MLT GlobeBacteriology: Anaerobes – page 2
56 Bacteriology MCQs on Anaerobes with answers and explanations.
Infant botulism has classically been linked to feeding babies:
Honey may contain C. botulinum spores, which germinate in the infant gut; it should not be given to babies under one year.
Which part of the human body has the highest numbers of anaerobic bacteria as normal flora?
Anaerobes vastly outnumber aerobes in the colon; the bladder, lower airways and CSF are normally sterile.
Which organism is an obligate anaerobe with spherical terminal spores?
C. tetani is a strict anaerobe with round terminal spores. B. anthracis is aerobic with central spores; Corynebacterium and Cutibacterium do not form spores.
Guests develop abdominal cramps and watery diarrhoea 10 hours after eating a meat stew that was cooled slowly and reheated. The likely cause is:
C. perfringens spores survive cooking, germinate as food cools and release enterotoxin in the gut. Botulism causes paralysis, not diarrhoea.
Which statement about Clostridium tetani is INCORRECT?
Canned-food illness is botulism (C. botulinum). C. tetani is acquired through wounds and is an obligate anaerobe that grows in cooked meat medium.
Which statement about Clostridium perfringens (C. welchii) is INCORRECT?
C. perfringens is non-motile, unlike most clostridia. It is capsulated in tissue, produces lecithinase (Nagler reaction) and seldom sporulates in vivo or in routine media.
Vincent's angina (acute necrotising ulcerative gingivitis/pharyngitis) is a synergistic infection involving:
A Gram stain of the ulcer shows fusiform bacilli with spirochaetes (e.g. Treponema vincentii). A pseudomembrane with toxin suggests diphtheria instead.
Clostridium tetani is best described as:
C. tetani is a motile, peritrichous anaerobe with round terminal spores ('drumstick'). C. perfringens is the non-motile clostridium.
The alpha toxin of Clostridium perfringens damages cell membranes because it is a:
Lecithinase hydrolyses membrane phospholipids, causing haemolysis and tissue necrosis; it gives opalescence on egg-yolk agar (Nagler reaction).
Bacteroides fragilis is intrinsically resistant to:
Aminoglycoside uptake needs oxygen-dependent transport, so all anaerobes are resistant. Metronidazole and carbapenems remain reliably active.
Litmus milk inoculated with an anaerobe shows clot formation torn apart by gas ('stormy fermentation'). The organism is:
C. perfringens ferments lactose, forming acid clot that is disrupted by vigorous gas production.
Pus from a deep pilonidal abscess typically yields:
Pilonidal and perianal abscesses are usually polymicrobial with anaerobes and streptococci. Anaerobic culture is essential.
The most appropriate specimen for Clostridioides difficile testing is:
Testing formed stool or asymptomatic patients detects colonisation, not disease. Two-step testing (GDH or PCR with a toxin assay) is preferred; test of cure is not recommended.
After a severe sore throat, a 19-year-old develops thrombophlebitis of the internal jugular vein and multiple lung abscesses. Anaerobic blood culture bottles grow pleomorphic gram-negative rods. The most likely organism is:
This is Lemierre syndrome, classically caused by F. necrophorum spreading from the pharynx to the jugular vein with septic emboli. Veillonella is a gram-negative coccus, and Clostridium is gram positive.
On egg yolk agar, half of the plate is covered with antitoxin. An isolate shows an opaque zone only on the half without antitoxin. This Nagler reaction identifies which toxin?
C. perfringens alpha toxin is a lecithinase that makes an opaque zone on egg yolk agar; specific antitoxin blocks it. The other toxins do not act on egg yolk lecithin.
A 3-month-old has constipation, a weak cry and poor feeding with floppy muscle tone. Which specimen is most useful to confirm infant botulism?
In infant botulism spores germinate in the gut and produce toxin there, so stool (or enema fluid) is tested for toxin and the organism. Serum toxin is often negative in infants.
A diarrhoeal stool gives a positive glutamate dehydrogenase (GDH) antigen and a negative toxin A/B immunoassay for Clostridioides difficile. The next step in a multistep algorithm is:
Discordant GDH-positive, toxin-negative results are resolved with NAAT for the toxin gene. GDH alone does not separate toxigenic from non-toxigenic strains, and repeating the insensitive toxin EIA adds little.
A woman with a long-term intrauterine device has a pelvic abscess. Pus contains yellow granules; the Gram stain shows branching gram-positive rods, and anaerobic colonies look like molar teeth. The organism is most likely:
Actinomyces israelii forms sulfur granules and molar-tooth colonies, and causes pelvic actinomycosis with IUDs. Nocardia is aerobic and partially acid-fast.
An anaerobic jar is checked 2 hours after setup and the resazurin indicator is still pink. What does this mean?
Resazurin is colourless when reduced (anaerobic) and pink when oxidized. A pink indicator shows oxygen is present, often from a leaking lid, a failed gas generator or an inactive catalyst.
One of four blood culture bottles grows a gram-positive diphtheroid-like rod in the anaerobic bottle after 5 days. It is catalase positive and indole positive. The most likely organism and interpretation is:
C. acnes is an aerotolerant anaerobe of skin that is catalase and indole positive and grows slowly. A single positive bottle usually means contamination. Clostridium and Actinomyces are catalase negative.