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 : Bacteria
What is the difference between gram-negative and gram-positive bacteria?
Gram-positive bacteria belong exclusively to the domain archaea
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
Gram-negative bacteria contain large amounts of peptidoglycan, while gram-positive bacteria contain only trace amounts
Gram-positive bacteria contain large amounts of peptidoglycan, while gram-negative bacteria contain only trace amounts
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?"
Vibrios
Spirilla
Cocci
Spirochetes
Bacilli
Bacilli
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?
Purple rod-shaped organisms
Pink comma-shaped organisms
Purple spiral-shaped organisms
Clusters of pink spheres
Chains of purple spheres
Chains of purple spheres
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 #1 : Identifying Bacteria
The Gram stain distinguishes between Gram-positive and Gram-negative bacteria. Which of the following is true about the Gram stain?
Peptidoglycan is stained through a Gram stain, and Gram-positive and Gram-negative bacteria have comparable virulence
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-positive bacteria are more virulent than Gram-negative bacteria
Peptidoglycan is stained through a Gram stains, and Gram-positive bacteria are more virulent than Gram-negative 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 stain, and Gram-negative bacteria are more virulent than Gram-positive bacteria
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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