1. When performing a gram stain on an unknown bacterium, the cells appear purple under the microscope. What does this indicate about the cell wall structure of the bacterium? - A) It has a thin peptidoglycan layer. - B) It lacks a peptidoglycan layer. - C) It has a thick peptidoglycan layer. - D) It has a lipid-rich outer membrane. Answer: C) It has a thick peptidoglycan layer. Rationale: Gram-positive bacteria stain purple due to a thick peptidoglycan layer in their cell walls that retains the crystal violet dye used in the staining process. 2. A microbiology student isolates an unknown microbe that ferments lactose. Which of the following media would best differentiate this microbe based on its ability to ferment lactose? - A) Blood agar - B) MacConkey agar - C) Mannitol salt agar - D) Sabouraud dextrose agar Answer: B) MacConkey agar. Rationale: MacConkey agar contains lactose and pH indicators that differentiate lactose fermenters, which produce acid and turn the medium pink, from non-fermenters. 3. An unknown bacterium tested negative for catalase. What does this suggest about its ability to neutralize reactive oxygen species? - A) It can effectively neutralize hydrogen peroxide. - B) It cannot effectively neutralize hydrogen peroxide. - C) It produces superoxide dismutase. - D) It has an aerobic metabolism. Answer: B) It cannot effectively neutralize hydrogen peroxide. Rationale: Catalase-negative bacteria lack the enzyme catalase, which breaks down hydrogen peroxide into water and oxygen, making them more susceptible to damage from reactive oxygen species. 4. Which of the following tests can be used to determine the motility of an unknown bacterium?