BIO 351 EXAM 3 (2026/2027) LAB QUESTIONS
AND CORRECT ANSWERS GRADED A+
What color are bacterial cells before beginning a gram stain?
Transparent/colorless
What is added as a mordant in gram staining?
Iodine
What is used to decolorize the crystal-violet iodine complex? What will be the
resultant color of bacteria?
Alcohol; Gram-positive bacteria will be purple and gram-negative bacteria will be
colorless
What is the counterstain used in gram staining? What bacterial cell will appear the
same color as the counterstain?
Safranin; Gram-negative
What is the resultant gram stain color for gram-positive and gram-negative
bacteria?
Gram-positive: Purple
Gram-negative: Pink
,What is the result of over-decolorization in gram staining?
Gram-positive cells stain pink (should stain purple)
What is the result of under-decolorization in gram staining?
Gram-negative cells stain purple (should stain pink)
What media is designed to enhance isolation by inhibiting growth of some
organisms while encouraging growth of others?
Selective media
What media is designed to contain indicators to expose differences between
organisms?
Differential Media
What is added to make medium selective (designed to prevent growth of specific
groups of microorganisms while allowing other organisms to grow)?
Inhibitors
What is added to make medium differential (differing abilities to perform a
specific chemical reaction differentiates organisms)?
, Substrates
Bacteria that undergo fermentation produce ___ byproducts which ___ the pH.
acid; lowers
Bacteria that cannot undergo fermentation utilize peptones or amino acids to
produce ___ byproducts which ___ the pH.
basic; raises
TSIA differentiates bacteria on the basis of
Sugar fermentation, sulfur reduction, and gas production
What is the importance of stabbing TSIA deep within the butt?
To provide aerobic and anaerobic growth environments
What three sugars are contained within TSIA?
Glucose, sucrose, and lactose
What does TSIA sand for?
Triple Sugar Iron Agar
AND CORRECT ANSWERS GRADED A+
What color are bacterial cells before beginning a gram stain?
Transparent/colorless
What is added as a mordant in gram staining?
Iodine
What is used to decolorize the crystal-violet iodine complex? What will be the
resultant color of bacteria?
Alcohol; Gram-positive bacteria will be purple and gram-negative bacteria will be
colorless
What is the counterstain used in gram staining? What bacterial cell will appear the
same color as the counterstain?
Safranin; Gram-negative
What is the resultant gram stain color for gram-positive and gram-negative
bacteria?
Gram-positive: Purple
Gram-negative: Pink
,What is the result of over-decolorization in gram staining?
Gram-positive cells stain pink (should stain purple)
What is the result of under-decolorization in gram staining?
Gram-negative cells stain purple (should stain pink)
What media is designed to enhance isolation by inhibiting growth of some
organisms while encouraging growth of others?
Selective media
What media is designed to contain indicators to expose differences between
organisms?
Differential Media
What is added to make medium selective (designed to prevent growth of specific
groups of microorganisms while allowing other organisms to grow)?
Inhibitors
What is added to make medium differential (differing abilities to perform a
specific chemical reaction differentiates organisms)?
, Substrates
Bacteria that undergo fermentation produce ___ byproducts which ___ the pH.
acid; lowers
Bacteria that cannot undergo fermentation utilize peptones or amino acids to
produce ___ byproducts which ___ the pH.
basic; raises
TSIA differentiates bacteria on the basis of
Sugar fermentation, sulfur reduction, and gas production
What is the importance of stabbing TSIA deep within the butt?
To provide aerobic and anaerobic growth environments
What three sugars are contained within TSIA?
Glucose, sucrose, and lactose
What does TSIA sand for?
Triple Sugar Iron Agar
