Self-study
P1
For nearly a century after the discovery in the 1880s that a bacterium, . ██████ █████████ ██████ ████████ ██████████ ████████ ████ ██ ████████ ██ ███ ██████████ ███████ ████ ███ █████ █████ ███ ████ █████████ █████████ ████████ ████ ███ ████████ ██████ ███████ ██████████████ ██ █████ ██████ ██ ████ ███ █████████████ █████ █████████ ███
Scientists' belief ·
Vibrio cholerae (VC), which causes cholara, travels only by human hosts
VC epidemics occur when bacteria spreads through human waste.
Puzzling phenomenon ·
VC pops up in places we thought it had been eliminated
And, where does it go in between epidemics?
P2
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Colwell's discovery ·
Found VC in Chesapeake Bay
Others were skeptical because they thought VC couldn't survive without a human host, and there were no cholera epidemics since 1911.
Colwell's hypothesis ·
VC can survive in seawater, and can't always be detected by traditional methods
Traditional methods involved growing bacteria in a petri dish. Perhaps we can't do this with VC.
Testing Colwell's hypothesis ·
New detection method finds VC in water
Used this method near New Orleans and in Asia. Method involves an antibody that produces light if VC is present.
P3
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Additional findings ·
VC can become dormant and stop reproducing
That's why it sometimes can't be cultured. In this dormant state, VC can survive away from humans. We don't know what awakens VC from a dormant state, but it might be changes in seawater temperature or salinity (saltiness).
Passage Style
Phenomenon-hypothesis (RC)
Single position
16.
It can be inferred from ███ ███████ ████ █████ ███ ██ ███ █████████ ████ ████████ ███ ███████████ ██ ███ ████████ ████████ ██
Question Type
Implied
The discrepancy is that antibody tests detected bacteria in 51 out of 52 samples, but culture techniques found them in only 7 out of the same 52 samples. The explanation for this discrepancy is presented in P3: “V. cholerae, like some other bacteria, goes into a dormant, sporelike state when environmental conditions do not favor reproduction; in this state, the bacterium's metabolic rate plummets and the bacterium shrinks some 15- to 300-fold. It stops reproducing and therefore cannot be cultured.”
a
V. cholerae cannot ██████ ██ █████ ██ █ █████ █████
Supported by the discussion of V. cholerae’s dormant state. In the dormant state, V. cholerae “stops reproducing and therefore cannot be cultured.” We’re told earlier that “cultured”
b
V. cholerae 's ███████ ██ ████ ████ ██████████ ██ ████████
Antibodies relate to the new detection technique Colwell used. It doesn’t explain why the traditional technique — cultures — didn’t detect bacteria as often as Colwell’s technique.
c
V. cholerae responds █████████ ██ ███████ ██ ███████████ ███ █████████
This doesn’t explain why cultured techniques didn’t detect V. cholerae as often as Colwell’s technique. If you like (C), you’re probably thinking that the temperature and salinity of the water might not have allowed V. cholerae to exit its dormant state. But that’s not what (C) says. The answer you’re actually looking for is something like “the temperature and salinity of the water from which V. cholerae was taken put the bacteria into a dormant state.” We also have no reason to think that V. cholerae “primarily” responds to temperature and salinity. These are just two factors thought to influence dormancy.
d
V. cholerae cannot ██ ████████ █████ ███████ █████ ████ ███████ █████ ████ █████ ██████ ██ ██████
This isn’t supported as an explanation, because we have no reason to believe that V. cholerae cultures can’t be taken from sources other than human tissue or waste. In fact, we know that cultured techniques found V. cholerae in 7 out of 52 water samples. There’s no indication that these water samples relate to human waste or tissue.
e
V. cholerae 's ████ ████████ ████████ ████████ █ ████████ ████ ██████████ █████ ███████████ ██████
This doesn’t explain why cultured techniques didn’t detect V. cholerae as often as Colwell’s technique. UV lights and fluorescing relate to Colwell’s technique — they wouldn’t explain why the cultured techniques didn’t detect V. cholerae in as many samples as Colwell’s technique did.