Biochemical Methods of Identification

Catalase test

This test differentiates Staphylococcus from Streptococcus. What is the name of this test?

Coagulase test

This test differentiates Staphylococcus aureus from coagulate-negative Staphylococcus. What is the name of this test?

Free coagulase

The coagulase tube test detects:

Bound coagulase

The coagulase slide test detects:

Novobiocin test

This test differentiates Staphylococcus saprophyticus from coagulate-negative Staphylococcus. What is the name of this test?

• S. aureus

• M. catarrhalis

• S. marcescens

Which organisms are DNAse positive?

Mannitol Salt Agar (MSA)

What is the standard selective and differential medium for Staphylococcus species?

7.5%

Mannitol Salt Agar (MSA) contains a high concentration of salt, specifically:

Staphylococcus aureus

A microbiology student inoculates a Mannitol Salt Agar (MSA) plate with a clinical isolate. After 24 hours of incubation at 37°C, the colonies appear golden-yellow, and the surrounding medium has changed from red to yellow. Which of the following organisms is most likely responsible for these observations?

Streptococcus pyogenes

A throat culture from a 7-year-old patient with suspected streptococcal pharyngitis is plated on a blood agar plate. Beta-hemolytic colonies grow and are tested for bacitracin and sulfamethoxazole-trimethoprim (SXT) susceptibility. The organism is sensitive to bacitracin and resistant to SXT. Which of the following organisms is most likely?

Enterococcus faecalis

A gram-positive, catalase-negative, gamma-hemolytic organism is isolated from a urine culture. The isolate grows in 6.5% NaCl and hydrolyzes esculin in the presence of bile. A PYR test is positive. Which of the following organisms is most likely?

P-dimethylcinnamaldehyde

L-pyrrolidonyl-beta-napthylamide (PYR) is hydrolyzed by PYRase, forming beta-naphthylamide, which produces a red color with the addition of which color developer?

Arrowhead

In the CAMP test, what pattern results when S. agalactiae is streaked perpendicularly to beta-hemolytic S. aureus?

Streptococcus pneumoniae

In the presence of optochin, colonies of what specific bacteria are selectively lysed?

P disk

Optochin is also known as:

>10

When using a 10μg Optochin disk, a zone of ___ mm is considered positive.

≥14

When using a 6μg Optochin disk, a zone of ___ mm is considered positive.

Bile Esculin Agar (BEA)

Which agar/broth differentiates group D Streptococcus and Enterococcus from other Lancefield groups?

6.5% Salt Broth

Which agar/broth differentiates group D streptococci from non-enterococci?

• Moraxella catarrhalis

• Neisseria elongata

A clinical microbiology lab receives a gram-negative diplococcus isolated from a respiratory specimen. The isolate is oxidase positive but shows no acid production in CTA tubes containing glucose, maltose, sucrose, or lactose. Which of the following organisms is/are the most likely identification?

Neisseria meningitidis

A gram-negative oxidase-positive diplococcus is isolated from a cerebrospinal fluid sample. Sugar fermentation testing in CTA tubes shows positive acid production in glucose and maltose but negative in lactose and sucrose. Which of the following organisms is the most likely identification?

• Glucose/Maltose (+)

• Lactose/Sucrose (-)

Neisseria lactamica

A gram-negative diplococcus isolated from a throat swab is oxidase positive. CTA sugar tubes show acid production in glucose, maltose, and lactose, but not sucrose. Which species is this most consistent with?

• Glucose/Maltose/Lactose (+)

• Sucrose (-)

Neisseria sicca

An isolate from a sputum specimen is a gram-negative diplococcus and oxidase positive. Sugar fermentation results from CTA tubes show acid production in glucose, maltose, and sucrose, but not lactose. Which species is this most likely?

• Glucose/Maltose/Sucrose (+)

• Lactose (-)

Neisseria gonorrhoeae

A patient presents with urethritis, and a gram-negative diplococcus is isolated from a urethral swab. The organism is oxidase positive and produces acid only in the glucose CTA tube. What is the most probable organism?

• Glucose (+) only

Oxidase test

A good test to differentiate Enterobacteriaceae (fermenters) from non-fermenters is the:

The organism produces cytochrome oxidase

A microbiologist performs an oxidase test using tetra-methyl-paraphenylenediamine dihydrochloride on a bacterial isolate. The reagent changes from colorless to dark purple within 30 seconds. What does this indicate?

Indophenol blue

Cytochrome oxidase, in the presence of atmospheric oxygen, oxidizes tetra-methyl-paraphenylenediamine dihydrochloride to form a colored compound called:

The organism reduces nitrate to nitrite

A gram-negative rod isolated from a wound culture is tested for nitrate reduction. After incubation, sulfanilic acid and N,N-dimethyl-1-naphthylamine are added, producing a red color immediately. What is the best interpretation?

Negative for nitrate reduction

During nitrate testing, after adding reagents, no red color appears. Zinc dust is added, and the medium turns red. What does this indicate about the organism’s nitrate reduction ability?

Add zinc dust and observe no color change

Which of the following steps confirms a true positive nitrate reduction test when no initial color develops after reagent addition?

Glucose

Which of the following sugars is fermented by all members of the Enterobacteriaceae family?

ONPG test

Which biochemical test is most useful in differentiating Citrobacter species (positive) from most Salmonella species (negative)?

H2S production

Which biochemical test best differentiates Salmonella species from Shigella species?

The organism ferments lactose/sucrose, produces hydrogen sulfide (H₂S), and gas.

An unknown gram-negative rod is inoculated on Triple Sugar Iron (TSI) agar. After incubation, the following results are observed:

• Slant: Yellow

• Butt: Yellow with black precipitate

• Bubbles: Present

Which of the following interpretations is correct?

Glucose fermented only, no lactose or sucrose fermentation

An unknown gram-negative rod produces the following TSI result:

• Slant: Pink

• Butt: Yellow

• No gas or black precipitate

Which of the following interpretations is correct?

Salmonella

A TSI agar slant shows the following:

• Slant: Yellow

• Butt: Yellow

• Black precipitate present

• Gas bubbles present

Which genus is most likely based on these results?

The organism did not ferment glucose, lactose, or sucrose

A gram-negative rod produces a TSI reaction of K/K (pink slant/pink butt) with no gas or H₂S production. This indicates:

Glucose fermentation only with gas production

What does a TSI result of K/A (pink slant/yellow butt) with gas production suggest?

Escherichia coli

Which of the following organisms is MR positive and VP negative?

KESH group (Klebsiella, Enterobacter, Serratia, Hafnia)

A gram-negative rod produces a pink-red color after the addition of alpha-naphthol and 40% KOH in the Voges-Proskauer test, but shows a yellow color in the Methyl Red test. Which organism group does this most likely belong to?

Mixed acids (lactic, acetic, formic, succinic)

What metabolic end products are responsible for a positive Methyl Red test in E. coli?

Diacetyl (from acetoin)

In the Voges-Proskauer test, what compound is detected by the formation of a pink-red color after adding alpha-naphthol and KOH?

Klebsiella pneumoniae

An unknown lactose-fermenting gram-negative rod is inoculated onto Simmons citrate agar. After incubation, there is growth with an intense blue color on the slant. Which of the following organisms is most consistent with this result?

• Proteus

• Cryptococcus

• H. pylori

• Ureaplasma

• Nocardia

• Klebsiella

• S. epidermidis

• S. saprophyticus.

Urease-positive microorganisms:

Green

Phenylalanine deaminase deaminates phenylalanine to phenylpyruvic acid, which reacts with ferric chloride in order to produce what color?

• Shigella

• Klebsiella

These are the only nonmotile Enterobacteriaceae

Yersinia pestis

The Enterobacteriaceae that is nonmotile at 37°C but motile at 22°C is:

• Proteus

   Morganella

   Providencia rettgeri

Which are Urease-positive organisms?