Lab Exam 2 Review

Flashcards from lecture notes.

E. Coli

produces gas by fermenting a carb.

Durham tube

A tube used to detect gas production in bacterial cultures; a positive result is indicated by an air bubble trapped inside the inverted tube.

Phenol red

Indicates if a bacteria can ferment a carbohydrate, resulting in acid production and a color change to yellow below pH 7.

Salmonella

Does not produce gas, no bubble in Durham tube, no fermentation, no change in pH.

Bacteria

Can grow on media and all have different reactions.

MacConkey agar

Selective and differential agar used to differentiate gram-negative enteric bacteria.

Selective property of MacConkey agar

Gram-negative bacteria grow, gram-positive do not.

Gram-negative lactose fermenters

Grow on MacConkey agar and ferment lactose, resulting in pink colonies.

S. epidermidis

Do not grow on MacConkey agar.

EMB (eosin methylene blue)

Selective and differential agar, with metallic green indicating rapid lactose fermentation.

Selective property of EMB

Gram-negative bacteria grow.

E. coli and Enterobacter

Rapid lactose fermenters, resulting in metallic green colonies on EMB.

P. aeruginosa

Non-lactose fermenters, resulting in not pink colonies on EMB.

Nitrate Reduction test

Used to differentiate the ability to reduce nitrate.

Naphthylamine & Sulfanilic Acid

Reagents added to detect nitrite after nitrate reduction.

Zinc (Zn++)

If no color change after adding reagents,____ is added to mimic the enzyme reaction.

Nitrate reductase

Enzyme: NO3 (Nitrate) -> NO2 (Nitrite)

Nitrite reductase

Enzyme: NO2 (Nitrite) -> NH3 (Ammonia)

E. Coli

Has nitrate reductase, converting nitrate to nitrite and turns red immediately.

Alicaligenes

Nothing happens until zinc is added, mimicking nitrate reductase and turning red synthetically.

Pseudomonas

Has both nitrate and nitrite reductase, converting nitrate to ammonia, never turning red.

TSI (Triple Sugar Iron) agar

Used for gram-negative enterics, tests ability to ferment sugars and produce H2S.

Yellow slant/butt in TSI

Fermenters (E. coli).

Red slant/butt in TSI

Non-fermenters (Alicaligenes).

FeS + H2

A H2S producer (Salmonella), black precipitate.

Urease Broth

Used to differentiate ability to rapidly hydrolyze urea.

Proteus

Only bacteria that make urease; positive >7 Purple presence of ammonia.

E. coli

No urease, negative <7 yellow.

SIM (Sulfide Indole Motility)

Used to identify ability to produce indole, reduce sulfur, and determine motility.

Kovacs reagent

Reagent added to SIM to produce a cherry red band if positive.

Klebsiella

Negative for indole.

Salmonella

Positive for H2S production.

Methyl Red (MR)

Used to differentiate based on usage of glucose.

E.coli

Positive (cherry red acidic).

Enterobacter

Negative (orange alkaline).

Simmons Citrate Agar Slant

Used to differentiate gram-negative enteric bacilli based on ability to utilize citrate as a carbon source; positive changes from green to blue.

Blood Agar Plate with Bacitracin

Used to identify Group A beta hemolytic strep (S. pyogenes).

Beta B

Complete RBC hemolysis (S. pyogenes).

Gammar

No hemolysis (Enterococcus).

Mannitol Salt Agar Plate

Often used to differentiate pathogenic Staph loccus epidermidis from non-pathogenic.

Growth + Yellow media

Salt tolerant & mannitol fermentation (S. aureus)

No growth

Not salt tolerant (S. epidermidis).

DNase Plate

Used to determine DNA hydrolysis; clearing zones indicate a positive result.

Catalase Test; positive (S. aureus).

Breaks down peroxide into water & no oxygen; bubbles

Lack of bubbles

Catalase negative (enterococcus).