Urine from a 30-year-old woman with cystitis grows more than 10^5 CFU/mL of one organism. On sheep blood agar the colonies are gray and beta-hemolytic; on MacConkey agar they are flat and pink with a pink halo. The laboratory's flowchart allows a presumptive report of Escherichia coli for a lactose-fermenting, oxidase-negative gram-negative rod that is spot indole positive. Oxidase on a blood agar colony is negative. The spot indole (DMACA) test, done on a pink colony from the MacConkey plate, shows no blue-green color. What should be done next?

Repeat spot indole on blood agar growth

Repeat spot indole on another MacConkey colony

Report E. coli on its beta-hemolysis alone

Report Klebsiella pneumoniae presumptively

Identification: colony morphology and rapid tests (coagulase, catalase, oxidase, indole, PYR)

25 ASCP MLS practice questions on Identification: colony morphology and rapid tests (coagulase, catalase, oxidase, indole, PYR), part of Microbiology. The first 25 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Hard
A 24-year-old man has a swollen, painful knuckle 3 days after punching another man in the mouth. Pus from the wound grows mixed flora and, at 48 hours, small gray colonies on sheep blood and chocolate agar in 5% CO2, some of which pit the agar. Nothing grows on MacConkey agar. Results for this gram-negative rod: nonhemolytic; oxidase positive; catalase negative; spot indole negative; ornithine decarboxylase positive; nitrate reduced. The most likely organism is:
1. ACapnocytophaga ochracea
2. BEikenella corrodens
3. CKingella kingae
4. DPasteurella multocida
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Answer: B · Eikenella corrodens
A clenched-fist injury carries human mouth flora into the joint, so oral organisms are expected. The oxidase-positive, catalase-negative result narrows the list to Eikenella and Kingella, because Pasteurella is catalase positive and the oral Capnocytophaga species are oxidase and catalase negative. Eikenella corrodens is nonhemolytic, ornithine decarboxylase positive and reduces nitrate, and about half of strains pit the agar; Kingella kingae is beta-hemolytic and ornithine negative. Eikenella needs CO2, grows slowly and often has a bleach-like odor. It is resistant to clindamycin, so the identification matters for therapy.
- A: Oral Capnocytophaga species are oxidase and catalase negative.
- C: Kingella kingae is beta-hemolytic and ornithine decarboxylase negative.
- D: Pasteurella multocida is catalase and indole positive and comes from animal bites.
Question 2 · Hard
A 58-year-old man with alcoholic cirrhosis is admitted 36 hours after eating raw oysters on a Gulf Coast trip. He is hypotensive and has hemorrhagic bullae on both legs. Blood cultures flag positive at 14 hours with curved gram-negative rods. Bench results: lactose-fermenting colonies on MacConkey agar; oxidase positive; spot indole positive; string test positive; growth in nutrient broth with 1% NaCl but none in broth without added NaCl; green colonies on TCBS agar. Which organism is most likely?
1. AAeromonas hydrophila
2. BVibrio cholerae non-O1
3. CVibrio vulnificus
4. DVibrio parahaemolyticus
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Answer: C · Vibrio vulnificus
An oxidase-positive, indole-positive fermenter with a positive string test points to a Vibrio, and failure to grow without added salt shows a halophilic species. That rules out Aeromonas and V. cholerae, which both grow in broth without NaCl; V. cholerae also ferments sucrose and forms yellow colonies on TCBS. Of the halophilic, sucrose-negative (green) vibrios, V. vulnificus ferments lactose, while V. parahaemolyticus does not. The setting is typical: raw oysters, liver disease and hemorrhagic bullae are the classic picture of V. vulnificus sepsis, which can kill within days. The result should be phoned to the clinician at once.
- A: Aeromonas grows without added salt and is usually string test negative.
- B: V. cholerae grows without added salt and forms yellow colonies on TCBS.
- D: V. parahaemolyticus is lactose negative and mainly causes gastroenteritis.
Question 3 · Hard
Urine from a 30-year-old woman with cystitis grows more than 10^5 CFU/mL of one organism. On sheep blood agar the colonies are gray and beta-hemolytic; on MacConkey agar they are flat and pink with a pink halo. The laboratory's flowchart allows a presumptive report of Escherichia coli for a lactose-fermenting, oxidase-negative gram-negative rod that is spot indole positive. Oxidase on a blood agar colony is negative. The spot indole (DMACA) test, done on a pink colony from the MacConkey plate, shows no blue-green color. What should be done next?
1. ARepeat spot indole on blood agar growth
2. BRepeat spot indole on another MacConkey colony
3. CReport E. coli on its beta-hemolysis alone
4. DReport Klebsiella pneumoniae presumptively
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Answer: A · Repeat spot indole on blood agar growth
Every finding except the indole fits E. coli, so the negative result should be questioned before it changes the identification. Colonies from MacConkey agar are unsuitable for the spot indole test, because the pink color of lactose fermenters masks the blue-green reaction and can give false negatives. The test should be repeated on growth from a medium without dyes, such as sheep blood agar, which supplies enough tryptophan. A positive result then supports the presumptive E. coli report, while a true negative would need a full identification. Beta-hemolysis supports E. coli but is not enough on its own.
- B: Any MacConkey colony gives the same unreliable color reaction.
- C: Beta-hemolysis supports E. coli but does not replace a valid indole result.
- D: The indole result is invalid, so it cannot be used to call the isolate Klebsiella.
Question 4 · Medium
Large beta-hemolytic colonies of a catalase-negative gram-positive coccus from a throat swab were tested with bacitracin and trimethoprim-sulfamethoxazole disks, as shown. What is the presumptive identification?
Figure: MedExams
1. AStreptococcus anginosus group
2. BStreptococcus agalactiae
3. CStreptococcus pyogenes
4. DGroup C or G streptococcus
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Answer: C · Streptococcus pyogenes
Any zone around the bacitracin (A) disk counts as susceptible, and there is a clear zone here, while there is no zone around the SXT disk. Bacitracin susceptible and SXT resistant is the pattern of Streptococcus pyogenes (group A). Group B is resistant to both, and groups C and G are bacitracin resistant but SXT susceptible. Because a few group B, C and G strains are bacitracin susceptible, the result is presumptive and is confirmed with PYR or Lancefield grouping.
- A: The S. anginosus group forms small colonies and is SXT susceptible.
- B: S. agalactiae is resistant to both bacitracin and SXT.
- D: Groups C and G are bacitracin resistant and SXT susceptible, the reverse of this plate.
Question 5 · Easy
An alpha-hemolytic, catalase-negative gram-positive coccus from the blood of a 70-year-old with pneumonia is tested with a 6-mm optochin disk in 5% CO2, as shown. The result supports:
Figure: MedExams
1. AEnterococcus faecalis
2. BStreptococcus pneumoniae
3. CAerococcus viridans
4. DViridans group streptococcus
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Answer: B · Streptococcus pneumoniae
The zone around the 6-mm optochin (P) disk measures about 20 mm against the 10-mm scale bar. With a 6-mm disk incubated in CO2, a zone of 14 mm or more is presumptive for Streptococcus pneumoniae, because optochin lyses pneumococci. Viridans streptococci are resistant or give zones under 14 mm. Zones of 7 to 13 mm need a bile solubility test before the isolate is called a pneumococcus.
- A: Enterococcus faecalis is optochin resistant.
- C: Aerococcus viridans is optochin resistant.
- D: Viridans streptococci are optochin resistant or give zones under 14 mm.
Question 6 · Medium
An oxidase test on Escherichia coli, done by rubbing the colony onto reagent-soaked filter paper with a nichrome wire loop, turns purple. The most likely cause is:
1. AIron in the loop oxidizing the reagent
2. BTesting a colony grown on blood agar
3. CReading the result within 10 seconds
4. DUsing reagent freshly prepared that day
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Question 7 · Hard
A swarming, oxidase-negative, lactose-negative gram-negative rod is H2S positive. Which single rapid test best separates Proteus vulgaris from Proteus mirabilis?
1. APhenylalanine deaminase
2. BRapid urease
3. CCatalase
4. DSpot indole
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Question 8 · Hard
A dog-bite wound grows beta-hemolytic, catalase-positive gram-positive cocci in clusters that are tube coagulase positive. Further tests are PYR positive and Voges-Proskauer negative. What is the most likely organism?
1. AStaphylococcus epidermidis
2. BStaphylococcus pseudintermedius
3. CStaphylococcus lugdunensis
4. DStaphylococcus aureus
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Question 9 · Easy
The catalase test separates staphylococci from streptococci. A positive result is seen as:
1. AA clot in rabbit plasma
2. BBubbles when hydrogen peroxide is added
3. CA purple color with oxidase reagent
4. DA red color with Kovac's reagent
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Question 10 · Medium
A suspected Staphylococcus aureus gives a negative slide coagulase test. Why should a tube coagulase test be done before reporting it as coagulase negative?
1. AThe slide test is positive only in MRSA
2. BThe slide test detects only free coagulase
3. CSome S. aureus strains lack clumping factor
4. DThe tube test gives results sooner
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Question 11 · Easy
Which bacterial enzyme does the oxidase test detect?
1. ATryptophanase
2. BCytochrome c oxidase
3. CCatalase
4. DPyrrolidonyl arylamidase
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Question 12 · Medium
A flat, dry, lactose-fermenting colony on MacConkey agar from a urine culture is an oxidase-negative gram-negative rod. Which rapid test result best supports a presumptive identification of Escherichia coli?
1. AA positive spot indole
2. BA positive PYR test
3. CA negative catalase test
4. DA positive rapid urease
Answer and explanation in LearnFree account
Question 13 · Medium
A nonhemolytic, catalase-negative gram-positive coccus in pairs and short chains is bile esculin positive and PYR negative. What is the most likely organism?
1. AEnterococcus faecalis
2. BStreptococcus agalactiae
3. CStreptococcus gallolyticus
4. DEnterococcus faecium
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Question 14 · Medium
Why is an oxidase test done before a gram-negative rod is set up in a kit or panel designed for Enterobacterales?
1. AIts database assumes oxidase-negative isolates
2. BThe oxidase result sets the kit's reading time
3. COxidase-positive rods do not grow in the kit
4. DThe oxidase reagent is also used in the kit
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Question 15 · Hard
A leg wound from a freshwater boating injury grows a beta-hemolytic gram-negative rod that also grows on MacConkey agar. It is oxidase positive, spot indole positive and DNase positive. Which organism is most likely?
1. AAeromonas hydrophila
2. BVibrio vulnificus
3. CPlesiomonas shigelloides
4. DPseudomonas aeruginosa
Answer and explanation in LearnFree account
Question 16 · Hard
An alpha-hemolytic, catalase-negative gram-positive coccus in pairs and short chains is vancomycin resistant. It is PYR negative and LAP negative, and it produces gas from glucose in MRS broth. Which organism is most likely?
1. AEnterococcus faecium
2. BViridans group streptococci
3. CLeuconostoc species
4. DPediococcus species
Answer and explanation in LearnFree account
Question 17 · Hard
A cat-bite wound grows small gray colonies on sheep blood and chocolate agar but nothing on MacConkey agar. The gram-negative coccobacillus is oxidase positive, catalase positive and spot indole positive. Which organism is most likely?
1. AEikenella corrodens
2. BHaemophilus influenzae
3. CCapnocytophaga canimorsus
4. DPasteurella multocida
Answer and explanation in LearnFree account
Question 18 · Medium
When a catalase test is done on a colony from sheep blood agar, care is taken not to pick up any agar because:
1. AHemolysis makes bubbles hard to see
2. BBlood agar inhibits bacterial catalase
3. CRed cells in the agar contain catalase
4. DColonies on blood agar are too small to test
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Question 19 · Hard
A liver abscess aspirate grows pinpoint beta-hemolytic colonies with a caramel odor. The gram-positive cocci in chains are catalase negative, PYR negative, Voges-Proskauer positive and bacitracin resistant. The organism is most likely:
1. AStreptococcus dysgalactiae
2. BStreptococcus pyogenes
3. CStreptococcus agalactiae
4. DStreptococcus anginosus group
Answer and explanation in LearnFree account
Question 20 · Hard
Urine from an 82-year-old man grows alpha-hemolytic, catalase-negative gram-positive cocci in clusters and tetrads. They are vancomycin susceptible, PYR negative and LAP positive. Which organism is most likely?
1. APediococcus species
2. BAerococcus urinae
3. CAerococcus viridans
4. DEnterococcus faecalis
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Question 21 · Hard
Gram-positive cocci in clusters from a wound are catalase positive. The slide test for clumping factor is positive, but the tube coagulase is negative at 4 and 24 hours. The isolate is PYR positive and ornithine decarboxylase positive. The most likely organism is:
1. AStaphylococcus saprophyticus
2. BStaphylococcus epidermidis
3. CStaphylococcus aureus
4. DStaphylococcus lugdunensis
Answer and explanation in LearnFree account
Question 22 · Medium
Catalase-positive gram-positive cocci in tetrads form bright yellow colonies and are positive in the modified oxidase (microdase) test. Which organism is most likely?
1. AStaphylococcus saprophyticus
2. BStaphylococcus epidermidis
3. CStaphylococcus aureus
4. DMicrococcus species
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Question 23 · Hard
A tube coagulase test on a beta-hemolytic staphylococcus is incubated at 35 °C and first read at 22 hours: no clot. A latex test for clumping factor and protein A is positive, and MALDI-TOF identifies Staphylococcus aureus. The most likely reason the tube test was negative is:
1. AThe inoculum was too heavy
2. BStaphylokinase lysed the clot
3. CThe plasma contained EDTA
4. DThe isolate lacks clumping factor
Answer and explanation in LearnFree account
Question 24 · Medium
A beta-hemolytic, catalase-negative gram-positive coccus in chains is PYR positive and bacitracin susceptible. What is it?
1. AStreptococcus agalactiae (group B)
2. BEnterococcus faecalis
3. CStreptococcus pyogenes (group A)
4. DStreptococcus pneumoniae
Answer and explanation in LearnFree account
Question 25 · Easy
Gram-positive cocci in clusters from a blood culture are catalase positive and give a positive latex test for clumping factor and protein A. What is the most likely organism?
1. AStaphylococcus aureus
2. BEnterococcus faecalis
3. CStaphylococcus epidermidis
4. DStreptococcus pyogenes
Answer and explanation in LearnFree account

Identification: colony morphology and rapid tests (coagulase, catalase, oxidase, indole, PYR)

Large beta-hemolytic colonies of a catalase-negative gram-positive coccus from a throat swab were tested with bacitracin and trimethoprim-sulfamethoxazole disks, as shown. What is the presumptive identification?

An alpha-hemolytic, catalase-negative gram-positive coccus from the blood of a 70-year-old with pneumonia is tested with a 6-mm optochin disk in 5% CO2, as shown. The result supports:

An oxidase test on Escherichia coli, done by rubbing the colony onto reagent-soaked filter paper with a nichrome wire loop, turns purple. The most likely cause is:

A swarming, oxidase-negative, lactose-negative gram-negative rod is H2S positive. Which single rapid test best separates Proteus vulgaris from Proteus mirabilis?

A dog-bite wound grows beta-hemolytic, catalase-positive gram-positive cocci in clusters that are tube coagulase positive. Further tests are PYR positive and Voges-Proskauer negative. What is the most likely organism?

The catalase test separates staphylococci from streptococci. A positive result is seen as:

A suspected Staphylococcus aureus gives a negative slide coagulase test. Why should a tube coagulase test be done before reporting it as coagulase negative?

Which bacterial enzyme does the oxidase test detect?

A flat, dry, lactose-fermenting colony on MacConkey agar from a urine culture is an oxidase-negative gram-negative rod. Which rapid test result best supports a presumptive identification of Escherichia coli?

A nonhemolytic, catalase-negative gram-positive coccus in pairs and short chains is bile esculin positive and PYR negative. What is the most likely organism?

Why is an oxidase test done before a gram-negative rod is set up in a kit or panel designed for Enterobacterales?

A leg wound from a freshwater boating injury grows a beta-hemolytic gram-negative rod that also grows on MacConkey agar. It is oxidase positive, spot indole positive and DNase positive. Which organism is most likely?

An alpha-hemolytic, catalase-negative gram-positive coccus in pairs and short chains is vancomycin resistant. It is PYR negative and LAP negative, and it produces gas from glucose in MRS broth. Which organism is most likely?

A cat-bite wound grows small gray colonies on sheep blood and chocolate agar but nothing on MacConkey agar. The gram-negative coccobacillus is oxidase positive, catalase positive and spot indole positive. Which organism is most likely?

When a catalase test is done on a colony from sheep blood agar, care is taken not to pick up any agar because:

A liver abscess aspirate grows pinpoint beta-hemolytic colonies with a caramel odor. The gram-positive cocci in chains are catalase negative, PYR negative, Voges-Proskauer positive and bacitracin resistant. The organism is most likely:

Urine from an 82-year-old man grows alpha-hemolytic, catalase-negative gram-positive cocci in clusters and tetrads. They are vancomycin susceptible, PYR negative and LAP positive. Which organism is most likely?

Gram-positive cocci in clusters from a wound are catalase positive. The slide test for clumping factor is positive, but the tube coagulase is negative at 4 and 24 hours. The isolate is PYR positive and ornithine decarboxylase positive. The most likely organism is:

Catalase-positive gram-positive cocci in tetrads form bright yellow colonies and are positive in the modified oxidase (microdase) test. Which organism is most likely?

A tube coagulase test on a beta-hemolytic staphylococcus is incubated at 35 °C and first read at 22 hours: no clot. A latex test for clumping factor and protein A is positive, and MALDI-TOF identifies Staphylococcus aureus. The most likely reason the tube test was negative is:

A beta-hemolytic, catalase-negative gram-positive coccus in chains is PYR positive and bacitracin susceptible. What is it?

Gram-positive cocci in clusters from a blood culture are catalase positive and give a positive latex test for clumping factor and protein A. What is the most likely organism?