Bacteriology: identification and medically important bacteria

36 Kerala PSC Lab Technician Gr II practice questions on Bacteriology: identification and medically important bacteria, 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 · Medium
An IMViC result of + + − − (indole, methyl red, Voges-Proskauer, citrate) is typical of:
1. AEscherichia coli
2. BEnterobacter cloacae
3. CCitrobacter freundii
4. DKlebsiella pneumoniae
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Answer: A · Escherichia coli
Escherichia coli forms indole from tryptophan and makes stable acids from glucose, so the methyl red test is positive. It does not produce acetoin (VP negative) and cannot use citrate as its sole carbon source. Klebsiella pneumoniae and Enterobacter give the opposite pattern (− − + +). Because methyl red and VP results are usually opposite, both positive suggests a mixed culture.
- B: Enterobacter cloacae is usually − − + +.
- C: Citrobacter freundii uses citrate and is usually indole negative.
- D: Klebsiella pneumoniae is usually − − + +.
Question 2 · Medium
After a course of clindamycin, a patient develops pseudomembranous colitis. Loss of normal gut flora has allowed overgrowth of:
1. ABacteroides fragilis
2. BClostridioides difficile
3. CEnterococcus faecalis
4. DClostridium perfringens
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Answer: B · Clostridioides difficile
Broad-spectrum antibiotics such as clindamycin, cephalosporins and fluoroquinolones suppress the colonic flora that normally holds C. difficile in check. Toxigenic strains then multiply and release toxins A and B, which damage the colonic lining and form pseudomembranes. Diagnosis is by detecting the toxins or toxin genes in a loose stool. The condition shows the protective role of normal flora (colonization resistance).
- A: B. fragilis is a normal gut anaerobe that causes abscesses after bowel spillage.
- C: E. faecalis causes urinary and wound infections, not pseudomembranous colitis.
- D: C. perfringens causes gas gangrene and food poisoning, not pseudomembranous colitis.
Question 3 · Medium
All of the following bacteria are oxidase positive EXCEPT:
1. APseudomonas aeruginosa
2. BVibrio cholerae
3. CNeisseria meningitidis
4. DEscherichia coli
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Answer: D · Escherichia coli
The oxidase test detects cytochrome c oxidase, which turns tetramethyl-p-phenylenediamine purple within seconds. E. coli and the other common Enterobacterales are oxidase negative. Pseudomonas, Vibrio and Neisseria are oxidase positive, so the test quickly separates them from enteric gram-negative rods. Colonies taken from media containing fermentable sugar or dyes can give unreliable results.
- A: P. aeruginosa is oxidase positive.
- B: V. cholerae is oxidase positive, which separates it from enteric rods.
- C: Neisseria species are oxidase positive.
Question 4 · Easy
What is the main normal flora of the vagina in women of reproductive age?
1. ACandida albicans
2. BLactobacillus species
3. CEscherichia coli
4. DStaphylococcus aureus
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Answer: B · Lactobacillus species
Under the influence of oestrogen, vaginal epithelial cells store glycogen, which lactobacilli (Doderlein's bacilli) ferment to lactic acid. The resulting acidic pH of about 4-4.5 discourages many pathogens. Loss of lactobacilli, as in bacterial vaginosis, lets the pH rise and other organisms overgrow. Before puberty and after menopause, the flora is mixed and the pH is higher.
- A: Candida is present in small numbers in some women; overgrowth causes thrush.
- C: E. coli may colonize the area but is not the dominant vaginal flora.
- D: S. aureus is carried by a minority of women and is not the main flora.
Question 5 · Easy
A 2-year-old has bouts of coughing that end in an inspiratory whoop. Which organism is the most likely cause?
1. AStreptococcus pneumoniae
2. BHaemophilus influenzae
3. CBordetella pertussis
4. DCorynebacterium diphtheriae
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Answer: C · Bordetella pertussis
Whooping cough is caused by Bordetella pertussis, a small gram-negative coccobacillus that attaches to the ciliated cells of the respiratory tract. Its toxins cause bouts of coughing that end with a whoop as air is drawn in. A pernasal (nasopharyngeal) swab is cultured on Bordet-Gengou or charcoal blood agar. Vaccination with DPT has made the disease much less common.
- A: S. pneumoniae causes lobar pneumonia with fever and a productive cough.
- B: H. influenzae causes meningitis, epiglottitis and pneumonia, not whooping cough.
- D: C. diphtheriae causes a throat membrane, not bouts of whooping cough.
Question 6 · Easy
Which bacterial enzyme turns Christensen's urea medium pink?
1. ACatalase
2. BOxidase
3. CCoagulase
4. DUrease
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Question 7 · Hard
Blood culture from a patient with 10 days of fever grows a non-lactose fermenter. Results: motile; TSI alkaline slant, acid butt, no gas, slight H2S; indole negative; citrate negative; urease negative. The most likely organism is:
1. ASalmonella Paratyphi A
2. BSalmonella Typhi
3. CShigella flexneri
4. DProteus mirabilis
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Question 8 · Hard
A 34-year-old man cleaned flooded drains in Alappuzha 10 days ago. He now has high fever, severe calf pain, red eyes without discharge, jaundice and low urine output. There is no eschar. Which organism is the most likely cause?
1. ASalmonella Typhi
2. BLeptospira interrogans
3. COrientia tsutsugamushi
4. DBurkholderia pseudomallei
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Question 9 · Easy
A farmer from Wayanad has fever, a painless black eschar in the groin and enlarged lymph nodes. Which organism is the most likely cause?
1. ARickettsia prowazekii
2. BOrientia tsutsugamushi
3. CLeptospira interrogans
4. DCoxiella burnetii
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Question 10 · Medium
A smear from a deep wound of a patient with lockjaw shows gram-positive bacilli with round terminal spores, like drumsticks. The organism is:
1. ABacillus anthracis
2. BClostridium tetani
3. CClostridium perfringens
4. DClostridium botulinum
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Question 11 · Medium
A Corynebacterium diphtheriae isolate from a child with a throat membrane must be tested for toxin production. Which in vitro test is used?
1. ASchick skin test
2. BElek gel precipitation test
3. CCAMP test
4. DNagler reaction
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Question 12 · Medium
On triple sugar iron agar, a red slant over a yellow butt shows that the organism:
1. AFerments glucose only
2. BFerments lactose or sucrose
3. CProduces hydrogen sulphide
4. DFerments none of the sugars
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Question 13 · Easy
Which of these gram-positive organisms does NOT produce catalase?
1. AListeria monocytogenes
2. BMicrococcus luteus
3. CStaphylococcus aureus
4. DStreptococcus pyogenes
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Question 14 · Hard
A 4-year-old has fever and frequent small stools with blood and mucus. Stool culture: non-lactose-fermenting colonies on MacConkey agar; non-motile; TSI alkaline slant, acid butt, no gas, no H2S; urease negative; citrate negative. The most likely organism is:
1. AKlebsiella pneumoniae
2. BShigella species
3. CSalmonella Typhi
4. DProteus mirabilis
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Question 15 · Easy
After Kovac's reagent is added to a peptone water culture, a red ring forms at the top. This is a positive:
1. ACitrate test
2. BVoges-Proskauer test
3. CIndole test
4. DMethyl red test
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Question 16 · Medium
Simmons citrate agar turns from green to blue when the organism:
1. AFerments glucose to acid
2. BSplits urea to ammonia
3. CProduces hydrogen sulphide
4. DUses citrate as its only carbon source
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Question 17 · Medium
The slide coagulase test detects:
1. ADNase produced by the cells
2. BHemolysin on the cell surface
3. CBound coagulase (clumping factor)
4. DFree coagulase in the medium
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Question 18 · Easy
The catalase test uses hydrogen peroxide at a strength of:
1. A10%
2. B3%
3. C30%
4. D0.3%
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Question 19 · Easy
Mycobacterium leprae has not been grown on artificial media; it is grown in the:
1. ALowenstein-Jensen medium
2. BRabbit kidney cells
3. CFootpad of mice
4. DChick embryo
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Question 20 · Easy
Gas gangrene is caused mainly by:
1. AClostridioides difficile
2. BClostridium perfringens
3. CClostridium botulinum
4. DClostridium tetani
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Question 21 · Easy
A positive oxidase test with tetramethyl-p-phenylenediamine gives what colour within 10 seconds?
1. ABright yellow
2. BPink red
3. CDeep purple
4. DGreen
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Question 22 · Easy
Rice-water stools with darting motility on a hanging drop suggest:
1. AVibrio cholerae
2. BEntamoeba histolytica
3. CShigella flexneri
4. DSalmonella Typhi
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Question 23 · Medium
A urine isolate from a young woman with cystitis is a gram-positive coccus in clusters: catalase positive, coagulase negative, resistant to novobiocin. The organism is:
1. AStaphylococcus saprophyticus
2. BEnterococcus faecalis
3. CStaphylococcus epidermidis
4. DStaphylococcus aureus
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Question 24 · Medium
Colonies of Haemophilus influenzae grow larger near a streak of Staphylococcus aureus on blood agar because the staphylococcus supplies:
1. AX factor (hemin)
2. BVitamin K
3. CExtra carbon dioxide
4. DV factor (NAD)
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Question 25 · Medium
Why should the catalase test not be done with colonies picked from blood agar?
1. ABlood removes the peroxide
2. BRed cells contain catalase
3. CBlood inhibits all enzymes
4. DColonies are too small
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Bacteriology: identification and medically important bacteria

An IMViC result of + + − − (indole, methyl red, Voges-Proskauer, citrate) is typical of:

After a course of clindamycin, a patient develops pseudomembranous colitis. Loss of normal gut flora has allowed overgrowth of:

All of the following bacteria are oxidase positive EXCEPT:

What is the main normal flora of the vagina in women of reproductive age?

A 2-year-old has bouts of coughing that end in an inspiratory whoop. Which organism is the most likely cause?

Which bacterial enzyme turns Christensen's urea medium pink?

Blood culture from a patient with 10 days of fever grows a non-lactose fermenter. Results: motile; TSI alkaline slant, acid butt, no gas, slight H2S; indole negative; citrate negative; urease negative. The most likely organism is:

A 34-year-old man cleaned flooded drains in Alappuzha 10 days ago. He now has high fever, severe calf pain, red eyes without discharge, jaundice and low urine output. There is no eschar. Which organism is the most likely cause?

A farmer from Wayanad has fever, a painless black eschar in the groin and enlarged lymph nodes. Which organism is the most likely cause?

A smear from a deep wound of a patient with lockjaw shows gram-positive bacilli with round terminal spores, like drumsticks. The organism is:

A Corynebacterium diphtheriae isolate from a child with a throat membrane must be tested for toxin production. Which in vitro test is used?

On triple sugar iron agar, a red slant over a yellow butt shows that the organism:

Which of these gram-positive organisms does NOT produce catalase?

A 4-year-old has fever and frequent small stools with blood and mucus. Stool culture: non-lactose-fermenting colonies on MacConkey agar; non-motile; TSI alkaline slant, acid butt, no gas, no H2S; urease negative; citrate negative. The most likely organism is:

After Kovac's reagent is added to a peptone water culture, a red ring forms at the top. This is a positive:

Simmons citrate agar turns from green to blue when the organism:

The slide coagulase test detects:

The catalase test uses hydrogen peroxide at a strength of:

Mycobacterium leprae has not been grown on artificial media; it is grown in the:

Gas gangrene is caused mainly by:

A positive oxidase test with tetramethyl-p-phenylenediamine gives what colour within 10 seconds?

Rice-water stools with darting motility on a hanging drop suggest:

A urine isolate from a young woman with cystitis is a gram-positive coccus in clusters: catalase positive, coagulase negative, resistant to novobiocin. The organism is:

Colonies of Haemophilus influenzae grow larger near a streak of Staphylococcus aureus on blood agar because the staphylococcus supplies:

Why should the catalase test not be done with colonies picked from blood agar?