MLT GlobeBacteriology: Anaerobes – page 3
56 Bacteriology MCQs on Anaerobes with answers and explanations.
In a reverse CAMP test, an anaerobic gram-positive rod is streaked at right angles to Streptococcus agalactiae. An arrowhead of enhanced hemolysis points toward the rod. The rod is most likely:
The alpha toxin (lecithinase) of C. perfringens acts with the CAMP factor of group B streptococci to give an arrowhead of hemolysis pointing toward the Clostridium streak.
Young colonies of an anaerobic gram-negative rod from a dental abscess show brick-red fluorescence under long-wave UV light before black pigment appears. This suggests:
Pigmented Prevotella and Porphyromonas species fluoresce brick-red under UV light before they develop black-brown pigment. B. fragilis and Fusobacterium do not fluoresce red.
A patient without trauma develops rapidly spreading gas gangrene. Blood cultures grow a swarming, spore-forming anaerobic gram-positive rod that is lecithinase negative. The patient should be investigated for:
Clostridium septicum causes non-traumatic (spontaneous) gas gangrene and bacteremia and is strongly associated with colorectal cancer and neutropenia. It swarms and is lecithinase negative.
Tetanospasmin causes muscle rigidity and spasms mainly by:
Tetanus toxin travels to the spinal cord and blocks release of the inhibitory transmitters glycine and GABA, so motor neurons fire without control, causing spastic paralysis.
Most members of the Bacteroides fragilis group are resistant to penicillin mainly because they:
B. fragilis group organisms produce chromosomal beta-lactamases that break down penicillin and many cephalosporins. Metronidazole and beta-lactam/beta-lactamase inhibitor combinations remain active against most isolates.
On cycloserine-cefoxitin-fructose agar (CCFA), Clostridioides difficile typically forms:
C. difficile forms yellow, flat, ground-glass colonies on CCFA with a horse-stable (p-cresol) odour and yellow-green fluorescence under UV light.
After 10 days of treatment for Clostridioides difficile infection, a patient's diarrhoea has stopped. The clinician asks for a repeat toxin test to prove cure. The best response is:
C. difficile spores, toxin and DNA can persist after clinical cure, so a test of cure is not recommended. Only unformed stool from symptomatic patients should be tested.
A vaginal Gram stain is scored by the Nugent method and gives a score of 8. This result indicates:
Nugent scores are 0–3 normal, 4–6 intermediate and 7–10 bacterial vaginosis. High scores reflect loss of lactobacilli and many Gardnerella, Bacteroides/Prevotella and curved Mobiluncus-type rods.
An anaerobic culture from a lung abscess grows thin gram-negative rods with pointed ends (spindle shape) that are indole positive. The most likely organism is:
F. nucleatum appears as thin, fusiform rods with tapered ends, is indole positive and forms bread-crumb-like colonies. Bacteroides are pleomorphic rods with rounded ends; Veillonella are cocci.
On egg yolk agar, an iridescent, pearly ('mother-of-pearl') layer on and around the colonies indicates:
Lipase breaks down free fats in egg yolk and forms an oily, iridescent sheen, as seen with C. botulinum and C. sporogenes. Lecithinase gives an opaque white zone in the medium.
In a GasPak anaerobic jar, what is the role of the palladium catalyst?
The generating envelope releases hydrogen and CO2; palladium catalyses the reaction of hydrogen with residual oxygen to form water.
Obligate anaerobes are intrinsically resistant to aminoglycosides such as gentamicin because:
Aminoglycoside entry depends on an oxygen-driven electron transport system that anaerobes lack; beta-lactamase does not affect aminoglycosides.
An anaerobic gram-negative rod from an abdominal abscess is resistant to vancomycin 5 µg, kanamycin 1 mg and colistin 10 µg special-potency disks. It grows on Bacteroides bile esculin agar with blackening. It belongs to the:
The B. fragilis group resists all three disks, grows in 20% bile and hydrolyzes esculin (black colonies). Fusobacterium is kanamycin and colistin susceptible, and pigmented Prevotella and Porphyromonas are bile sensitive.
An anaerobic gram-positive coccus from a wound is inhibited by a sodium polyanethol sulfonate (SPS) disk. It is most likely:
P. anaerobius is characteristically susceptible to SPS, giving a zone of 12 mm or more. Other anaerobic gram-positive cocci are resistant, and Veillonella is a gram-negative coccus.
A spore-forming gram-positive rod grows on chocolate agar in CO2 as well as anaerobically, but forms spores only under anaerobic conditions. It is catalase negative. The most likely organism is:
Clostridium tertium is aerotolerant but forms spores only anaerobically and is catalase negative. Bacillus species are usually catalase positive and form spores aerobically; C. perfringens does not grow in air.
An anaerobic, black-pigmented gram-negative rod is susceptible to vancomycin 5 µg but resistant to kanamycin 1 mg and colistin 10 µg special-potency disks. It is indole positive. It most likely belongs to the genus:
Porphyromonas is unusual among gram-negative anaerobes because it is vancomycin susceptible. Prevotella and Bacteroides are vancomycin resistant; Fusobacterium is kanamycin and colistin susceptible.