) It causes the life-threatening lung infection tuberculosis.
( ) It is the cause of a type of pneumonia known as Legionnaire's disease.

Question 30

Cytochalasin D (CyD) is a drug that binds to the plus end of actin filaments and prevents actin polymerization. Having identified mutations in actin that confer resistance to CyD, a researcher sets out to study the role of the actin cytoskeleton in the invasion of mammalian host cells by two intracellular parasites: the protozoan Toxoplasma gondii and the bacterium Salmonella enterica. She infects wild-type or CyD-resistant host cells with either wild-type T. gondii, CyD-resistant T. gondii, or wild-type S. enterica, each in the presence (+) or absence (-) of CyD, and measures parasite internalization and intracellular proliferation as a result. The findings are summarized in the table below, in which High or Low levels of proliferation are indicated. According to these results, which column (A to C) do you think corresponds to infection with wild-type T. gondii? Which column corresponds to CyD-resistant T. gondii? Which row (D or E) corresponds to the wild-type host? Your answer would be a three-letter string composed of letters A to E only, e.g. ABE.

Question 31

In which of the following groups of viruses is an RNA-dependent RNA polymerase packaged as a structural protein in each newly made viral particle?

A) DNA viruses
B) Viruses with a positive [+] strand RNA genome
C) Viruses with a negative [-] strand RNA genome
D) Retroviruses
E) Viruses with a part single- and part double-stranded DNA genome

Question 32

Trypanosoma cruzi uses two alternative strategies to invade its host cells. To examine the contribution of each of these strategies to the pathogenicity, you culture mammalian cells that can be infected with this pathogen. The cells are engineered to express green fluorescent protein (GFP) fused to a plasma membrane protein, as well as red fluorescent protein (RFP) fused to a lysosome-specific transmembrane protein. You then either add only tiny latex beads (as a negative control) or add T. cruzi protozoa (to infect the cells). After 10 minutes, you fix the cells and examine them under a fluorescence microscope. You observe that a significant fraction (~30%) of phagosomes/vacuoles in sample 1 are RFP-positive (i.e. show RFP fluorescence at their membrane), whereas all phagosomes in sample 2 are RFP-negative. Which sample (1 or 2) is the one infected with T. cruzi? In this infected sample, would you expect the RFP-negative trypanosome-containing vacuoles to be GFP-positive or GFP-negative?

A) Sample 1; GFP-positive
B) Sample 1; GFP-negative
C) Sample 2; GFP-positive
D) Sample 2; GFP-negative