Over-decolorizing a Gram smear with acetone-alcohol will most likely make:

Gram-positive cells look Gram-negative

Gram-negative cells look Gram-positive

Bacteriology: staining, culture and identification

75 Lab technician recruitment practice questions on Bacteriology: staining, culture and identification, 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 · Easy
Which culture medium is routinely used to grow Mycobacterium tuberculosis?
1. AMacConkey agar
2. BLoeffler's serum slope
3. CLowenstein-Jensen medium
4. DThayer-Martin medium
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Answer: C · Lowenstein-Jensen medium
Lowenstein-Jensen medium is an egg-based medium with malachite green to suppress contaminants. Colonies of M. tuberculosis appear in 2 to 8 weeks as dry, rough, buff colonies. Liquid systems such as MGIT give faster results.
- A: MacConkey agar is for enteric Gram-negative bacilli.
- B: Loeffler's serum slope is for Corynebacterium diphtheriae.
- D: Thayer-Martin medium is for Neisseria gonorrhoeae.
Question 2 · Medium
Over-decolorizing a Gram smear with acetone-alcohol will most likely make:
1. AAcid-fast cells look red
2. BGram-positive cells look Gram-negative
3. CCapsules stain violet
4. DGram-negative cells look Gram-positive
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Answer: B · Gram-positive cells look Gram-negative
Prolonged decolorization washes the crystal violet-iodine complex out of Gram-positive cells too. They then take up safranin and appear pink. Under-decolorizing has the opposite effect.
- A: Acid-fastness is not judged in a Gram stain.
- C: Capsules are not stained by the Gram method.
- D: Gram-negative cells looking violet is the result of under-decolorizing.
Question 3 · Medium
Adding 5 to 10% sheep blood to nutrient agar makes the medium:
1. ASelective
2. BAn enrichment broth
3. CA transport medium
4. DEnriched
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Answer: D · Enriched
Blood agar is nutrient agar with 5 to 10% sheep blood added to support fastidious organisms. It also shows haemolysis. It does not inhibit any group of bacteria.
- A: Selective media contain inhibitors, as MacConkey has bile salts.
- B: Enrichment broths are liquid and favour one organism over others.
- C: Transport media such as Stuart's keep organisms alive without growth.
Question 4 · Easy
A urease-positive organism turns Christensen's urea medium:
1. APink
2. BBlue
3. CYellow
4. DBlack
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Answer: A · Pink
Urease splits urea to ammonia, which raises the pH. The phenol red indicator turns from yellow-orange to pink. Proteus gives a rapid strong reaction; Klebsiella a slower one.
- B: Blue is the citrate-positive colour.
- C: Yellow is the colour of an acid or negative medium.
- D: Black indicates hydrogen sulphide.
Question 5 · Hard
A stool culture shows pale colonies on MacConkey that are oxidase negative, TSI K/A without H2S, non-motile and urease negative. The most likely organism is:
1. ASalmonella Typhimurium
2. BPseudomonas aeruginosa
3. CProteus mirabilis
4. DShigella
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Answer: D · Shigella
A non-lactose fermenter that ferments glucose only, makes no H2S and is non-motile and urease negative fits Shigella. Serotyping with antisera confirms it. Salmonella is usually motile with H2S.
- A: Salmonella Typhimurium is motile and H2S positive.
- B: Pseudomonas is oxidase positive and gives K/K.
- C: Proteus is urease positive, motile and H2S positive.
Question 6 · Easy
After ZN staining, tubercle bacilli and the background look:
1. AGreen on a black background
2. BRed on a blue background
3. CViolet on a pink background
4. DBlue on a red background
Answer and explanation in LearnFree account
Question 7 · Medium
What does steaming the slide achieve in the hot ZN method?
1. ATo kill the counterstain
2. BTo dissolve the mucus in sputum
3. CTo drive the dye through the waxy wall
4. DTo fix the bacteria to the slide
Answer and explanation in LearnFree account
Question 8 · Easy
An enteric isolate with IMViC + + - - is most likely:
1. ACitrobacter freundii
2. BEscherichia coli
3. CEnterobacter aerogenes
4. DKlebsiella pneumoniae
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Question 9 · Easy
Which is an enrichment medium for Salmonella from faeces?
1. ASelenite F broth
2. BRobertson's cooked meat
3. CBrain heart infusion
4. DAlkaline peptone water
Answer and explanation in LearnFree account
Question 10 · Medium
Complete clearing of blood around colonies on blood agar is called:
1. AAlpha haemolysis
2. BGamma haemolysis
3. CAlpha-prime haemolysis
4. DBeta haemolysis
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Question 11 · Medium
Under NTEP, a ZN smear with 1 to 10 AFB per oil-immersion field is graded as:
1. AScanty positive
2. BGrade 3+
3. CGrade 2+
4. DGrade 1+
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Question 12 · Medium
In the triple sugar iron (TSI) test, an alkaline slant and acid butt with blackening suggests:
1. AShigella sonnei
2. BPseudomonas aeruginosa
3. CEscherichia coli
4. DSalmonella Typhimurium
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Question 13 · Hard
Rice-water stool from an adult is plated on TCBS after enrichment. Yellow colonies grow that are oxidase positive and show darting motility. The likely organism is:
1. AVibrio parahaemolyticus
2. BVibrio cholerae
3. CAeromonas hydrophila
4. DCampylobacter jejuni
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Question 14 · Easy
MacConkey agar contains which sugar to distinguish lactose fermenters?
1. AGlucose
2. BSucrose
3. CLactose
4. DMannitol
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Question 15 · Easy
Clumping of cells in plasma on a glass slide (bound coagulase) is shown by:
1. AMicrococcus luteus
2. BStaphylococcus saprophyticus
3. CStaphylococcus aureus
4. DEnterococcus faecalis
Answer and explanation in LearnFree account
Question 16 · Medium
The day's E. coli ATCC 25922 zone for one drug falls outside its QC range. The right action is:
1. AReport if yesterday was in range
2. BWithhold that drug's results and investigate
3. CWiden the acceptable range
4. DReport results with a remark
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Question 17 · Medium
In the citrate test on Simmons medium, a positive result shows as:
1. ABlack precipitate
2. BBlue colour with growth
3. CYellow colour with growth
4. DRed ring on top
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Question 18 · Easy
Kovac's reagent is used to detect:
1. AUrease
2. BHydrogen sulphide
3. CCitrate use
4. DIndole
Answer and explanation in LearnFree account
Question 19 · Easy
What is the role of Gram's iodine in the Gram stain?
1. ACounterstain
2. BPrimary stain
3. CDecolorizer
4. DMordant
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Question 20 · Easy
Which is the most common method of sterilizing culture media containing agar?
1. AHot air oven at 160 degrees Celsius
2. BAutoclaving at 121 degrees Celsius
3. CMembrane filtration
4. DUltraviolet light
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Question 21 · Medium
Which is the best sample for culture of Salmonella Typhi in the first week of enteric fever?
1. ABlood
2. BStool
3. CWidal serum
4. DUrine
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Question 22 · Hard
A sputum ZN smear shows 6 AFB in 100 oil-immersion fields. How should it be reported under NTEP?
1. ANegative
2. BScanty, 6 AFB seen
3. C1+
4. D2+
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Question 23 · Hard
A Gram-positive rod from a neonate's blood culture shows tumbling motility at 25 degrees Celsius, narrow beta haemolysis and is catalase positive. The likely organism is:
1. ABacillus subtilis
2. BLactobacillus
3. CListeria monocytogenes
4. DErysipelothrix rhusiopathiae
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Question 24 · Hard
A pus isolate gives golden colonies with beta haemolysis, is catalase positive, slide coagulase negative but tube coagulase positive at 4 hours. What should be reported?
1. AStreptococcus pyogenes
2. BCoagulase-negative staphylococcus
3. CMicrococcus
4. DStaphylococcus aureus
Answer and explanation in LearnFree account
Question 25 · Hard
A throat swab grows beta-haemolytic colonies that are catalase negative. Which pair of tests best confirms group A streptococcus?
1. AOptochin sensitive and bile soluble
2. BBile esculin positive and grows in 6.5% NaCl
3. CBacitracin sensitive and PYR positive
4. DCAMP positive and hippurate positive
Answer and explanation in LearnFree account

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Bacteriology: staining, culture and identification

Which culture medium is routinely used to grow Mycobacterium tuberculosis?

Over-decolorizing a Gram smear with acetone-alcohol will most likely make:

Adding 5 to 10% sheep blood to nutrient agar makes the medium:

A urease-positive organism turns Christensen's urea medium:

A stool culture shows pale colonies on MacConkey that are oxidase negative, TSI K/A without H2S, non-motile and urease negative. The most likely organism is:

After ZN staining, tubercle bacilli and the background look:

What does steaming the slide achieve in the hot ZN method?

An enteric isolate with IMViC + + - - is most likely:

Which is an enrichment medium for Salmonella from faeces?

Complete clearing of blood around colonies on blood agar is called:

Under NTEP, a ZN smear with 1 to 10 AFB per oil-immersion field is graded as:

In the triple sugar iron (TSI) test, an alkaline slant and acid butt with blackening suggests:

Rice-water stool from an adult is plated on TCBS after enrichment. Yellow colonies grow that are oxidase positive and show darting motility. The likely organism is:

MacConkey agar contains which sugar to distinguish lactose fermenters?

Clumping of cells in plasma on a glass slide (bound coagulase) is shown by:

The day's E. coli ATCC 25922 zone for one drug falls outside its QC range. The right action is:

In the citrate test on Simmons medium, a positive result shows as:

Kovac's reagent is used to detect:

What is the role of Gram's iodine in the Gram stain?

Which is the most common method of sterilizing culture media containing agar?

Which is the best sample for culture of Salmonella Typhi in the first week of enteric fever?

A sputum ZN smear shows 6 AFB in 100 oil-immersion fields. How should it be reported under NTEP?

A Gram-positive rod from a neonate's blood culture shows tumbling motility at 25 degrees Celsius, narrow beta haemolysis and is catalase positive. The likely organism is:

A pus isolate gives golden colonies with beta haemolysis, is catalase positive, slide coagulase negative but tube coagulase positive at 4 hours. What should be reported?

A throat swab grows beta-haemolytic colonies that are catalase negative. Which pair of tests best confirms group A streptococcus?