GRE Subject Test: Biology : Identifying Bacteria

Study concepts, example questions & explanations for GRE Subject Test: Biology

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Example Questions

Example Question #1 : Identifying Bacteria

What is the difference between gram-negative and gram-positive bacteria?

Possible Answers:

Gram-positive bacteria belong exclusively to the domain archaea

Gram-negative bacteria contain large amounts of peptidoglycan, while gram-positive bacteria contain only trace amounts

Gram-negative bacteria belong exclusively to the domain archaea

Gram-positive bacteria contain large amounts of peptidoglycan, while gram-negative bacteria contain only trace amounts

Correct answer:

Gram-positive bacteria contain large amounts of peptidoglycan, while gram-negative bacteria contain only trace amounts

Explanation:

The main reaction that happens during a gram stain is the detection of the presence of peptidoglycan. Most bacterial cell walls contain peptidoglycan, but in varying amounts. If the cell contains large amounts of peptidoglycan, it will retain the crystal violet dye and display a purple coloration (this is gram-positive). If the cell does not contain a large quantity of peptidoglycan, it will display the counterstain and appear pink (this is gram-negative).

The gram-stain does not dictate whether or not a species belongs to the domain archaea. Archaea cell walls do not contain peptidoglycan and will not react to the stain. Note that archaea and bacteria are different domains altogether, though they share some similar characteristics.

Example Question #2 : Identifying Bacteria

Which type of bacteria is referred to as "rod-shaped?"

Possible Answers:

Vibrios

Bacilli

Spirochetes

Cocci

Spirilla

Correct answer:

Bacilli

Explanation:

There are three principal forms of bacteria. Spherical bacteria are termed cocci. Rod-shaped bacteria are called bacilli. Spiral bacteria are rigid (spirilla), flexible (spirochetes), or curved (vibrios). 

Example Question #3 : Identifying Bacteria

When seen under the microscope using a gram stain, the bacteria that causes strept throat whould have which of the following appearances?

Possible Answers:

Clusters of pink spheres

Chains of purple spheres

Purple rod-shaped organisms

Pink comma-shaped organisms

Purple spiral-shaped organisms

Correct answer:

Chains of purple spheres

Explanation:

Streptococcus, the causative agent of strep throat, is a genus of gram-postitive (purple staining) cocci (spheres), in which the cells tend to form chains. Bacteria are described by a combination of their response to gram stain and their appearance. For example, "gram-positive streptococcus" indicate chains of spheres that stain purple. 

Example Question #4 : Identifying Bacteria

The Gram stain distinguishes between Gram-positive and Gram-negative bacteria. Which of the following is true about the Gram stain?

Possible Answers:

Peptidoglycan is stained through a Gram stain, and Gram-positive and Gram-negative bacteria have comparable virulence

Lipopolysaccharides are stained through a Gram stain, and Gram-positive bacteria are more virulent than Gram-negative bacteria

Peptidoglycan is stained through a Gram stain, and Gram-negative bacteria are more virulent than Gram-positive bacteria

Lipopolysaccharides are stained through a Gram stain, and Gram-negative bacteria are more virulent than Gram-positive bacteria

Peptidoglycan is stained through a Gram stains, and Gram-positive bacteria are more virulent than Gram-negative bacteria

Correct answer:

Peptidoglycan is stained through a Gram stain, and Gram-negative bacteria are more virulent than Gram-positive bacteria

Explanation:

The peptidoglycan layer of bacteria retains the Gram stain. In general, Gram-negative bacteria are more virulent than Gram-positive bacteria. This is due in part to an outer layer of lipopolysaccharides present on Gram-negative bacteria that invokes immune responses (particularly via a component called lipid-A), and are somewhat more difficult to target with antibiotics.

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