MicroBio Lab Exam #2 With Questions And
Answers Latest Update 2026
What are the three main types of culture media? Nutritive, selective, and differential.
What is the purpose of nutritive media? Supports growth of many microorganisms.
What is selective media? Inhibits some microbes while allowing others.
Distinguishes organisms based on visible
What is differential media?
changes.
What makes media selective? Salt, dyes, antibiotics.
What makes media differential? Substrates + pH indicators.
How do pH indicators work? Change color with acid/base.
What does acid production indicate? Fermentation.
What is Mannitol Salt Agar (MSA)? Selective and differential medium.
Why is MSA selective? High salt.
Why is MSA differential? Mannitol + phenol red.
What color = mannitol fermentation? Yellow.
What organisms grow on MSA? Halotolerant bacteria.
What is a halotolerant organism? Tolerates high salt.
What is EMB agar? Selective and differential medium.
How is EMB selective? Dyes inhibit Gram+.
How is EMB differential? Lactose fermentation.
What does E. coli look like on EMB? Green metallic sheen.
What is MacConkey agar? Selective and differential medium.
What does MAC select for? Gram− bacteria.
,What inhibits Gram+ on MAC? Bile salts + crystal violet.
What does MAC differentiate? Lactose fermentation.
What color = lactose fermenter on MAC? Pink/red.
What is blood agar? Differential medium.
What does blood agar detect? Hemolysis.
What is hemolysis? RBC breakdown.
What is alpha hemolysis? Green partial.
What is beta hemolysis? Clear complete.
What is gamma hemolysis? No change.
What causes hemolysis? Hemolysins.
What is the catalase test? Detects catalase enzyme.
What does catalase do? Breaks H₂O₂ → bubbles.
Catalase + result? Bubbles.
Catalase − result? No bubbles.
Staph catalase result? Positive.
Strep catalase result? Negative.
What are staphylococci? Gram+ cocci clusters.
What are streptococci? Gram+ cocci chains.
Key difference staph vs strep? Catalase test.
What is coagulase? Clots plasma.
Which bacteria is coagulase+? Staphylococcus aureus.
What is a virulence factor? Trait that increases pathogenicity.
What pigment does S. aureus make? Staphyloxanthin.
What toxin causes beta hemolysis in S. aureus? Alpha toxin.
Why can staph grow on MSA? Halotolerant.
, Which staph ferments mannitol? S. aureus.
Which does not? S. epidermidis.
What is bacitracin test? Differentiates strep groups.
Bacitracin sensitive? Group A.
Which is Group A? Streptococcus pyogenes.
Which is Group B? Streptococcus agalactiae.
What is a dichotomous key? Stepwise ID tool.
What is an indicator organism? Signals contamination.
What is a coliform? Gram− lactose fermenter.
Why are coliforms important? Indicate fecal contamination.
What family are enterics? Enterobacteriaceae.
Key traits of Enterobacteriaceae? Gram−, ferment glucose.
What is TSIA? Differential medium.
What does TSIA test? Sugar fermentation, gas, H₂S.
Sugars in TSIA? Glucose, lactose, sucrose.
pH indicator in TSIA? Phenol red.
Yellow in TSIA? Acid.
Red in TSIA? Alkaline.
What does K/A mean? Glucose only.
What does A/A mean? Glucose + lactose/sucrose.
What does K/K mean? No fermentation.
What indicates gas in TSIA? Cracks/lift.
What indicates H₂S? Black precipitate.
Why is glucose low in TSIA? Detect glucose-only fermenters.
What is citrate test? Use citrate as carbon source.
Positive citrate? Blue.
Negative citrate? Green.
Answers Latest Update 2026
What are the three main types of culture media? Nutritive, selective, and differential.
What is the purpose of nutritive media? Supports growth of many microorganisms.
What is selective media? Inhibits some microbes while allowing others.
Distinguishes organisms based on visible
What is differential media?
changes.
What makes media selective? Salt, dyes, antibiotics.
What makes media differential? Substrates + pH indicators.
How do pH indicators work? Change color with acid/base.
What does acid production indicate? Fermentation.
What is Mannitol Salt Agar (MSA)? Selective and differential medium.
Why is MSA selective? High salt.
Why is MSA differential? Mannitol + phenol red.
What color = mannitol fermentation? Yellow.
What organisms grow on MSA? Halotolerant bacteria.
What is a halotolerant organism? Tolerates high salt.
What is EMB agar? Selective and differential medium.
How is EMB selective? Dyes inhibit Gram+.
How is EMB differential? Lactose fermentation.
What does E. coli look like on EMB? Green metallic sheen.
What is MacConkey agar? Selective and differential medium.
What does MAC select for? Gram− bacteria.
,What inhibits Gram+ on MAC? Bile salts + crystal violet.
What does MAC differentiate? Lactose fermentation.
What color = lactose fermenter on MAC? Pink/red.
What is blood agar? Differential medium.
What does blood agar detect? Hemolysis.
What is hemolysis? RBC breakdown.
What is alpha hemolysis? Green partial.
What is beta hemolysis? Clear complete.
What is gamma hemolysis? No change.
What causes hemolysis? Hemolysins.
What is the catalase test? Detects catalase enzyme.
What does catalase do? Breaks H₂O₂ → bubbles.
Catalase + result? Bubbles.
Catalase − result? No bubbles.
Staph catalase result? Positive.
Strep catalase result? Negative.
What are staphylococci? Gram+ cocci clusters.
What are streptococci? Gram+ cocci chains.
Key difference staph vs strep? Catalase test.
What is coagulase? Clots plasma.
Which bacteria is coagulase+? Staphylococcus aureus.
What is a virulence factor? Trait that increases pathogenicity.
What pigment does S. aureus make? Staphyloxanthin.
What toxin causes beta hemolysis in S. aureus? Alpha toxin.
Why can staph grow on MSA? Halotolerant.
, Which staph ferments mannitol? S. aureus.
Which does not? S. epidermidis.
What is bacitracin test? Differentiates strep groups.
Bacitracin sensitive? Group A.
Which is Group A? Streptococcus pyogenes.
Which is Group B? Streptococcus agalactiae.
What is a dichotomous key? Stepwise ID tool.
What is an indicator organism? Signals contamination.
What is a coliform? Gram− lactose fermenter.
Why are coliforms important? Indicate fecal contamination.
What family are enterics? Enterobacteriaceae.
Key traits of Enterobacteriaceae? Gram−, ferment glucose.
What is TSIA? Differential medium.
What does TSIA test? Sugar fermentation, gas, H₂S.
Sugars in TSIA? Glucose, lactose, sucrose.
pH indicator in TSIA? Phenol red.
Yellow in TSIA? Acid.
Red in TSIA? Alkaline.
What does K/A mean? Glucose only.
What does A/A mean? Glucose + lactose/sucrose.
What does K/K mean? No fermentation.
What indicates gas in TSIA? Cracks/lift.
What indicates H₂S? Black precipitate.
Why is glucose low in TSIA? Detect glucose-only fermenters.
What is citrate test? Use citrate as carbon source.
Positive citrate? Blue.
Negative citrate? Green.
