V23-18

Question

Listeria monocytogenes (Lm) is a foodborne bacterial zoonotic pathogen that can cause listeriosis, a severe infection with a high case fatality rate in immunocompromised individuals. Molecular studies have revealed the clonal population structure of Lm and the global spread of clonal complex 1, known as Lm-CC1 (originally named epidemic clone ECI). Defined by multilocus sequence typing (MLST), this widespread clonal group was first identified in an Italian soldier suffering from meningitis during World War I (WWI). Notably, Lm-CC1 is the most prevalent clinical clonal complex in several countries and actually corresponds to 20% of all of Lm clinical isolates deposited at the National Center for Biotechnology Information (NCBI). Lm-CC1 belongs to Lm major lineage I and evolved from a subgroup of serotype 4b ancestry.
 
While there is no proven interhuman horizontal transmission of listeriosis, it was only in 1983 that the foodborne transmission of human listeriosis was formally established. Since then, Lm-CC1 has been reported in different food matrices, which constitute a major source of human listeriosis, including heavily contaminated dairy products. Previous studies have also demonstrated the hypervirulence of Lm-CC1, and its higher efficiency in gut colonization and fecal shedding, compared to hypovirulent Lm clones. Moreover, increasing evidence shows that bovines, which are frequent Lm asymptomatic carriers and contribute to Lm enrichment in soils, are the main source of disease and constitute a reservoir for Lm-CC1. In addition to Lm subclinical infections that may contaminate milk, the long-term persistence of Lm in cattle manure–amended soils also poses serious risks of transmission to fresh produce.
 
Understanding the global evolution of Lm-CC1, which is now spread over all continents, as well as its emergence and dissemination across different spatial levels is critical to understand Lm population dynamics and to develop better control strategies, particularly in countries with aging and/or immunosuppressed populations who are most at risk for severe infection. However, the complex movement of livestock and food products associated with asymptomatic intestinal colonization complicates traditional epidemiological investigations aiming at deciphering Lm epidemiology by linking isolates in space and time. Here, we took a population biology approach to fill this knowledge gap and conducted the largest genomic Lm-CC1 study to date, combining genomic and evolutionary approaches to decipher its evolutionary history and pattern of emergence and spread.

In the passage, the author is primarily concerned with:

A. Tracing the historical origins of Lm-CC1.
B. Discussing the genetic makeup of Lm-CC1.
C. Describing the symptoms of listeriosis.
D. Emphasizing the role of asymptomatic carriers in listeriosis transmission.
E. Exploring the global prevalence and implications of Lm-CC1.

bb · Answer

Official Solution:

Listeria monocytogenes (Lm) is a foodborne bacterial zoonotic pathogen that can cause listeriosis, a severe infection with a high case fatality rate in immunocompromised individuals. Molecular studies have revealed the clonal population structure of Lm and the global spread of clonal complex 1, known as Lm-CC1 (originally named epidemic clone ECI). Defined by multilocus sequence typing (MLST), this widespread clonal group was first identified in an Italian soldier suffering from meningitis during World War I (WWI). Notably, Lm-CC1 is the most prevalent clinical clonal complex in several countries and actually corresponds to 20% of all of Lm clinical isolates deposited at the National Center for Biotechnology Information (NCBI). Lm-CC1 belongs to Lm major lineage I and evolved from a subgroup of serotype 4b ancestry.
 
While there is no proven interhuman horizontal transmission of listeriosis, it was only in 1983 that the foodborne transmission of human listeriosis was formally established. Since then, Lm-CC1 has been reported in different food matrices, which constitute a major source of human listeriosis, including heavily contaminated dairy products. Previous studies have also demonstrated the hypervirulence of Lm-CC1, and its higher efficiency in gut colonization and fecal shedding, compared to hypovirulent Lm clones. Moreover, increasing evidence shows that bovines, which are frequent Lm asymptomatic carriers and contribute to Lm enrichment in soils, are the main source of disease and constitute a reservoir for Lm-CC1. In addition to Lm subclinical infections that may contaminate milk, the long-term persistence of Lm in cattle manure–amended soils also poses serious risks of transmission to fresh produce.
 
Understanding the global evolution of Lm-CC1, which is now spread over all continents, as well as its emergence and dissemination across different spatial levels is critical to understand Lm population dynamics and to develop better control strategies, particularly in countries with aging and/or immunosuppressed populations who are most at risk for severe infection. However, the complex movement of livestock and food products associated with asymptomatic intestinal colonization complicates traditional epidemiological investigations aiming at deciphering Lm epidemiology by linking isolates in space and time. Here, we took a population biology approach to fill this knowledge gap and conducted the largest genomic Lm-CC1 study to date, combining genomic and evolutionary approaches to decipher its evolutionary history and pattern of emergence and spread.
 
 In the passage, the author is primarily concerned with:

A. Tracing the historical origins of Lm-CC1.
B. Discussing the genetic makeup of Lm-CC1.
C. Describing the symptoms of listeriosis.
D. Emphasizing the role of asymptomatic carriers in listeriosis transmission.
E. Exploring the global prevalence and implications of Lm-CC1.

A. Tracing the historical origins of Lm-CC1.
 
The passage does bring up history once (that Lm-CC1 was first seen in a WWI soldier) but that’s just a side note. The author isn’t really “tracing origins” throughout, they’re much more focused on its global spread and risks. B. Discussing the genetic makeup of Lm-CC1.
 
We do get some quick mentions of how it’s defined (lineage, serotype, MLST typing), but there’s no deep dive into DNA or genetic composition. That’s background, not the main story.
 
C. Describing the symptoms of listeriosis.
 
Other than one line about listeriosis being severe and dangerous in immunocompromised people, there’s really no symptom talk here. The passage is about the bacteria’s spread, not how the disease shows up in patients.
 
D. Emphasizing the role of asymptomatic carriers in listeriosis transmission.
 
Yes, cattle as silent carriers come up, and it’s an important point, but it’s just one piece of the bigger puzzle. The whole passage doesn’t revolve around carriers, so this option is too specific.
 
E. Exploring the global prevalence and implications of Lm-CC1.
 
The passage discusses the worldwide distribution of Lm-CC1, its role in listeriosis, and the need for better understanding and control strategies, indicating that the primary concern is exploring the global prevalence and implications of Lm-CC1. This aligns with choice E.

Answer: E

evenlamer · Answer

I don’t quite agree with the solution. "conducted the largest genomic Lm-CC1 study to date, combining genomic and evolutionary approaches to decipher its evolutionary history and pattern of emergence and spread."

"Understanding the global evolution of Lm-CC1" and "evolutionary history and pattern of emergence and spread" - this part is not explained by E)

prevalence is NOT the same as evolutionary history

bb · Answer

Thank you for the comment and feedback!

I think you may be reading/using the word “prevalence” very literally and expecting every aspect (like evolutionary history) to appear verbatim in the correct choice.

However, on GMAT main-idea questions things are a bit vague sometimes and the correct answer can (and often does) use broader terms such as implications, global significance, spread, etc.—that implicitly cover evolution, emergence, and the reasons it’s a public health concern. It is simply the test’s way of combining the key ideas under one broader conceptual umbrella and summarizing things and rephrasing so that the language differs and requires critical thinking. So this is a bona fide GMAT question.

abhijatyadonu · Answer

I did not quite understand the solution. Emphasizing the role of asymptomatic carriers in listeriosis transmission

How come this option is not correct as it is given in 2nd para

Kapoor99 · Answer

Agree to this since it talks quite a bit around how the dairy products maybe infected, the land the graze on etc. 
Why is that not the answer?

alrightalright · Answer

As per my understanding, since the question asks us to figure out what the author is primarily concerned with, we have to figure out what is it that the author is trying to convey through the entire passage. The answer choice D is only mentioned in paragraph 2 and does not give you much understanding of the main point of the passage, or what the author is trying to convey as a whole, which is why it may be correct. 
Such an answer (choice D) may be correct if it is an inference question.

hr1212 · Answer

I’ve expanded the solution by adding more reasoning to explain why each incorrect choice is wrong.

V23-18

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