ACT science practice test 26

Directions: Each passage is followed by several questions. After reading a passage, choose the best answer to each question and fill in the corresponding oval on your answer document. You may refer to the passages as often as necessary.

You are NOT permitted to use a calculator on this test.


Bacteria species are differentiated into two large groups, gram-positive and gram-negative, based on the properties of their cell walls. Peptidoglycan, a sugar-amino acid polymer, is a structural component of the cells walls of both types of bacteria, though the peptidoglycan layer is significantly thicker in gram-positive bacteria. Gram-negative bacteria have an extra lipid bilayer, called the outer membrane, that surrounds the entire cell. Figure 1.3 shows a structural comparison of the cell walls of gram-positive and gram-negative bacteria.

Figure 1.3

Gram staining is a technique used to identify to which group a particular bacteria species belongs based on its ability to retain a dye when rinsed with a solvent. First, the primary stain, crystal violet, is applied to the bacteria culture. An iodine solution is then added to form a complex with the crystal violet inside the cells. A decolorizer (ethyl alcohol or acetone) is added next. In gram-positive bacteria, the decolorizer dehydrates and shrinks the thick peptidoglycan layer. This traps the large crystal violet-iodine complex inside the cell, staining the cell purple. In gram-negative bacteria, the decolorizer degrades the outer membrane. This prevents the thin peptidoglycan layer from retaining the crystal violet-iodine complex, and the dye is washed out of the cell. A counterstain (safranine or fuchsin) is then added to the culture, giving decolorized gram-negative cells a red color. The counterstain is lighter colored than the primary stain, so it does not affect the outcome for gram-positive cells. After the staining procedure is completed, the treated cells are examined under a microscope to determine their color, thus identifying the group to which they belong.

Gram staining is typically the first test in a series of laboratory tests used to identify an unknown bacteria sample. Table 1.6 is a dichotomous key of characteristics that can be used to identify members of five common bacteria genera.

TABLE 1.6 Bacteria Dichotomous Key

1. Which statement accurately describes a structural difference between gram-positive and gram-negative bacteria?

A. Gram-positive bacteria have a thicker layer of peptidoglycan but lack an outer membrane.
B. Both types of bacteria have a cell wall, but gram-negative bacteria lack a cell membrane.
C. Gram-negative bacteria have an outer membrane instead of a peptidoglycan layer.
D. The outer membrane is located beneath the peptidoglycan layer in gram-positive bacteria.

2. Which structural feature is present in both gram-positive and gram-negative cells?

A. Porins
B. Lipoteichoic acid
C. Periplasmic space
D. Lipopolysaccharides

3. Which of the following statements is most logically supported by the presence of porins in gram-negative bacteria?

A. Cells walls are not permeable, so all substances entering a bacteria cell must travel through porins.
B. The lipopolysaccharide and phospholipid bilayer is less permeable than peptidoglycan.
C. In bacteria cells, a thicker peptidoglycan layer is more permeable than a thin peptidoglycan layer.
D. Gram-negative bacteria transport larger molecules into their cells than do gram-positive bacteria.

4. Which substance does not act as a tissue stain in the Gram staining technique?

A. Safranine
B. Crystal violet
C. Fuchsin
D. Ethyl alcohol

5. According to the Gram staining technique, a bacteria species is identified as gram-negative if its cells:

A. appear purple after the staining procedure.
B. have not been exposed to any stain.
C. appear colorless after the staining procedure.
D. appear red after the staining procedure.

6. In the Gram staining technique, which step must be performed before the addition of the iodine solution?

A. Staining with safranine
B. Washing with acetone
C. Staining with crystal violet
D. Washing with ethyl alcohol

7. Based on the information about the Gram staining technique, the most logical reason for applying a counterstain is to:

A. intensify the appearance of gram-positive cells under a microscope.
B. prevent the primary stain from affecting gram-negative cells.
C. counteract the effects of the primary stain on gram-positive cells.
D. allow gram-negative cells to be seen more easily under a microscope.

8. Based on the information about the Gram staining technique, it is most reasonable to expect a chain of which type of molecule to degrade in the presence of ethyl alcohol?

A. Lipids
B. Nucleotides
C. Sugars
D. Amino acids

9. Based on the information in Table 1.6, bacteria belonging to which genus would appear purple after a Gram staining test?

A. Streptococcus
B. Escherichia
C. Pseudomonas
D. Enterobacter

10. In Table 1.6, Steps 2 and 3 list the same cell shape characteristics because:

A. gram-positive and gram-negative bacteria can both be rod- or sphere-shaped.
B. gram-positive bacteria can switch between rod and sphere shapes.
C. cell shape depends on the results of the bacteria's Gram staining test.
D. the cell shape of many gram-positive and gram-negative bacteria is unknown.

11. Of the five bacteria genera listed in Table 1.6, how many have a cell wall composed of a thick peptidoglycan layer?

A. One
B. Two
C. Three
D. Five

12. Based on the information in Table 1.6, which genera contains gram-negative, rod-shaped bacteria that do not ferment lactose?

A. Pseudomonas
B. Enterobacter
C. Staphylococcus
D. Escherichia

13. Based on the information in Table 1.6, which characteristic is shared by Pseudomonas and Enterobacter bacteria?

A. Gram-positive cells
B. Lactose fermentation
C. Use of citric acid as sole carbon source
D. Rod-shaped cells

14. A laboratory technician is examining a bacteria sample belonging to the genus Escherichia under a microscope and notes that the sample remains colorless after performing the Gram staining procedure. It is most reasonable to assume that an error occurred during the:

A. application of the primary stain.
B. application of the counterstain.
C. decolorization of the cells.
D. bonding of iodine to the primary stain.

15. Since gram-negative bacteria are generally more resistant to antibiotics such as penicillin, Gram staining can be used to inform appropriate antibiotic treatment for patients with bacterial infections. Based on the information in Table 1.6, infections caused by bacteria belonging to which genera would be most effectively treated with penicillin?

A. Staphylococcus and Streptococcus
B. Enterobacter and Escherichia
C. Staphylococcus and Enterobacter
D. Streptococcus and Escherichia