Lab Exam 2 Review

What does the Oxidase Test detect?

The enzyme cytochrome c oxidase.

What are the three test mediums used in the IMViC series?

SIM media, MR-VP broth, and Citrate media.

What type of bacteria are members of the Enterobacteriaceae family?

They are a diverse group of Gram-negative bacilli.

What does gamma-hemolysis look like on TSAb plates?

No color change around colonies, indicating red blood cells are not broken down.

What do Solutions A and B detect in the Nitrate Test?

They detect nitrite (NO₂), turning the media red.

What is the function of differential media?

To distinguish between different types of microorganisms.

What is the purpose of the Bile Esculin Hydrolysis test?

To identify Group D (Enterococcus species and S. bovis) organisms from other Streptococcus groups.

Are Staphylococci catalase-positive or catalase-negative?

Catalase-positive.

What does it mean if the media turns red after adding Solutions A and B in the Nitrate Test?

The bacteria produce nitrate reductase and reduce nitrate to nitrite.

What does a red layer after adding Kovac's reagent in a SIM test signify?

An indole-positive result, indicating tryptophanase hydrolyzed tryptophan.

What does the Oxidase Test differentiate?

Gram-negative aerobic/microaerophilic rods and cocci (+) from Gram-negative facultative anaerobic rods (-).

What is the role of methylene blue in EMB agar?

It inhibits the growth of Gram-positive bacteria.

What is phenol red's function in MSA?

It serves as a pH indicator.

How does phenylethyl alcohol inhibit Gram-negative bacteria?

By interfering with their DNA synthesis.

What indicates a negative result in the Bile Esculin Hydrolysis test?

No color change, indicating Streptococcus species.

What enzyme can enteric bacteria have that reduces nitrate?

The enzyme nitrate reductase.

What does a yellow color on an MSA plate indicate?

Mannitol is fermented, suggesting a potential pathogenic Staphylococcus species.

What color does MSA media turn if mannitol is fermented?

Yellow.

What is the purpose of the Catalase Test?

To differentiate Staphylococcus and Micrococcus from Streptococcus and Enterococcus.

What color indicates a positive indole test in the top layer of SIM media?

The top layer will appear red.

What happens to Streptococcus species in the 6.5% NaCl test?

They fail to grow at this salt concentration.

What color does the reagent strip turn in the presence of cytochrome c oxidase?

The strip is oxidized and turns purple/blue.

What is Mannitol Salt Agar (MSA) used for?

It is a selective and differential medium used for Staphylococcus species.

What enzyme degrades hydrogen peroxide to water and oxygen gas?

Catalase.

What type of bacteria does Eosin Methylene Blue (EMB) agar select for and differentiate?

Gram-negative bacteria.

What color does the Methyl Red indicator turn at a pH of 6 or greater?

Yellow-orange, indicating a Methyl Red negative result.

What indicates a positive result in the Coagulase Test?

The plasma becomes solid after incubation.

What reagent is added to detect indole production after incubation in the indole test?

Kovac's reagent (para-dimethylaminobenzaldehyde and amyl alcohol).

What indicates a positive result in the Bile Esculin Hydrolysis test?

Dark brown color, indicating Enterococcus species.

Are Streptococci catalase-positive or catalase-negative?

Catalase-negative.

What enzyme is necessary for organisms to convert tryptophan to indole, pyruvic acid, and ammonia?

Tryptophanase.

What characteristics do Streptococcus and Enterococcus share in a Gram stain and Catalase test?

Both are Gram-positive cocci in pairs/chains and are catalase-negative.

What serves as the final electron acceptor in anaerobic respiration during nitrate reduction?

Nitrate serves as the final electron acceptor instead of oxygen.

What does the indole test determine?

If an organism can degrade tryptophan and produce indole.

What indicates a positive result in the Catalase Test?

The emission of bubbles (oxygen gas).

What is the purpose of using selective media?

To inhibit the growth of certain organisms that may be present in a sample.

What enzyme do organisms need to utilize citrate?

Citrate permease.

What does the acronym IMViC stand for?

Indole, Methyl Red, Voges-Proskauer, and Citrate utilization.

What is the purpose of zinc dust in the Nitrate Test?

To chemically reduce nitrate to nitrite.

What does blackening of the media in a SIM test indicate?

H₂S was produced, indicating a positive result for sulfur reduction.

What is the primary purpose of the IMViC series of tests?

To differentiate members of the Enterobacteriaceae family.

How does MSA selectively inhibit certain organisms?

Its 7.5% NaCl concentration inhibits all but salt-tolerant Staphylococcus species.

What does a metallic green sheen on EMB agar indicate?

Vigorous lactose and/or sucrose fermentation, typical of fecal coliforms like E. coli.

What does nitrate reductase reduce nitrate (NO₃) to?

Nitrite (NO₂).

What type of bacteria does Phenylethyl Alcohol agar (PEA) select for?

Gram-positive bacteria.

What color does the Methyl Red indicator turn at a pH below 4?

Red, indicating a Methyl Red positive result.

What does beta-hemolysis look like on TSAb plates?

A clear zone around colonies, indicating complete breakdown of red blood cells.

What is the purpose of the Coagulase Test?

To differentiate Staphylococcus aureus from S. epidermidis and other coagulase-negative species.

What does a red color on an MSA plate indicate?

The organism does not ferment mannitol and is a non-pathogenic Staph species.

What does the Nitrate Reductase test determine?

If an organism can reduce nitrate through anaerobic respiration.

What is the role of mannitol in MSA?

It is a fermentable carbohydrate used to detect acid-producing Staphylococcus species.

What does the Citrate Test determine?

If an organism can utilize citrate as its sole carbon and energy source.

What indicates a positive result in the 6.5% NaCl test?

Turbid media, indicating growth (typical of Enterococcus species).

What does alpha-hemolysis look like on TSAb plates?

A brownish-green zone around colonies, indicating incomplete breakdown of red blood cells.

What can Gram-negative bacteria ferment on EMB agar?

Lactose and/or sucrose.

What does no growth on an MSA plate indicate?

The organism is inhibited by the 7.5% NaCl concentration.

What does 'No Blackening of media' indicate in a SIM reaction?

No H₂S was produced (negative for sulfur reduction).

What are the products of the Mixed Acid fermentation detected by Methyl Red?

Ethanol and a mixture of stable acids.

What does growth on an MSA plate indicate?

The organism is a salt-tolerant Staphylococcus species.

What indicates a positive result for the Citrate Test?

Growth on the slant and a blue color change.

What does a red color after zinc dust addition indicate in the Nitrate Test?

Nitrate was NOT reduced by the organism (zinc reduced it instead).

What does beige broth after zinc dust addition indicate in the Nitrate Test?

Nitrate was reduced beyond nitrite to other nitrogenous compounds.

When will zinc dust not turn the media red (remaining beige)?

If nitrate is no longer present in the media.

What causes the Nitrate Test media to remain beige after adding zinc dust?

The organism reduced nitrate beyond nitrite to compounds like N₂.

In the Nitrate Test, what does red color after zinc dust mean?

The organism is negative for nitrate reduction.

In the Nitrate Test, what does beige broth after zinc dust mean?

The organism is positive for nitrate reduction beyond nitrite.

Why is mineral oil added to decarboxylase media?

To generate an anaerobic environment.

Under what conditions is amino acid decarboxylase activity optimal?

An acidic environment.

What color does the pH indicator turn in a positive decarboxylase test?

Purple.

What is the purpose of the Phenylalanine Deaminase test?

To identify organisms that possess the enzyme phenylalanine deaminase.

What enzyme removes the amino group (—NH₂) from an amino acid?

Deaminase (e.g., phenylalanine deaminase).

What is added to the medium after incubation in the Phenylalanine Deaminase test?

10-12% ferric chloride (FeCl₃).