microbio practical 2 TCC SE Dr. Brookins

What is the interpreted result if bubbles form immediately after catalase reagent (hydrogen peroxide) is added to growth?

Catalase positive

What is the interpreted result if NO bubbles form after hydrogen peroxide is added to growth?

Catalase negative

What gas forms the bubbles seen in a positive catalase test?

Oxygen

Which products result from catalase breaking down hydrogen peroxide?

Water and oxygen gas

Which genus is catalase NEGATIVE?

Streptococcus

Which pair of genera is generally catalase POSITIVE?

Staphylococcus and Micrococcus

A Gram-positive coccus is catalase positive. What does this result support?

Staphylococcus or Micrococcus

A Gram-positive coccus is catalase negative. What does this result support?

Streptococcus

In the catalase slide test what is tested?

Growth placed on a microscope slide

How is the catalase slant test performed?

Add 2-3 drops of reagent to growth on an incubated nutrient agar slant

A positive catalase result alone is best described as:

Supporting but not independently proving an identification

What is the interpreted result if the oxidase test turns blue or blue-purple within the valid reading time?

Oxidase positive

What is the interpreted result if there is no color change on the oxidase test?

Oxidase negative

Which enzyme does the oxidase test detect?

Cytochrome c oxidase

Within what time must a direct paper or swab oxidase test be read?

0 to 3 minutes

Within what time must an impregnated oxidase strip be read?

Within the first minute

A blue color appears on an oxidase test after the stated time has passed. How should it be interpreted?

Possible false positive; do not accept it

Which is a disadvantage of the direct oxidase test?

It kills the tested culture

Which oxidase method preserves the original culture?

Indirect test on bibulous paper a strip or a swab

Which organism is an expected oxidase POSITIVE example?

Pseudomonas aeruginosa

Which organism is an expected oxidase POSITIVE example?

Alcaligenes faecalis

Which organism is an expected oxidase NEGATIVE example?

Escherichia coli

What is the interpreted result if rabbit plasma forms a clot or solidifies in a coagulase test?

Coagulase positive

What is the interpreted result if rabbit plasma remains liquid in a coagulase test?

Coagulase negative

Which organism is the key coagulase-positive Staphylococcus?

Staphylococcus aureus

A coagulase-positive result best supports identification as:

S. aureus

Why is mannitol salt agar (MSA) selective?

High salt concentration favors salt-tolerant organisms

What does MSA differentiate?

Mannitol fermentation

What is the interpretation of growth with YELLOW medium on MSA?

Salt tolerant and ferments mannitol

What is the interpretation of growth with RED or PINK medium on MSA?

Salt tolerant but no mannitol fermentation

What is the interpretation of NO growth on MSA?

Inhibited by salt

What does a novobiocin disk test differentiate?

S. epidermidis from S. saprophyticus

Which species is usually novobiocin SUSCEPTIBLE?

Staphylococcus epidermidis

Which species is usually novobiocin RESISTANT?

Staphylococcus saprophyticus

What does a clear zone around a novobiocin disk indicate?

Inhibition of growth (susceptible)

What does growth right up to a novobiocin disk indicate?

Resistance

In what unit is the novobiocin zone of inhibition measured?

Millimeters

A catalase-positive coccus is coagulase negative grows with yellow medium on MSA? Which test best separates S. epidermidis from S. saprophyticus?

Novobiocin susceptibility

Which hemolysis produces green or brown discoloration around colonies on blood agar?

Alpha hemolysis

Which hemolysis produces a clear zone around colonies on blood agar?

Beta hemolysis

Which hemolysis shows no visible change in the blood agar?

Gamma hemolysis

Alpha hemolysis represents what kind of red blood cell breakdown?

Incomplete

Beta hemolysis represents what kind of red blood cell breakdown?

Complete

How can you better see clearing or discoloration on blood agar?

Hold the plate toward a light source

What is the interpreted result if the medium blackens on a bile esculin slant?

Bile esculin positive (esculin hydrolyzed)

What is the interpreted result if a bile esculin slant remains unchanged?

Bile esculin negative

A positive bile esculin test supports identification as:

Enterococcus

What is the interpreted result if there is growth or turbidity in 6.5% NaCl broth?

Salt tolerant; supports Enterococcus

What is the interpreted result if there is NO growth in 6.5% NaCl broth?

Not salt tolerant; consistent with Streptococcus

Which test helps distinguish Enterococcus from Streptococcus?

Growth in 6.5% NaCl broth

Which set of tests is used for Streptococcus/Enterococcus identification in Exercise 25?

Hemolysis bile esculin salt tolerance low-temperature growth tellurite growth

What is the interpreted result if there is growth on a plate incubated at 10 degrees Celsius?

Low-temperature growth positive; supports Enterococcus

What is the interpreted result if there is NO growth on a plate incubated at 10 degrees Celsius?

Low-temperature growth negative; consistent with Streptococcus

What is the interpreted result if an organism grows on tellurite medium?

Tellurite tolerant (tellurite growth positive)

What is the interpreted result if an organism does NOT grow on tellurite medium?

Tellurite growth negative

What three things does SIM medium test?

Sulfur reduction indole production and motility

What is the interpreted result if a black precipitate forms in SIM?

H2S production positive

What is the interpreted result if there is no blackening in SIM?

H2S negative

What is the interpreted result if a pink-red layer forms after adding Kovac reagent to SIM?

Indole positive

What is the interpreted result if a yellow layer forms after adding Kovac reagent to SIM?

Indole negative

Which enzyme produces indole from tryptophan?

Tryptophanase

Which reagent is added to SIM to detect indole?

Kovac reagent

What is the interpreted result if growth spreads away from the stab line in SIM?

Motility positive

What is the interpreted result if growth is confined to the stab line in SIM?

Motility negative

Why should motility be read before adding Kovac reagent to SIM?

Reagent can obscure the original appearance

A SIM tube shows black precipitate a pink-red layer after Kovac reagent and growth spreading away from the stab line. How is it read?

H2S positive indole positive motility positive

A SIM tube shows no blackening a yellow layer after Kovac reagent and growth confined to the stab line. How is it read?

H2S negative indole negative motility negative

What is the interpreted result if the broth turns red after adding methyl red reagent?

Methyl red positive (stable acid from glucose fermentation)

What is the interpreted result if the broth is yellow after adding methyl red reagent?

Methyl red negative

What does a positive methyl red test indicate?

Stable acidic products from glucose fermentation

What is the interpreted result if the broth turns red after adding VP A and VP B?

Voges-Proskauer positive

Which reagents are used in the Voges-Proskauer test?

Alpha-naphthol and potassium hydroxide

Why is the VP tube gently agitated after reagents are added?

To supply oxygen

What is the interpreted result if the VP tube does not turn red?

Voges-Proskauer negative

What is the interpreted result if Simmons citrate medium turns blue with growth?

Citrate positive

What is the interpreted result if Simmons citrate remains green with little or no growth?

Citrate negative

Which medium tests use of citrate as the sole carbon source?

Simmons citrate

After adding nitrate reagents A and B the broth turns red immediately. What is the result?

Nitrate reductase positive; testing is complete

After adding nitrate reagents A and B there is NO color. What is the next step?

Add zinc dust

After adding zinc the broth turns red. What is the result?

Nitrate reductase negative

After adding zinc there is NO color. What is the result?

Positive: nitrate reduced beyond nitrite

What does zinc dust do in the nitrate test?

Chemically reduces remaining nitrate to nitrite

What do nitrate reagents A and B detect?

Nitrite

Which statement about nitrate results is correct?

Red after zinc is the negative result

What role does nitrate play in the nitrate reductase test?

Electron acceptor

Nitrate reduced to products beyond nitrite is recorded as:

Positive nitrate reduction

What is the interpreted result if a decarboxylase broth is purple after growth?

Decarboxylase positive

What is the interpreted result if a decarboxylase broth is yellow after growth?

Decarboxylase negative

A purple decarboxylase broth indicates formation of what?

An alkaline amine

A yellow decarboxylase broth indicates what?

Acidic fermentation products

Which pH indicator is in decarboxylase media?

Brom cresol purple

What is the purpose of mineral oil in decarboxylase broth?

Creates reduced-oxygen conditions

Lysine decarboxylation produces which product?

Cadaverine

Ornithine decarboxylation produces which product?

Putrescine

What three things does MIO medium test?

Motility indole and ornithine decarboxylase

What is the interpreted result if growth spreads away from the stab line in MIO?

Motility positive

What is the interpreted result if growth is confined to the stab line in MIO?

Motility negative

What is the interpreted result if a red or pink-red surface layer forms after Kovac reagent in MIO?

Indole positive

In MIO in what order should results be read?

Motility and ornithine before adding Kovac reagent

A purple MIO medium (reduced oxygen) indicates what?

Ornithine decarboxylase positive