Microbio Midterm Pos/Neg

Methyl red: positive result

Red color

Methyl red: negative result

No change (stays yellow)

Methyl red: how many drops of reagent?

4 drops

Methyl red: what does a positive result mean?

Organism does mixed-acid fermentation of glucose and makes a stable strong acid

Voges-Proskauer: positive result

Red/pink on top of the solution

Voges-Proskauer: negative result

Remains yellow

Voges-Proskauer: reagents used

15 drops Barritt's A and 5 drops Barritt's B

Voges-Proskauer: procedure after adding reagents

Swirl to mix thoroughly then rest 10-20 minutes before reading

Voges-Proskauer: what does a positive result mean?

Organism uses the butanediol pathway and makes acetoin

Nitrate reduction: gas bubble in Durham tube

Positive - nitrate reduced to nitrogen gas

Nitrate reduction: no gas then red after reagents A and B

Positive - nitrate reduced to nitrite

Nitrate reduction: no gas then no red after A and B then no red after zinc

Positive - nitrate reduced past nitrite (to ammonia or other nitrogenous product)

Nitrate reduction: no gas then no red after A and B then red after zinc

Negative - nitrate was not reduced (zinc converted the leftover nitrate to nitrite)

Nitrate reduction: when do you add zinc powder?

Only when there is no gas and no red color after reagents A and B

Nitrate reduction: why is red after zinc a NEGATIVE?

Zinc reduces nitrate to nitrite - so nitrate was still present and the organism never reduced it

Glucose fermentation: yellow broth

Positive - acid produced (fermentation)

Glucose fermentation: red/pink broth

Negative - no fermentation

Glucose fermentation: bubble in Durham tube

Gas produced

Glucose fermentation: no bubble in Durham tube

No gas produced

Lactose fermentation: reddish pink broth

Negative - no fermentation

Lactose fermentation: yellow broth

Positive - acid produced (fermentation)

Lactose fermentation: bubble in Durham tube

Gas produced

Lactose fermentation: no bubble in Durham tube

No gas produced

Mannitol fermentation: yellow

Positive - acid produced (fermentation)

Mannitol fermentation: red/pink

Negative - no fermentation

Carbohydrate fermentation: what does yellow mean?

Acid produced - the organism fermented the sugar

Carbohydrate fermentation: what does a bubble mean?

Gas was produced

Citrate slant: positive result

Fully or mostly blue

Citrate slant: negative result

Green (or only a little blue)

Citrate test: what does a positive result mean?

Organism uses citrate as its only carbon source

Phenylalanine deaminase: reagent

5 drops of 10% ferric chloride (FeCl3)

Phenylalanine deaminase: positive result

Green color change

Phenylalanine deaminase: negative result

No change (no green)

Indole: reagent

5 drops of Kovac's reagent

Indole: positive result

Red on top of the solution

Indole: negative result

No change

Indole: what does a positive result mean?

Organism breaks down tryptophan using tryptophanase

Urease: positive result

Turns pink

Urease: negative result

No color change (remains orange)

Urease: what does a positive result mean?

Organism breaks urea down to ammonia

Starch hydrolysis plate: what do you add before reading?

Iodine

Starch hydrolysis plate: positive result

Clear border around the bacterial swipe

Starch hydrolysis plate: negative result

No border (iodine stains the whole plate)

Starch hydrolysis: what enzyme does a positive show?

Amylase - the organism digested starch

Casein hydrolysis (milk plate): positive result

Clear border around the bacterial swipe

Casein hydrolysis (milk plate): negative result

No border

Casein hydrolysis: how do you see the result?

Look at the plate under the colony counter light if needed

Casein hydrolysis: what enzyme does a positive show?

Caseinase (protease) - the organism digested milk protein

Gram stain: purple cells

Gram-positive

Gram stain: pink cells

Gram-negative

Motility agar: positive result

Turbidity extending beyond the stab line

Motility agar: negative result

Growth only along the stab line

Motility agar: uninoculated control

Stays clear with no turbidity

Soft agar: growth only at the top

Obligate aerobe

Soft agar: growth just below the surface

Microaerophile

Soft agar: growth throughout but thicker at the top

Facultative anaerobe

Soft agar: even growth throughout the tube

Aerotolerant anaerobe

Soft agar: no growth at the surface and growth at the bottom

Obligate anaerobe

Kirby-Bauer: positive sign of antibiotic effectiveness

Clear zone of inhibition around the disk

Kirby-Bauer: no zone around the disk

Organism is resistant to that antibiotic

Antiseptic/disinfectant disk test: effective agent

Zone of inhibition around the disk (measured in mm)

Which tests use a color change to the broth or slant itself?

Methyl red, citrate, urease, carbohydrate fermentation, phenylalanine deaminase (after FeCl3)

Which tests need a reagent added after incubation?

Methyl red, VP, nitrate (A + B then zinc), phenylalanine deaminase, indole, starch (iodine)

Which test needs no reagent and is read under the counter light?

Casein hydrolysis

Which tests show a RED positive?

Methyl red, VP (red/pink on top), indole (red on top), nitrate (red after A and B)

Which tests show a clear zone or border as positive?

Starch hydrolysis, casein hydrolysis, Kirby-Bauer zone of inhibition

Which tests show a GREEN positive?

Phenylalanine deaminase (green = positive) - but for citrate green = negative

Which tests show a BLUE positive?

Citrate slant

Which tests show a PINK positive?

Urease (pink = positive) - but for carbohydrate fermentation pink/red = negative