Listeria monocytogenes studies
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Listeria monocytogenes studies
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[post_date] => 2024-12-23 17:54:40
[post_date_gmt] => 2024-12-23 22:54:40
[post_content] => Practice Passage (Question 1-6)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
Listeria monocytogenes is a Gram-positive, rod-shaped bacterium responsible for listeriosis, a severe food-borne human infection with an overall mortality rate of 30%. It has evolved efficient strategies to survive in the intestine and cross the intestinal, blood–brain and placental barriers, leading to gastroenteritis, septicemia, central nervous system infections, and mother-to-child infections.
L. monocytogenes spreads from cell to cell aided by the listerial membrane surface protein ActA, encoded by the gene actA. The bacterial membrane-bound ActA protein recruits the host cell’s Arp2/3 protein complex, which helps to polymerize actin filaments at the posterior end of the bacterium. Nucleation with the Arp2/3 complex generates branched arrays of filament that grow towards the membrane to which they are tethered, elongating the actin strand and creating a sort of comet tail propels the bacterium forward. This function allows the bacterium to move throughout the host cell cytoplasm as well as to invade neighboring eukaryotic cells.
Figure 1 ActA protein, tethered to the bacterial membrane, stimulates actin filament nucleation with the Arp2/3 complex; attribution: Benjamin A Smith et al, CC-BY 4.0.
Researchers interested in combating the danger of L. monocytogenes contamination of refrigerated foods performed the following experiments.
Experiment 1
Researchers studied the peculiar property of L. monocytogenes to grow and multiply at lower temperatures than other bacteria. Two types of foodborne bacteria, L. monocytogenes and Escherichia coli, were cultured in a medium and refrigerated at 4 degrees Celsius. The colonies of the two species were kept separate to prevent any horizontal gene transfer. The number of bacteria was then measured every few weeks and a growth curve was plotted.
Figure 2 L. monocytogenes and E. coli bacteria growth curves
Experiment 2
Researchers investigated CK-0944636, an Arp2/3 inhibitor that binds Arp2/3 within its hydrophobic core and inhibits the formation of the actin comet tail. Researchers cultured different L. monocytogenes strains, with or without actA gene disruption, in media containing freely soluble actin monomers with or without CK-0944636. The results are shown in Figure 2.
Figure 3 The rates of actin filament polymerization in µM⁻¹s⁻¹ for three different strains of L. monocytogenes
Data adapted from: “Dortet L, Mostowy S, Louaka AS, Gouin E, Nahori M-A, et al. (2011). Recruitment of the Major Vault Protein by InlK: A Listeria monocytogenes Strategy to Avoid Autophagy. PLoS Pathog 7(8): e1002168.
[post_title] => Listeria monocytogenes studies
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[question] => The growth exhibited in weeks 0–3 of the bacterial growth curves (Figure 2) is attributed to which process?
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[answer] => 2
[description] => Reason for Correct Answer:
Mitosis only occurs in eukaryotic cells.
Conjugation is the horizontal transfer of genetic material through cell–cell contact; it does not result in bacterial division.
Transformation is the genetic alteration of a bacterial cell resulting from the transfer of foreign DNA; it does not result in bacterial division.
Binary fission is the asexual reproduction process by which prokaryotes like L. monocytogenes and E. coli divide.
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[answers] => Array
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[each_answer] => A. Mitosis
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[each_answer] => B. Binary Fission
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[each_answer] => C. Transformation
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[each_answer] => D. Conjugation
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[1] => Array
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[quiz_unique_key] => 1403770772
[question] => Which of the following features is consistent with Gram-positive bacteria such as Listeria monocytogenes?
[value] => Array
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[answer] => 1
[description] => Reason for Correct Answer:
Gram-negative bacteria can be distinguished from Gram-positive bacteria through the presence of lipopolysaccharides as well as their thin peptidoglycan cell wall sandwiched between an inner cell membrane and an outer cell membrane.
The purple color of Gram-positive bacteria upon Gram staining does not result from bacterial enzymes; rather, it results from the stain being retained by the cell wall on the outside of the membrane.
Gram-positive bacteria have a thick cell wall that is primarily composed of a peptidoglycan layer consisting of sugar and amino acid cross-links. This peptidoglycan layer is responsible for retaining the purple color of the Gram stain when the bacterial cell is stained using the Gram staining method. In contrast, Gram-negative bacteria’s thinner peptidoglycan layer, surrounded by an outer membrane, does not retain the purple stain and appears pink after staining.
https://commons.wikimedia.org/wiki/File:OSC_Microbio_03_03_CellWalls.jpg
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[0] => Array
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[each_answer] => A. They have a cell wall composed of sugar and amino acid cross-links.
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[each_answer] => B. They have enzymes on their outer membrane that induce a purple color when stained.
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[2] => Array
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[each_answer] => C. They have two cell wall layers that trap purple gram stain coloration.
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[each_answer] => D. They have a plasma membrane consisting of endotoxins like lipopolysaccharides.
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[2] => Array
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[quiz_unique_key] => 1403770772
[question] => What conclusions about the rates of actin filament polymerization in L. monocytogenes can we draw from Figure 3?
[value] => Array
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[answer] => 2
[description] => Reason for Correct Answer:
Look at Figure 3 to compare how the presence of CK-0944636 and/or the disruption of the actA gene affected actin polymerization.
Figure 3 shows that the combination of CK-0944636 and a disrupted actA gene fully inhibited actin polymerization. It also shows that treatment with CK-0944636 decreased the actin polymerization rate more than disruption of the actA gene.
According to the passage, CK-0944636 is an inhibitor of Arp2/3.
Because Figure 3 shows that inhibition of the Arp2/3 protein decreased actin polymerization more than the disruption of the actA gene, this suggests that the Arp2/3 protein normally contributes more positively to the actin polymerization rate than ActA protein.
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[answers] => Array
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[0] => Array
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[each_answer] => A. ActA protein contributes more positively to the actin polymerization rate than Arp2/3 protein.
)
[1] => Array
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[each_answer] => B. Arp2/3 protein contributes more positively to the actin polymerization rate than ActA protein.
)
[2] => Array
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[each_answer] => C. CK-0944636 only inhibits actin polymerization in the absence of the ActA protein.
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[each_answer] => D. CK-0944636 only inhibits actin polymerization in the presence of the ActA protein.
)
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[quiz_unique_key] => 1403770772
[question] => Actin filaments are composed of actin monomers that self-assemble in only one direction; therefore, the filaments have two distinct ends – these are termed the ‘plus’ and ‘minus’ ends. Actin monomers add to the plus end at a faster rate than they add to the minus end. Which of the following arrows points to an actin plus end?
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[description] => Reason for Correct Answer:
Actin filaments should grow towards their plus ends, where they are adding actin faster than their minus ends.
Look at Figure 1 to see in which directions the filaments are growing.
In Figure 1, you can see that the filaments are getting longer on the side that faces the bacterial membrane (both the main filament and the newly formed branch). The extent of growth in the opposite direction is minimal.
The passage also states that “nucleation with the Arp2/3 complex generates branched arrays of filament that grow towards the membrane to which they are tethered.”
Therefore, the ends of the filaments that are directed toward the bacterial membrane are the plus ends. There are two plus ends in the figure, as shown here; however, only one plus end is correctly identified with an arrow in Choice A.
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[each_answer] => A.
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[each_answer] => B.
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[each_answer] => C.
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[each_answer] => D.
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[quiz_unique_key] => 1403770772
[question] => Which of the following statements best explains the bacterial growth curve’s behavior after week 3?
[value] => Array
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[answer] => 4
[description] => Reason for Correct Answer:
The bacterial growth curves are shown in Figure 2. After week 3, the growth curves for the two bacterial species become relatively stationary when compared with the exponential growth prior to week 3.
Nothing in the passage or the growth curve indicates that temperature was decreasing or even varying during the experiment.
If E. coli were to become more cold-resistant due to conjugation or selection, the graph would indicate an exponential rise in bacterial number, matching – or perhaps surpassing – the number of L. monocytogenes bacteria. This is not indicated by the growth curve.
The stationary phase on bacterial growth curves occurs when the growth rate slows down and the number of new cells produced equals the number of cells that die. This equilibrium is usually a result of the depletion of essential nutrients, as well as the accumulation of toxic waste products, or other limiting factors. During this phase, the growth curve levels off, forming a plateau. Bacterial cells in this phase are in a state of metabolic dormancy, and there is minimal net growth.
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[each_answer] => A. Conjugation between the two bacterial species occurs, allowing E. coli to develop resistance to cold temperatures.
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[each_answer] => B. E. coli becomes more resistant to cold than L. monocytogenes due to evolutionary selection factors.
)
[2] => Array
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[each_answer] => C. VDecreasing temperatures impede the growth of new L. monocytogenes and E. coli, causing the bacterial species to form endosomes.
)
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[each_answer] => D. The number of new L. monocytogenes and E. coli is limited by the availability of essential nutrients.
)
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[question] => Compounds that disrupt the ability of L. monocytogenes to use the host’s actin cytoskeleton are potential antivirulence drugs; these drugs could complement the action of conventional antibiotics. Which of the following proteins would be the most suitable target for an antivirulence drug against L. monocytogenes, and why?
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[answer] => 4
[description] => Reason for Correct Answer:
According to the passage, the actin cytoskeleton “plays a central role in diverse biological functions, including the maintenance of cell structure, cell migration, and organelle transport”.
Also, according to the passage, Arp2/3 is “an essential factor for actin assembly”.
Putting the previous two pieces of information together, it is safe to assume that targeting Arp2/3 would have a negative effect on the host cell – it could potentially be toxic to both the host and the pathogen.
Although Arp2/3 inhibition slowed the rate of actin polymerization more than actA disruption, this does not necessarily mean that actA inhibition would be ineffective in reducing L. monocytogenes virulence (the relationship between the rate of actin polymerization and bacterial virulence would need to be known, and this is not provided).
ActA is the most suitable drug target because it is not part of the host but is specific to the target pathogen, and, according to the passage, “enables the bacterium to hijack the host cell’s actin cytoskeleton.”
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[each_answer] => A. Actin filaments, because they are essential for the movement of the bacterium
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[each_answer] => B. Arp2/3, because it is an essential factor for actin assembly
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[each_answer] => C. Arp2/3, because it helps to recruit ActA
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[each_answer] => D. ActA, because it is a bacterial protein
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Figure 1 ActA protein, tethered to the bacterial membrane, stimulates actin filament nucleation with the Arp2/3 complex; attribution: Benjamin A Smith et al, CC-BY 4.0.
Researchers interested in combating the danger of L. monocytogenes contamination of refrigerated foods performed the following experiments.
Experiment 1
Researchers studied the peculiar property of L. monocytogenes to grow and multiply at lower temperatures than other bacteria. Two types of foodborne bacteria, L. monocytogenes and Escherichia coli, were cultured in a medium and refrigerated at 4 degrees Celsius. The colonies of the two species were kept separate to prevent any horizontal gene transfer. The number of bacteria was then measured every few weeks and a growth curve was plotted.
Figure 2 L. monocytogenes and E. coli bacteria growth curves
Experiment 2
Researchers investigated CK-0944636, an Arp2/3 inhibitor that binds Arp2/3 within its hydrophobic core and inhibits the formation of the actin comet tail. Researchers cultured different L. monocytogenes strains, with or without actA gene disruption, in media containing freely soluble actin monomers with or without CK-0944636. The results are shown in Figure 2.
Figure 3 The rates of actin filament polymerization in µM⁻¹s⁻¹ for three different strains of L. monocytogenes
Data adapted from: “Dortet L, Mostowy S, Louaka AS, Gouin E, Nahori M-A, et al. (2011). Recruitment of the Major Vault Protein by InlK: A Listeria monocytogenes Strategy to Avoid Autophagy. PLoS Pathog 7(8): e1002168.
