New SAT Reading Practice Test 66: Microbiomes

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Microbiomes

Germs make us sick. It's an elementary
truth that we teach to our children. It's why
we wash our hands before eating. It's why we
pasteurize our milk, and refrigerate our food.
05When they do make us sick, our ability to
safely exterminate them is nothing short of a
modern miracle. Beginning with penicillin in
1928, antibiotics forever transformed the way
we both treat and prevent infectious disease.
10Today, moreover, one can stroll down any
cleaning supply aisle at a supermarket, and
discover a bevy of products boasting of their
broad-spectrum antimicrobial activity.
For better or for worse, our culture of
15"germophobia" was hard-won by its proponents.
From the time it was first proposed in
the 16th century, the germ theory of disease
faced three hundred years' worth of influential
naysayers, and it was not until the
20late 1800's that the theory began to gain the
pervasive public vindication it enjoys today.
However, an emerging body of research indicates
that we have been perhaps overzealous
in our crusade to eradicate the germs that
25live within us.
The "human microbiome" refers collectively
to the microscopic organisms that
naturally colonize the human body, and the
application of dynamic ecological theories
30to this biome represents a rapidly expanding
field of study. Comprised of fungi, viruses,
archaea, and perhaps 1,000 species of bacteria,
the population of this microbiome is
thought to outnumber our own cells by as
35much as ten to one. What's more, much like
our own cells, a significant portion of these
organisms play crucial roles in our metabolic
and immunological processes.
For example, Oxalobacter formigenes,
40which colonizes the colon, is a primary
source of the enzyme oxalyl-CoA decarboxylase,
which allows us to safely eliminate
dietary oxalate. Without this enzyme, calcium
oxalate salts tend to accumulate in the
45kidney tubules, and eventually precipitate as
renal stones. Other colonic bacteria catalyze
the reduction of bilirubin into urobilinogen:
a reaction critical to our digestion of
fats, and absorption of fat-soluble vitamins.
50Interestingly, many bacteria within our
gastrointestinal tracts also directly synthesize
several vitamins in excess of their own metabolic
needs, and, as a result, represent an
important source of both vitamin B12, which
55is necessary for the production of new red
blood cells, and vitamin K, which is a cofactor
in the synthesis of several blood clotting
factors.
The benefits we gain from a balanced,
60thriving microbiome are diverse, and we are
only just beginning to appreciate their true
complexity, though perhaps no single function
it serves is more significant than its role
in regulating our immune systems. There are
65numerous mechanisms by which the microbiome
helps protect us from disease. Some
species, for instance, secrete special proteins,
known as "bacteriocins," that are directly
toxic to pathogenic bacteria, but harmless
70to our own cells. One particularly impressive
member of the microbiome, Lactobacillus,
produces a powerful bacteriocin called
reuterin, as well as lactic acid and hydrogen
peroxide, which inhibit the growth of disease-
75causing organisms by lowering local pH
and damaging lipid membranes respectively.
Of no less importance, there are a number
of more indirect, ecologically-oriented ways
in which the microbiome confers protection
80to its host. Abundant colonization of
our bodies by benign microorganisms, for
example, inhibits the overgrowth of more
dangerous ones through the sheer depletion
of microbial nutrients. This notion of ecological
85balance has been of particular interest
to scientists studying the microbiome, as it
hinges upon both the variable diversity of
species that colonize an individual, as well as
factors that affect the dynamism of a microbiotic
90population. Age, geography, diet, and
stress have all been implicated in influencing
both the composition and balance of the
microbiome. None, however, has been shown
to have so drastic an effect as the use of antibiotics,
95which, unlike bacteriocins, tend to be
just as deadly to disease-causing bacteria as
they are to those that colonize us naturally.
Admittedly, antibiotics save lives. Yet
our decision to use them must be weighed
100carefully, as artificially upsetting the ecological
balance of our bodies can have dire
consequences. There is perhaps no greater
example of this than Clostridium difficile, a
colonic bacterium that is both highly resistant
105to antibiotics, and an increasing cause
of hospital-acquired disease. When properly
counterbalanced by its neighboring species,
C. difficile is harmless to humans. However,
following the administration of antibiotics,
110and the elimination of its ecological
competitors, C. diff proliferation proceeds
unchecked, resulting in a frequently fatal
infection for which very few effective treatments
exist.

The table illustrates variations in microbiota for individuals following either vegetarian or omnivorous diets across three geographical demographics.

Diversity of Human Microbiomes

1. The general structure of the passage is best described as

  • A. a broad introduction followed by specific illustrations.
  • B. a logical, point-by-point argument.
  • C. an interesting look followed by in-depth persuasion.
  • D. a sequence of technical examples.

2. As used in line 1, the word "elementary" most closely means

  • A. scholastic.
  • B. medical.
  • C. healthy.
  • D. fundamental.

3. The paragraph in lines 14-25 most directly serves to

  • A. articulate that while society has now embraced germ theory, taking the theory too far may be detrimental.
  • B. argue that germophobia has continued to be a major obstacle to scientific progress.
  • C. point out the shortcomings of germ theory by presenting the valid concerns of germophobics.
  • D. present the many ways that germ theory has concrete applications to everyday life.

4. What is the overall purpose of the paragraph in lines 39-58?

  • A. To recommend specific bacteriological treatments to common gastrointestinal illnesses
  • B. To provide concrete examples of the utility of some bacteria to our metabolic and immunological processes
  • C. To address the objections of those who are inherently skeptical towards the existence of bacteria
  • D. To explain the metabolic processes whereby bacteria lead to the creation of vitamins B12 and K

5. Which of the following does the passage explicitly state illustrates the potential danger in overusing antibiotics?

  • A. Oxalobacter formigenes
  • B. Bilirubin
  • C. Lactobacillus
  • D. Clostridium difficile

6. Which option gives the best evidence for the answer to the previous question?

  • A. Lines 39-43 ("For . . . oxalate")
  • B. Lines 46-49 ("Other . . . vitamins")
  • C. Lines 70-76 ("One . . . respectively")
  • D. Lines 108-114 ("However . . . exist")

7. As used in line 87, the word "hinges" most closely means

  • A. fulcrums.
  • B. analyzes.
  • C. depends.
  • D. joints.

8. The information in the table would be most helpful to the study of which of the following concepts mentioned in the passage?

  • A. Germophobia
  • B. Ecological balance
  • C. Blood clotting
  • D. Pathogenic bacteria

9. Which option gives the best evidence for the answer to the previous question?

  • A. Lines 14-21 ("For better . . . today")
  • B. Lines 50-58 ("Interestingly . . . factors")
  • C. Lines 66-76 ("Some . . . respectively")
  • D. Lines 84-93 ("This . . . microbiome")

10. Based on the information in the table, analysis of the variations of which two microorganisms would be most helpful in roughly determining the country of residence of a randomly selected human test subject?

  • A. Actinobacteria and Bacteroides
  • B. Bifidobacteria and Clostridia
  • C. Enterococci and Lactobacilli
  • D. Methanogens and Yeasts

11. Based on the information in the table, knowing the percentage of each of the following microorganisms would be useful in determining whether someone had a vegetarian or omnivorous diet, no matter his or her geographical location, EXCEPT:

  • A. Bacteroides.
  • B. Enterococci.
  • C. Lactobacilli.
  • D. Yeasts.