SAT Subject Biology Practice Question 362: Answer and Explanation

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Question: 362

12. Most bacteria can be grown in the laboratory on agar plates containing glucose as their only carbon source. Some bacteria require additional substances, such as amino acids, to be added to the growth medium. Such bacteria are termed auxotrophs. These bacteria are denoted by the amino acid they require followed by a "-" in superscript (e.g., arg-). Bacteria that do not require that particular amino acid can be indicated by a "+" in superscript.

Different strains of bacteria were grown on several plates containing a variety of nutrients. Figure 1 shows the colonies (numbered) that grew on each plate. The supplements in each plate are indicated.

In a second experiment, Colony 1 was mixed with soft agar and spread over a plate so that an even lawn of bacteria grew. Bacterial lawns appear cloudy on agar plates. A single drop of an unknown organism was placed in the center of the

A. The unknown organism is bacterial Colony 2, and these bacteria are eating the bacteria from Colony 1 forming the lawn.
B. The unknown organism is a virus that is infecting the bacteria and causing them to lyse (killing them).
C. The drop placed in the center of the lawn contained a strong acid that destroyed the bacteria at that spot.
D. Bacteria are very delicate and the disturbance caused them to die.
E. The unknown organism began producing threonine, which is toxic to Colony 1.

Correct Answer: B


B Again, try to predict an answer first. Clear spots on lawns of bacteria are due to infection by viruses that cause lysis of the bacteria. Even if this was not obvious to you, you should have been able to eliminate the other choices. There is no reason to assume Colony 2 would prey on Colony 1 (A is wrong); strong acid would lyse and destroy the bacteria immediately, not after 24 hours (C is wrong); bacteria are not delicate, they can grow just about anywhere, under any conditions (D is wrong); and there is no data to support the fact that threonine may be toxic to the bacteria or that the "unknown organism" was producing it.

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