Anaerobic bacteria

14 DOH Abu Dhabi (HAAD) lab licensing practice questions on Anaerobic bacteria, part of Microbiology. The first 14 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

1. Question 1 · Hard

   A 44-year-old woman who has had an intrauterine device for 9 years has pelvic pain and a tubo-ovarian mass. Pus from the drained abscess contains small yellow granules. A crushed granule shows branching, beaded gram-positive rods, and a modified Kinyoun stain (1% sulfuric acid decolorizer) is negative. After 7 days, rough white colonies with a 'molar tooth' appearance grow on anaerobic blood agar, with scant growth in 5% CO2 and none in air. The isolate is catalase negative and indole negative. The most likely organism is:

   1. ALactobacillus species
   2. BCutibacterium acnes
   3. CActinomyces israelii
   4. DNocardia species

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   Answer: C · Actinomyces israelii

   Sulfur granules containing branching gram-positive rods, in a woman with a long-term IUD, suggest pelvic actinomycosis. The negative modified acid-fast stain, the negative catalase and the preference for anaerobic growth separate Actinomyces from Nocardia, which is partially acid-fast, catalase positive and aerobic. A. israelii forms the classic rough 'molar tooth' colonies after several days. Cutibacterium acnes is catalase and indole positive and does not form granules, and lactobacilli do not branch. Anaerobic cultures for Actinomyces should be held for 7 to 14 days.

   • A: Lactobacilli are non-branching rods and do not form sulfur granules.
   • B: C. acnes is catalase and indole positive and does not form granules.
   • D: Nocardia is aerobic, catalase positive and partially acid-fast.

2. Question 2 · Hard

   Pus from an intra-abdominal abscess grows an anaerobic gram-negative rod. Results: colonies over 1 mm on BBE agar with a black surrounding zone; resistant to vancomycin, kanamycin and colistin disks; spot indole negative; catalase positive. Which organism is most likely?

   1. ABacteroides thetaiotaomicron
   2. BBilophila wadsworthia
   3. CBacteroides fragilis
   4. DFusobacterium mortiferum

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   Answer: C · Bacteroides fragilis

   Members of the B. fragilis group grow in 20% bile and most hydrolyse esculin, so they form colonies over 1 mm with blackening on BBE agar, and they are resistant to all three special-potency disks. A negative spot indole separates B. fragilis from the indole-positive B. thetaiotaomicron. B. fragilis is the anaerobe most often recovered from intra-abdominal infection and anaerobic bacteremia. Most strains make beta-lactamase, so penicillin is unreliable, while metronidazole and carbapenems usually remain active.

   • A: B. thetaiotaomicron is spot indole positive.
   • B: Bilophila forms tiny colonies on BBE and is kanamycin and colistin susceptible.
   • D: Fusobacterium mortiferum is kanamycin and colistin susceptible.

3. Question 3 · Medium

   A 67-year-old man with no injury or surgery develops fever and spreading muscle necrosis of the thigh. Blood cultures grow Clostridium septicum. Which investigation should this finding prompt?

   1. AStool testing for C. difficile toxin
   2. BA search for occult colorectal or blood cancer
   3. CTesting household contacts for carriage
   4. DEchocardiography for infective endocarditis

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   Answer: B · A search for occult colorectal or blood cancer

   C. septicum causes spontaneous (nontraumatic) gas gangrene and bacteremia, and it is strongly linked to colorectal cancer and hematologic malignancy, including neutropenia. The organism is thought to enter the blood through ulcerated bowel mucosa, often in the cecum. Finding it in blood without trauma should therefore lead to colonoscopy or another search for cancer. On blood agar it often swarms, which can make subculture plates hard to read.

   • A: C. difficile toxin testing does not explain clostridial myonecrosis.
   • C: C. septicum infection is not spread between household contacts.
   • D: Endocarditis is not the usual source of C. septicum bacteremia.

4. Question 4 · Hard

   An anaerobic gram-negative coccobacillus from a dental abscess forms black colonies after 5 days. Special-potency disks: vancomycin 5 µg susceptible, kanamycin 1 mg resistant, colistin 10 µg resistant. It does not grow on KVLB agar. The genus that fits best is:

   1. AFusobacterium
   2. BPorphyromonas
   3. CBacteroides
   4. DPrevotella

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   Answer: B · Porphyromonas

   Porphyromonas is unusual among gram-negative anaerobes in being susceptible to the 5 µg vancomycin disk, while it is resistant to kanamycin and colistin. Its vancomycin susceptibility is also why it fails to grow on KVLB agar, whereas pigmented Prevotella species are vancomycin resistant and grow well there. Both genera make black or brown pigment from heme, and most Porphyromonas species are also indole positive. Special-potency disks are used only for identification and are never reported as susceptibility results.

   • A: Fusobacterium is kanamycin and colistin susceptible and is not pigmented.
   • C: The Bacteroides fragilis group is resistant to all three disks and is not pigmented.
   • D: Prevotella is vancomycin resistant and grows on KVLB agar.

5. Question 5 · Hard

   Anaerobic rods from a wound stain pink on repeated Gram stains. Special-potency disks: vancomycin susceptible, kanamycin susceptible, colistin resistant. What is the best interpretation?

   1. AA Bacteroides fragilis group rod
   2. BA Porphyromonas species
   3. CA Fusobacterium species
   4. DA gram-positive rod staining pink

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   Answer: D · A gram-positive rod staining pink

   Gram-positive anaerobes are typically vancomycin susceptible and colistin resistant, while most gram-negative anaerobes are vancomycin resistant. Some clostridia and other gram-positive anaerobes lose crystal violet easily, especially in older cultures, so the disk pattern helps confirm the true Gram reaction. Kanamycin results vary among gram-positive anaerobes and do not change this reading. A spore stain, or survival after heat or alcohol shock, can then show whether the isolate is a spore-former.

   • A: The B. fragilis group is resistant to all three disks.
   • B: Porphyromonas is kanamycin and colistin resistant.
   • C: Fusobacterium is vancomycin resistant and colistin susceptible.

6. Question 6 · Easy

   Kanamycin-vancomycin laked blood (KVLB) agar is used in anaerobic culture to select for:

   1. AAnaerobic gram-negative rods such as Prevotella
   2. BSpore-forming rods such as Clostridium
   3. CFacultative gram-negative rods such as E. coli
   4. DAnaerobic gram-positive cocci such as Peptostreptococcus

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7. Question 7 · Hard

   Five periprosthetic tissue samples are taken at revision of a painful shoulder replacement. On day 9 of extended incubation, three samples grow the same catalase-positive, indole-positive anaerobic gram-positive rod. How should this be interpreted?

   1. AColonisation of the implant, not infection
   2. BCutibacterium acnes prosthetic joint infection
   3. CContamination, since growth took over 5 days
   4. DSkin contamination, since C. acnes is skin flora

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8. Question 8 · Medium

   An abscess aspirate in an anaerobic transport vial will not be plated for about 3 hours. Until then it should be held:

   1. AIn a 35 °C incubator
   2. BIn a refrigerator at 2 to 8 °C
   3. CAt room temperature
   4. DFrozen at -20 °C

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9. Question 9 · Hard

   A 19-year-old student had a sore throat a week ago. He now has fever, a tender swollen neck and septic lung emboli. Anaerobic bottles grow pleomorphic gram-negative rods, some long and filamentous with rounded ends. The isolate is indole positive and lipase positive on egg yolk agar. Which organism is most likely?

   1. AFusobacterium necrophorum
   2. BFusobacterium nucleatum
   3. CPrevotella melaninogenica
   4. DBacteroides fragilis

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10. Question 10 · Medium

    On egg yolk agar, the opaque zone around Clostridium perfringens is absent on the half of the plate spread with antitoxin. This Nagler test detects:

    1. AProteolytic activity
    2. BLecithinase from alpha toxin
    3. CHemolysis by theta toxin
    4. DLipase acting on egg yolk fats

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11. Question 11 · Easy

    For the primary non-selective plate in an anaerobic culture, laboratories commonly use:

    1. ABrucella blood agar with hemin and vitamin K
    2. BBacteroides bile esculin agar with gentamicin
    3. CPhenylethyl alcohol blood agar
    4. DKanamycin-vancomycin laked blood agar

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12. Question 12 · Medium

    For aerotolerance testing, a colony from an anaerobic plate is subcultured to blood agar incubated anaerobically and to chocolate agar in 5% CO2. Growth appears only on the anaerobic plate. The isolate is:

    1. AA facultative anaerobe
    2. BA capnophilic organism
    3. CAn aerotolerant anaerobe
    4. DAn obligate anaerobe

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13. Question 13 · Easy

    Gram-positive rods with round terminal spores that swell the cell, giving a drumstick or tennis-racket shape, are typical of:

    1. AClostridioides difficile
    2. BClostridium perfringens
    3. CClostridium septicum
    4. DClostridium tetani

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14. Question 14 · Medium

    Which approach is commonly used to diagnose Clostridioides difficile infection?

    1. ABlood cultures taken during a fever spike
    2. BGDH and toxin tests on unformed stool, PCR if discordant
    3. CAnaerobic culture of a formed stool sample on selective agar
    4. DA Gram stain looking for gram-positive rods

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