MLT GlobeBacteriology: Gram-positive bacilli
58 Bacteriology MCQs on Gram-positive bacilli with answers and explanations.
Which group of Gram-positive bacteria forms branching filaments resembling fungal hyphae?
Actinomycetes grow as branching filaments that may fragment into rods, hence 'filamentous bacteria'. Spirochetes are helical; Bacillus forms straight rods.
Albert's stain of Corynebacterium diphtheriae shows bluish-black granules that stain differently from the dye colour. These volutin granules are also called:
Volutin (polyphosphate) granules stain metachromatically, a feature used to identify C. diphtheriae. They are storage inclusions, not spores or flagella.
Albert's, Ponder's and Neisser's stains are used to show metachromatic granules in:
These stains highlight volutin granules at the poles of C. diphtheriae, supporting a presumptive diagnosis of diphtheria.
In an Albert-stained smear of Corynebacterium diphtheriae, the volutin (metachromatic) granules appear:
Albert stain shows green bacilli containing bluish-black metachromatic granules, a feature supporting C. diphtheriae in throat smears.
The historical Schick test, an intradermal toxin injection, was used to assess immunity to:
A small dose of diphtheria toxin was injected; no reaction meant circulating antitoxin (immunity). It is now obsolete, replaced by serology.
A grey pharyngeal membrane that bleeds when scraped off, with a 'bull-neck' swelling, suggests:
Diphtheria pseudomembrane is adherent and bleeds on removal. Candida plaques wipe off easily; mononucleosis gives exudative tonsillitis.
A throat smear is stained with Albert stain to look for metachromatic granules. This screens for:
Albert stain shows green rods with bluish-black volutin granules, typical of C. diphtheriae. Confirmation needs culture and toxin testing.
After growth on Loeffler serum medium, methylene blue staining of Corynebacterium diphtheriae shows:
Loeffler medium promotes polyphosphate storage granules that stain red-purple (metachromatic) with methylene blue. Terminal spores suggest Clostridium, and safety-pin staining suggests Yersinia or Burkholderia.
'Cold enrichment' at 4 °C to recover an organism from mixed specimens such as food or placenta is used for:
Listeria grows at refrigerator temperature while most competing flora do not, so holding the sample at 4 °C enriches it. This also explains its spread through refrigerated foods.
Several people vomit 2 hours after eating fried rice that was kept warm for many hours. The most likely cause is a heat-stable emetic toxin from:
B. cereus spores survive cooking, and the emetic toxin (cereulide) forms in stored rice and causes vomiting within hours. C. perfringens causes diarrhoea after 8–16 hours.
A wet mount of vaginal discharge (pH 5.0, fishy odour with KOH) shows squamous cells so heavily covered with small coccobacilli that their borders are hidden. The main organism involved is:
These are clue cells, typical of bacterial vaginosis, in which Gardnerella vaginalis and anaerobes replace lactobacilli. Lactobacilli keep vaginal pH low and do not form clue cells.
A Gram stain of Corynebacterium species typically shows:
Corynebacteria are pleomorphic, club-shaped gram-positive rods that snap apart during division, forming palisades and V or L shapes ('Chinese letters').
In the Schaeffer-Fulton spore stain, endospores appear:
Malachite green is driven into spores with heat, and safranin counterstains the vegetative cells. Spores appear green inside pink-red cells. In a Gram stain, spores appear only as unstained areas.
On a Gram stain, Bacillus anthracis typically appears as:
B. anthracis forms large, square-ended Gram-positive rods in long chains, often described as a 'bamboo stick' appearance.
Which Gram-positive rods (Döderlein's bacilli) keep the healthy vaginal pH acidic?
Lactobacilli produce lactic acid from glycogen, keeping vaginal pH low and protecting against pathogens.
Cutaneous anthrax typically produces which lesion?
The 'malignant pustule' of cutaneous anthrax becomes a painless black necrotic eschar surrounded by oedema.
The diphtheria component of the DTP vaccine is a:
Diphtheria vaccine is toxin treated with formaldehyde (toxoid), which induces antitoxin antibodies without causing disease.
Respiratory diphtheria spreads from person to person mainly by:
C. diphtheriae is carried in the throat and spread by droplets and close contact.
Spores of which organism are used as biological indicators to check that a steam autoclave is working?
G. stearothermophilus spores are highly heat resistant, so their killing shows that sterilising conditions were reached.
Anthrax is primarily a disease of which animals, passing to humans by contact with them or their products?
Anthrax is a zoonosis of herbivores that eat spores from soil; humans are infected from animals, hides, wool or meat.