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Science · Drill 5

Science practice 5

15 questions ~9 min recommended
⏱ 00:00
Score
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Gram-Positive

Teichoic acid

Lipoteichoic acid

Peptidoglycan

Cell membrane +

Gram-Negative

Lipopolysaccharides

Porins

Outer membrane layer

Peptidoglycan

Periplasmic space

Cell membrane -

Lipoproteins

Membrane protein

Phospholipids

Membrane protein

1a. Gram-positive cells

1b. Gram-negative cells

2a. Rod-shaped cells

2b. Sphere-shaped cells

3a. Rod-shaped cells

3b. Sphere-shaped cells

4a. Produces catalase

4b. Does not produce catalase

5a. Ferments lactose

5b. Does not ferment lactose

6a. Can use citric acid as sole carbon source

6b. Cannot use citric acid as sole carbon source

Go to Step 2

Go to Step 3

Gram-positive bacilli

Go to Step 4

Go to Step 5

Gram-negative cocci

Staphylococcus spp.

Streptococcus spp.

Go to Step 6

Pseudomonas spp.

Enterobacter spp.

Escherichia spp.

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?

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

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

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

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

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

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

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?

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

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

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

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

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

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:

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?

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