MLT GlobeBacteriology: Anaerobes
56 Bacteriology MCQs on Anaerobes with answers and explanations.
A Gram-stained smear from a deep wound shows Gram-positive rods with round terminal spores giving a 'drumstick' shape. Which organism is suggested?
C. tetani has spherical terminal spores wider than the cell, giving drumstick (or tennis-racket) forms. The other organisms do not form spores.
A crushed leg wound shows crepitus, myonecrosis and Gram-positive boxcar-shaped rods without visible spores. Which organism causes this gas gangrene?
C. perfringens alpha-toxin (lecithinase) destroys muscle and produces gas. C. difficile causes colitis, C. tetani causes tetanus.
Several people develop descending flaccid paralysis after eating home-canned vegetables. Which organism is responsible?
Improperly canned low-acid food allows C. botulinum spores to germinate anaerobically and produce botulinum neurotoxin. C. perfringens food poisoning follows reheated meat dishes and causes diarrhoea.
McIntosh and Fildes' jar is used to culture which type of organism?
The jar removes oxygen using hydrogen and a catalyst, creating anaerobic conditions for organisms such as clostridia. Modern labs use gas-generating packs.
Which Clostridium species is NOT a cause of gas gangrene?
C. perfringens, C. novyi and C. septicum are histotoxic clostridia causing myonecrosis. C. botulinum produces a neurotoxin causing botulism.
A farmer develops lockjaw and muscle spasms a week after a soil-contaminated puncture wound. Which organism is responsible?
Tetanospasmin from C. tetani blocks inhibitory neurotransmitter release, causing trismus and spastic paralysis. The other clostridia cause gas gangrene.
Which spore-forming bacillus grows aerobically, not anaerobically?
Bacillus species are aerobic or facultative spore formers; Clostridium species are anaerobes.
Which of these is an obligate anaerobe?
Clostridia are strict anaerobes. B. anthracis and Listeria are facultative; Nocardia is aerobic.
On blood agar, an anaerobic gram-positive rod shows an inner zone of complete and outer zone of partial haemolysis. Identify it.
The double zone results from theta toxin (complete) and alpha toxin (partial). It is characteristic of C. perfringens.
Which specimen is acceptable for anaerobic culture?
Anaerobic culture needs specimens collected without contact with normal flora, such as aspirates or tissue sent in anaerobic transport. Sputum, voided urine and surface swabs are contaminated with anaerobic flora.
A colony on anaerobic blood agar shows an inner zone of complete hemolysis surrounded by an outer zone of partial hemolysis. This double-zone hemolysis is typical of:
C. perfringens shows double-zone hemolysis: the inner complete zone is from theta toxin and the outer partial zone is from alpha toxin.
Botulinum toxin causes flaccid paralysis by:
Botulinum neurotoxin cleaves SNARE proteins so acetylcholine cannot be released at the neuromuscular junction, causing descending flaccid paralysis.
To confirm that a colony from an anaerobic blood agar plate is an obligate anaerobe, the laboratory should:
In aerotolerance testing, the colony is subcultured both aerobically (chocolate agar in CO2) and anaerobically. Growth only under anaerobic conditions confirms an obligate anaerobe.
Obligate anaerobes are damaged by oxygen mainly because they lack:
Without superoxide dismutase and catalase, obligate anaerobes cannot remove superoxide and hydrogen peroxide formed when oxygen is present, so these toxic products damage them.
Pseudomembranous colitis following antibiotic therapy is caused by:
Antibiotics disturb the gut flora, allowing toxigenic C. difficile to overgrow and produce toxins A and B.
Methylene blue is used as an indicator in anaerobic jars. Under properly anaerobic conditions it becomes:
Methylene blue is blue when oxidised and turns colourless when reduced under anaerobic conditions.
Which anaerobe is most frequently isolated from intra-abdominal abscesses and other anaerobic infections?
B. fragilis is the most commonly isolated anaerobic pathogen, especially after bowel perforation.
A foul, putrid smell from pus or a wound specimen most strongly suggests infection with:
Anaerobes produce volatile fatty acids and amines that give a characteristic foul odour.
Which antibiotic is active against most obligate anaerobes and is widely used to treat anaerobic infections?
Metronidazole is activated only inside anaerobic cells, so Bacteroides, Clostridium and most other obligate anaerobes are susceptible; gentamicin, aztreonam and colistin have no useful anaerobic activity.
In a tube of fluid thioglycollate medium, where do obligate anaerobes grow?
Oxygen is lowest at the bottom of thioglycollate broth, so obligate anaerobes grow there; obligate aerobes grow at the top.