Lab Midterm Flash questions

What are the components of a light microscope?

Eyepiece, nose piece, stage, body, light, base.

What are the magnification levels of different microscope lenses?

• 4x = Scanning lens

• 10x = Low power objective lens

• 40x = High power objective lens

• 100x = Oil immersion lens

What is the purpose of simple staining?

To visualize bacterial morphology using a single dye

What are the reagents and steps of Gram staining?

• Crystal violet (1 min, primary stain)

• Iodine (1 min, mordant)

• Alcohol (10-15 sec, decolorizer)

• Safranin (1 min, counterstain)

How do Gram-positive and Gram-negative bacteria appear after Gram staining?

• Gram-positive: Purple (thick peptidoglycan layer)

• Gram-negative: Pink (thin peptidoglycan layer)

What is the main characteristic of a negative stain?

Stains the background, not the cells; preserves shape and size.

What is the purpose of an endospore stain?

Identifies bacterial endospores, which resist heat and chemicals.

What is the reagent order for an acid-fast stain?

• Carbol fuchsin + steam (5 min, primary stain)

• Acid-alcohol (10 sec, decolorizer)

• Methylene blue (1 min, counterstain)

What does Mannitol Salt Agar (MSA) select and differentiate for?

• Selective for: Staphylococcus (high salt tolerance)

• Differential for: Mannitol fermentation

  • Positive: Yellow (S. aureus)

  • Negative: Pink/red

How does E. coli appear on MacConkey Agar?

Pink (lactose fermentation positive).

What is the function of Fluid Thioglycollate Broth?

Tests oxygen sensitivity of bacteria using sodium thioglycolate to remove O₂.

What are the results of an endospore stain?

• Positive: Green endospores inside red cells (e.g., Bacillus cereus)

• Negative: No green spores (e.g., E. coli, S. aureus)

What is a capsule stain used for?

Identifies bacterial capsules (virulence factor). Capsules appear as a clear halo around cells.

What are the reagents used in a capsule stain?

Congo red or India ink (stains background), Crystal violet (counterstain).

How does Klebsiella pneumoniae appear in a capsule stain?

Capsule positive – clear halo around the cell.

What is the purpose of DNase agar?

Tests the ability of bacteria to produce DNase, which breaks down DNA.

How do Gram-positive and Gram-negative bacteria appear on DNase agar?

• Gram-positive: Blue → Pink (DNase positive)

• Gram-negative: Blue → Blue (DNase negative)

What does starch agar test for?

Ability to produce alpha-amylase, which breaks down starch.

How do bacteria appear on starch agar after iodine is added?

• Positive (starch breakdown): Clear zone around colony

• Negative (no starch breakdown): Bluish/orange color

What is Chocolate Agar used for?

Enriched medium for fastidious bacteria like Neisseria and Haemophilus.

What are the results of E. coli and S. aureus on Chocolate Agar?

Both can grow, but do not require Chocolate Agar.

What does Eosin Methylene Blue (EMB) Agar test for?

• Selective for: Gram-negative bacteria

• Differential for: Strong lactose fermenters

How does E. coli appear on EMB agar?

Green metallic sheen (strong lactose fermenter).

How does S. aureus appear on EMB agar?

No growth (Gram-positive bacteria are inhibited).

What does Xylose Lysine Deoxycholate (XLD) agar differentiate?

Identifies Salmonella and Shigella species.

How do different bacteria appear on XLD agar?

• Shigella: Red colonies (uses lysine).

• Salmonella: Red with black center (ferments xylose, then lysine).

• E. coli: Yellow (ferments sucrose/lactose).

What is the function of resazurin in Fluid Thioglycollate Broth?

It is a redox indicator that turns pink in the presence of oxygen.