The triple sugar iron (TSI) tube shown was read after 20 hours at 35 °C. How is the reaction recorded?

A/A, gas positive, H2S negative

A/A, gas negative, H2S positive

K/A, gas positive, H2S negative

K/K, gas negative, H2S negative

Identification: conventional biochemicals and commercial kits

15 ASCP MLS practice questions on Identification: conventional biochemicals and commercial kits, part of Microbiology. The first 15 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
Three ventilated patients in one ICU have the same gram-negative rod in tracheal aspirates. At 35 °C, the colonies are cream colored and ferment lactose late on MacConkey agar. A commercial kit gives a low-discrimination profile, so conventional tests are set up: oxidase negative; motile; indole negative; Voges-Proskauer positive; citrate positive; urease negative; lysine decarboxylase positive; ornithine decarboxylase positive; arginine dihydrolase negative; DNase positive; gelatin liquefied; L-arabinose not fermented. The isolate is most likely:
1. AKlebsiella aerogenes
2. BSerratia marcescens
3. CEnterobacter cloacae complex
4. DHafnia alvei
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Answer: B · Serratia marcescens
A motile, VP-positive, citrate-positive profile with both lysine and ornithine decarboxylase points to Serratia or Klebsiella aerogenes. Two extracellular enzymes, DNase and gelatinase, together with failure to ferment arabinose, separate Serratia marcescens from K. aerogenes, which is DNase negative, ferments arabinose and ferments lactose promptly. Enterobacter cloacae complex is lysine negative and arginine positive, and Hafnia alvei is usually citrate negative and DNase negative. S. marcescens often lacks its red pigment at 35 °C, so a cream colony does not exclude it. A cluster in ventilated patients should be reported to infection prevention.
- A: K. aerogenes is DNase negative and ferments arabinose and lactose promptly.
- C: E. cloacae complex is lysine decarboxylase negative and arginine positive.
- D: Hafnia alvei is usually citrate negative and DNase negative.
Question 2 · Hard
A 10-year-old has fever, diarrhea and right lower quadrant pain; at surgery the appendix is normal but the mesenteric nodes are enlarged. His stool grows small lactose-negative, oxidase-negative colonies on MacConkey agar after 48 hours, with the reactions shown. What is the most likely organism?
Figure: MedExams
1. AProvidencia rettgeri
2. BYersinia enterocolitica
3. CMorganella morganii
4. DProteus mirabilis
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Answer: B · Yersinia enterocolitica
The isolate is A/A on TSI without gas or H2S, hydrolyses urea, and is motile at 25 °C (growth spreads from the stab) but not at 35 °C (growth stays on the stab line). Motility at 22 to 25 °C but not at 35 to 37 °C is characteristic of Yersinia enterocolitica, and sucrose fermentation explains the acid slant in a lactose-negative organism. Mesenteric adenitis that mimics appendicitis fits the same organism. Morganella, Proteus and Providencia are urease positive too, but they are motile at 35 °C and give an alkaline slant (K/A) on TSI.
- A: Providencia rettgeri is motile at 35 °C and gives K/A on TSI.
- C: Morganella morganii is motile at 35 °C and gives K/A on TSI.
- D: Proteus mirabilis is motile at 35 °C, produces H2S and gives K/A on TSI.
Question 3 · Medium
Refer to the identification flowchart shown. Which test belongs in the dashed box?
Figure: MedExams
1. AHydrogen sulfide
2. BLysine decarboxylase
3. CMethyl red
4. DMotility
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Answer: D · Motility
Escherichia coli separates from Klebsiella and Enterobacter on its IMViC pattern. Klebsiella pneumoniae and the Enterobacter cloacae complex share indole negative, VP positive and citrate positive results, and motility separates them: Enterobacter is motile, while Klebsiella has no flagella. The test in the box must be positive for Enterobacter and negative for Klebsiella, as the branch labels show. Ornithine decarboxylase would also work, because it is positive for E. cloacae and negative for K. pneumoniae.
- A: Neither organism produces hydrogen sulfide.
- B: Lysine decarboxylase is positive for K. pneumoniae and negative for E. cloacae, the reverse of the branches.
- C: Both organisms are usually methyl red negative.
Question 4 · Easy
The triple sugar iron (TSI) tube shown was read after 20 hours at 35 °C. How is the reaction recorded?
Figure: MedExams
1. AA/A, gas negative, H2S positive
2. BK/A, gas positive, H2S negative
3. CK/K, gas negative, H2S negative
4. DA/A, gas positive, H2S negative
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Answer: D · A/A, gas positive, H2S negative
A yellow slant and a yellow butt are recorded as A/A: the organism fermented glucose and also lactose, sucrose or both, making enough acid to keep the slant acid at 18 to 24 hours. Bubbles and a crack in the butt show gas. There is no black precipitate, so no H2S was produced. Escherichia coli and Klebsiella give this pattern; read the tube at 18 to 24 hours, because an early reading can show A/A for a glucose-only fermenter.
- A: This tube shows gas and no black precipitate, the reverse of this option.
- B: K/A needs a red (alkaline) slant; this slant is yellow.
- C: K/K means a red slant and butt, with no fermentation at all.
Question 5 · Medium
An oxidase-negative, lactose-negative gram-negative rod from a stool culture gives the reactions shown. The reactions fit:
Figure: MedExams
1. ACitrobacter freundii
2. BSalmonella enterica
3. CProteus mirabilis
4. DEdwardsiella tarda
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Answer: B · Salmonella enterica
The TSI is K/A with gas and a black butt (H2S), urea is not hydrolysed (yellow), the indole ring stays yellow (negative), citrate is used (blue), lysine is decarboxylated (purple, with a yellow control tube) and the organism is motile. This is the typical pattern of non-typhoidal Salmonella enterica. Citrobacter freundii shares the TSI, citrate and motility results but is lysine decarboxylase negative. That is why lysine decarboxylase, or lysine iron agar, is a key screen for Salmonella among H2S-positive stool isolates.
- A: Citrobacter freundii is lysine decarboxylase negative, but this lysine tube is purple.
- C: Proteus mirabilis hydrolyses urea rapidly, but this urea tube is yellow.
- D: Edwardsiella tarda is indole positive and citrate negative.
Question 6 · Hard
In a numeric-code identification kit, tests are scored in groups of three, with positives in the first, second and third test of each group worth 1, 2 and 4. The groups read + – +, – + + and + + +. The profile number is:
1. A537
2. B223
3. C765
4. D567
Answer and explanation in LearnFree account
Question 7 · Medium
A gram-negative rod forms colorless colonies on MacConkey agar at 24 hours but is ONPG positive. This means the organism:
1. AWas tested from an impure colony
2. BLacks beta-galactosidase, has permease
3. CCannot ferment lactose at all
4. DHas beta-galactosidase but lacks permease
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Question 8 · Easy
The Voges-Proskauer test detects:
1. AUse of citrate as the only carbon source
2. BMixed acids from glucose fermentation
3. CIndole from tryptophan breakdown
4. DAcetoin from glucose fermentation
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Question 9 · Hard
A mucoid, lactose-fermenting gram-negative rod gives: oxidase negative, indole negative, VP positive, citrate positive, nonmotile, ornithine decarboxylase negative, urease positive. What is the most likely organism?
1. AKlebsiella oxytoca
2. BKlebsiella aerogenes
3. CKlebsiella pneumoniae
4. DEnterobacter cloacae complex
Answer and explanation in LearnFree account
Question 10 · Medium
Which IMViC pattern (indole, methyl red, Voges-Proskauer, citrate) is typical of Klebsiella pneumoniae?
1. A+ + – +
2. B– + – +
3. C+ + – –
4. D– – + +
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Question 11 · Easy
In a Moeller decarboxylase test, which result in the amino acid tube shows a positive reaction?
1. APurple color after incubation
2. BYellow color after incubation
3. CRed ring after adding reagent
4. DBlack precipitate in the butt
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Question 12 · Hard
A non-swarming, lactose-negative gram-negative rod from urine gives: phenylalanine deaminase positive, urease positive, indole positive, H2S negative, ornithine decarboxylase positive. The organism is most likely:
1. AMorganella morganii
2. BProteus mirabilis
3. CProvidencia stuartii
4. DProteus vulgaris
Answer and explanation in LearnFree account
Question 13 · Hard
An isolate from an ICU patient is a lavender-tinged, non-lactose-fermenting gram-negative rod. Results: oxidase negative, motile, glucose weakly oxidized, maltose strongly oxidized, DNase positive, lysine decarboxylase positive. Which organism is most likely?
1. AStenotrophomonas maltophilia
2. BPseudomonas aeruginosa
3. CAcinetobacter baumannii
4. DBurkholderia cepacia complex
Answer and explanation in LearnFree account
Question 14 · Medium
Alpha-hemolytic gram-positive diplococci are optochin susceptible and bile soluble. What is the organism?
1. AMoraxella catarrhalis
2. BViridans group streptococci
3. CStreptococcus pneumoniae
4. DEnterococcus faecium
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Question 15 · Medium
A non-lactose fermenting gram-negative rod is oxidase positive, smells grape-like and produces a blue-green pigment. What is it?
1. AShigella sonnei
2. BPseudomonas aeruginosa
3. CSalmonella enterica
4. DProteus vulgaris
Answer and explanation in LearnFree account

Identification: conventional biochemicals and commercial kits

A 10-year-old has fever, diarrhea and right lower quadrant pain; at surgery the appendix is normal but the mesenteric nodes are enlarged. His stool grows small lactose-negative, oxidase-negative colonies on MacConkey agar after 48 hours, with the reactions shown. What is the most likely organism?

Refer to the identification flowchart shown. Which test belongs in the dashed box?

The triple sugar iron (TSI) tube shown was read after 20 hours at 35 °C. How is the reaction recorded?

An oxidase-negative, lactose-negative gram-negative rod from a stool culture gives the reactions shown. The reactions fit:

In a numeric-code identification kit, tests are scored in groups of three, with positives in the first, second and third test of each group worth 1, 2 and 4. The groups read + – +, – + + and + + +. The profile number is:

A gram-negative rod forms colorless colonies on MacConkey agar at 24 hours but is ONPG positive. This means the organism:

The Voges-Proskauer test detects:

A mucoid, lactose-fermenting gram-negative rod gives: oxidase negative, indole negative, VP positive, citrate positive, nonmotile, ornithine decarboxylase negative, urease positive. What is the most likely organism?

Which IMViC pattern (indole, methyl red, Voges-Proskauer, citrate) is typical of Klebsiella pneumoniae?

In a Moeller decarboxylase test, which result in the amino acid tube shows a positive reaction?

A non-swarming, lactose-negative gram-negative rod from urine gives: phenylalanine deaminase positive, urease positive, indole positive, H2S negative, ornithine decarboxylase positive. The organism is most likely:

An isolate from an ICU patient is a lavender-tinged, non-lactose-fermenting gram-negative rod. Results: oxidase negative, motile, glucose weakly oxidized, maltose strongly oxidized, DNase positive, lysine decarboxylase positive. Which organism is most likely?

Alpha-hemolytic gram-positive diplococci are optochin susceptible and bile soluble. What is the organism?

A non-lactose fermenting gram-negative rod is oxidase positive, smells grape-like and produces a blue-green pigment. What is it?