BIOL 251 Microbiology Lab Exam Practice

3

Ocular lens

No. ___

Where you look through (10x magnification)

4-5

Objective lenses

No. ___ & ___

4x, 10x, 40x, 100x magnification.

11

Stage

No. ___

Holds the slide.

12

Coarse focus

No. ___

Big movements to focus (low power).

13

Fine focus

No. ___

Small adjustments (high power).

8

Illuminator

No. ___

Illuminates specimen.

7

Condenser

No. ___

Focuses light on specimen.

Adjusts light intensity.

No. ___

Adjusts light intensity.

Parfocal lens

Lens system that stays in focus when you change magnification.

Resolution

Magnification

_____: Ability to distinguish two points as separate.

_____: How much larger the image appears.

Objective × Ocular

How to calculate total magnification?

Reduces light refraction at 100x, increases resolution

What is the purpose of using immersion oil with a 100X objective?

Aseptic tecnique

Inoculation

Inoculum

_____ _____: Prevents contamination.

_____: Transferring microbes.

_____: The sample being transferred.

Quadrant streak method

Which method is used to obtain pure cultures?

Differential

Selective

_____ Media: focus on differentiating between multiple microbes

_____ Media: focus on isolating a specific microbe,

indicating hemolytic activity, allowing differentiation between bacterial species based on their ability to lyse red blood cells

What is the function of blood agar?

Alpha

Beta

Gamma

_____: Partial (green zone).

_____: Complete (clear zone).

_____: None.

Beta

Alpha

Gamma

_____ hemolysis: Total destruction of RBCs → clear, sharp zone around colonies.

_____ hemolysis: Partial breakdown → greenish or brown discoloration around colonies.

_____ hemolysis: No breakdown → media around colonies looks unchanged.

Mannitol

Yellow

Pink

How is MSA selective and differential?

• Selective for: Staphylococci (high salt).

• Differential for: _____ fermentation (yellow = positive).

• Interpretation:

  • _____: Ferments mannitol (e.g., S. aureus).

  • _____: Does not ferment (e.g., S. epidermidis).

yellow

no

MSA Interpretation

• Growth + _____ color: Positive for mannitol fermentation and tolerant to high salt.

  • Staphylococcus aureus

• Growth + _____ color change (pink/red): Salt-tolerant but does not ferment mannitol.

  • Staphylococcus epidermidis

• No growth: Not salt-tolerant (inhibited by high NaCl).

  • E. coli, Streptococcus spp.

negative

Lactose

How is EMB agar selective and differential?

Selective for: Gram-_____.

Differential for: _____ fermentation.

Strong

Weak

Positive

EMB Interpretation

• Dark purple to black colonies with green metallic sheen: _____ lactose fermenter – produces acid that precipitates dyes

  • E. coli

• Pink/purple colonies without metallic sheen: _____ lactose fermenter

  • Enterobacter aerogenes

• Colorless or transparent colonies: Non-lactose fermenter

  • Salmonella, Shigella

• No growth: Gram-_____ bacteria inhibited by dyes

  • Staphylococcus, etc.

negative

Lactose

Pink

Colorless

How is MacConkey agar selective and differential?

• Selective for: Gram-_____.

• Differential for: _____ fermentation.

• Interpretation:

  • _____ colonies: Lactose fermenters.

  • _____: Non-fermenters.

• Clinical Use: Detecting enteric pathogens.

Lactose

non

positive

MacConkey Interpretation

• Pink to red colonies: _____ fermenter – acid production lowers pH, turns colonies pink

  • E. coli, Enterobacter

• Colorless or pale colonies: _____-lactose fermenter – no acid, no color change

  • Salmonella, Shigella

• No growth: Gram-_____ bacteria inhibited by bile salts and crystal violet

  • Staphylococcus, etc.

30-300

Colony forming units

What is the standard colony range?

What is CFU?

CFU/mL = Colonies × (1 ÷ Dilution factor)

Kirby-Bauer

What is the name of the method used to test antimicrobial sensitivity?

gelatinase

Liquid, solid

Name of the enzyme that breaks down gelatin?

What do the positive and negative test results look like?

Clear zone, no clearing

Amylase

What do the positive and negative test results look like after a starch hydrolysis test?

Name of the enzyme that is involved in this test?

Clear zone, no clearing

Caseinase

What do the positive and negative test results look like after a casein hydrolysis test?

Name of the enzyme that is involved in this test?

Sugar fermentation & gas production

Detects gas production

Over-incubation can cause false negatives (due to reversion)

What is tested by the phenol red broth experiment?

What is the purpose of using Durham tube?

Why is incubation time critical in this experiment?

Glucose, lactose, sucrose fermentation; H₂S production.

Creates anaerobic area for glucose fermentation.

Black precipitate = anaerobic respiration.

What is tested by the triple-sugar iron test?

What is the importance of stabbing the butt during inoculation?

What does the production of hydrogen sulfide indicate in terms of respiration?

Casein (protein), lactose (sugar)

Fermentation, coagulation, peptonization, reduction

In the litmus milk test, what are the two main energy sources that bacteria utilize in milk?

Indole, Methyl Red, Voges-Proskauer, Citrate

What is IMViC in its full form?

Tryptophan breakdown, Tryptophanase

What is tested by the Indole test? Enzyme involved?

MR: Mixed acid fermentation

VP: Butanediol fermentation

What is tested by the MR & VP tests?

If bacteria use citrate as sole carbon source, citrate permease

What is tested by the Citrate test? Enzyme involved?

Urea → Ammonia + CO₂

Urease

Ammonia ↑ pH (turns media pink)

What is the basis of the urease test?

What is the name of the enzyme involved?

Urea hydrolysis releases ammonia – what happens to pH? Increase or Decrease?