Systemic bacteriology: Gram-positive organisms

65 Kerala MSc MLT Entrance practice questions on Systemic bacteriology: Gram-positive organisms, part of Microbiology. The first 25 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 · Medium

   As pneumococcal colonies age, their centres sink, giving a draughtsman (carrom coin) appearance. This is caused by:

   1. ADrying of the agar under each colony
   2. BAutolysis by the organism's own amidase
   3. CLoss of the capsule in older colonies
   4. DAlpha-haemolysin diffusing into the agar

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   Answer: B · Autolysis by the organism's own amidase

   Pneumococci have an autolytic enzyme, an amidase, that breaks down their own cell wall as cultures age. Cells in the centre of a colony lyse first, so the centre collapses and the colony looks like a draughtsman or carrom coin. The same autolysin is activated by bile salts in the bile solubility test. Viridans streptococci lack this autolysis and keep domed colonies.

   • A: Drying of the agar affects the whole plate, not each colony centre.
   • C: Capsule loss gives rough colonies, not a central depression.
   • D: Alpha haemolysis colours the agar green but does not flatten the colony.

2. Question 2 · Hard

   A 25-year-old woman develops high fever, vomiting, hypotension and a diffuse red rash 2 days after nasal packing for septoplasty. Blood cultures are sterile, but the nasal pack grows Staphylococcus aureus. How does this illness arise?

   1. ABacteraemia missed because cultures were too few
   2. BImmune complexes formed against cell wall antigens
   3. CExfoliative toxin splitting the epidermal layers
   4. DToxin from a local focus acting as a superantigen

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   Answer: D · Toxin from a local focus acting as a superantigen

   This is staphylococcal toxic shock syndrome. TSST-1 or an enterotoxin produced in the packed nose is absorbed and acts as a superantigen, linking MHC class II molecules to T cell receptors outside the antigen-binding site and activating huge numbers of T cells. The resulting cytokine storm causes fever, rash, shock and multi-organ failure. Blood cultures are usually negative in staphylococcal TSS, and the focus is often a tampon, packing or wound.

   • A: Sterile blood cultures are typical of staphylococcal TSS, not a sampling error.
   • B: Immune complex disease develops over weeks, not 2 days.
   • C: Exfoliative toxins cause scalded skin syndrome, not shock with a red rash.

3. Question 3 · Medium

   Vomiting starts 2 to 3 hours after a meal of milk sweets in staphylococcal food poisoning. The short incubation is because:

   1. AThe cocci multiply rapidly in the small gut
   2. BThe toxin is heat-labile and quickly absorbed
   3. CThe enterotoxin is already formed in the food
   4. DThe cocci invade the intestinal mucosa

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   Answer: C · The enterotoxin is already formed in the food

   Staphylococcus aureus multiplies in food left at room temperature and releases enterotoxin into it. The toxin is heat-stable, so reheating does not destroy it, and illness begins 1 to 6 hours after eating. The enterotoxin acts on the gut and the vomiting centre, so vomiting dominates and fever is unusual. Food handlers with skin lesions or nasal carriage are the usual source.

   • A: Illness is caused by preformed toxin, not by growth in the gut.
   • B: The enterotoxin is heat-stable, which is why reheating does not help.
   • D: S. aureus food poisoning is an intoxication, not an invasive infection.

4. Question 4 · Medium

   Which sequel of Streptococcus pyogenes infection can follow skin infection (pyoderma) as well as pharyngitis?

   1. ARheumatic heart disease
   2. BSydenham chorea
   3. CAcute rheumatic fever
   4. DAcute glomerulonephritis

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   Answer: D · Acute glomerulonephritis

   Post-streptococcal glomerulonephritis can follow both throat and skin infections, because nephritogenic strains include skin types. Acute rheumatic fever, with its carditis and chorea, follows pharyngitis only. Both are non-suppurative sequelae that appear after a latent period, about 1 to 3 weeks for nephritis. Anti-DNase B is a better marker than ASO after skin infection.

   • A: Rheumatic heart disease results from rheumatic fever, which follows pharyngitis.
   • B: Sydenham chorea is a manifestation of rheumatic fever.
   • C: Rheumatic fever follows throat infection, not skin infection.

5. Question 5 · Easy

   Tetanospasmin produces spastic paralysis by:

   1. ABlocking acetylcholine release at end plates
   2. BBlocking release of glycine and GABA
   3. CInactivating elongation factor 2 in cells
   4. DRaising cyclic AMP inside nerve cells

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   Answer: B · Blocking release of glycine and GABA

   Tetanospasmin travels back along motor nerves to the spinal cord and brainstem. There it cleaves synaptobrevin in inhibitory interneurons, so glycine and GABA are not released. Without inhibition the motor neurons fire unchecked, causing rigidity and spasms such as trismus and opisthotonus. Botulinum toxin acts on the same protein family but at the neuromuscular junction, causing flaccid paralysis.

   • A: Blocking acetylcholine release is botulinum toxin, which causes flaccid paralysis.
   • C: Inactivating EF-2 is the action of diphtheria toxin.
   • D: Raising cAMP is the action of cholera and related enterotoxins.

6. Question 6 · Hard

   A 7-year-old has fever, sore throat, a red strawberry tongue and a fine rough rash that spares the area around the mouth. Throat culture: beta-haemolytic catalase-negative cocci, bacitracin sensitive, PYR positive. The rash is due to:

   1. AImmune complexes against M protein
   2. BStreptococcal pyrogenic exotoxins
   3. CStreptolysin O released in the skin
   4. DBacteraemia with cocci in the skin

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7. Question 7 · Easy

   In the Quellung reaction, pneumococci are serotyped by:

   1. AApparent capsular swelling with type antiserum
   2. BAgglutination of latex particles in saline
   3. CLysis of the cells by bile salts
   4. DInhibition around an optochin disc

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8. Question 8 · Hard

   A 9-year-old unvaccinated boy has a grey membrane on the tonsils and neck swelling. A throat swab stained by Albert's method shows green bacilli with bluish-black granules. Why should this smear not be reported as confirmed diphtheria?

   1. AGranules prove only that the strain is non-toxigenic
   2. BCommensal diphtheroids can look similar
   3. CSmears are positive only after antitoxin therapy
   4. DAlbert's stain cannot show granules in smears

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9. Question 9 · Hard

   Large gram-positive bacilli without spores are seen in exudate from a crushed leg after a road accident. Anaerobic culture shows a double zone of haemolysis, and the Nagler reaction is positive. Which further result is expected?

   1. ARound terminal spores in the culture
   2. BSwarming growth over the whole plate
   3. CStormy fermentation of litmus milk
   4. DActive motility in semi-solid medium

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10. Question 10 · Easy

    Holding a specimen at 4 °C for several weeks before subculture (cold enrichment) helps to isolate:

    1. AEnterococcus faecalis
    2. BStreptococcus agalactiae
    3. CListeria monocytogenes
    4. DBacillus cereus

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11. Question 11 · Medium

    In a hanging drop, Listeria monocytogenes shows tumbling motility at 25 °C but is hardly motile at 37 °C. Why?

    1. AIt moves only in the absence of oxygen
    2. BIts motility depends on actin from host cells
    3. CIt forms flagella mainly at lower temperatures
    4. DIts flagella are shed when it is heated

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12. Question 12 · Medium

    Colonies of Corynebacterium diphtheriae on potassium tellurite agar are grey to black because the organism:

    1. AMakes a dark melanin-like pigment
    2. BTakes up iron from the serum added
    3. CProduces hydrogen sulphide from cystine
    4. DReduces tellurite to metallic tellurium

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13. Question 13 · Easy

    The capsule of Bacillus anthracis is made of:

    1. ATeichoic acid
    2. BHyaluronic acid
    3. CMycolic acid
    4. DPoly-D-glutamic acid

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14. Question 14 · Medium

    Infant botulism differs from food-borne botulism in that:

    1. AIt is caused by Clostridioides difficile
    2. BToxin is absorbed from an infected wound
    3. CPreformed toxin in food is swallowed
    4. DSpores germinate and make toxin in the gut

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15. Question 15 · Medium

    Why is the laboratory of limited value in confirming a clinical diagnosis of tetanus?

    1. AC. tetani is often not isolated and can be found without disease
    2. BGram stain of the wound always shows mixed flora only
    3. CTetanus toxin cannot be detected in any animal test
    4. DThe organism dies as soon as the wound is sampled

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16. Question 16 · Hard

    A 50-year-old with rheumatic mitral valve disease has fever 3 weeks after a tooth extraction. Three blood cultures grow alpha-haemolytic, catalase-negative gram-positive cocci in chains. Optochin: no zone; bile solubility: negative; bile esculin: negative; 6.5% NaCl: no growth. The likely organism is:

    1. AStreptococcus pneumoniae
    2. BStreptococcus gallolyticus
    3. CEnterococcus faecalis
    4. DViridans streptococci

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17. Question 17 · Hard

    A 28-year-old pregnant woman at 30 weeks has fever and myalgia after eating soft cheese. Blood culture grows short gram-positive bacilli. Catalase positive, esculin hydrolysed, beta-haemolysis on blood agar, tumbling motility at 25 °C, CAMP positive. The isolate is:

    1. AListeria monocytogenes
    2. BErysipelothrix rhusiopathiae
    3. CCorynebacterium diphtheriae
    4. DStreptococcus agalactiae

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18. Question 18 · Medium

    Lancefield grouping of beta-haemolytic streptococci is based on the:

    1. APattern of haemolysis on blood agar
    2. BCapsular hyaluronic acid
    3. CM protein on the cell surface
    4. DCarbohydrate antigen of the cell wall

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19. Question 19 · Hard

    An 80-year-old catheterised man has fever. Urine and blood grow non-haemolytic, catalase-negative gram-positive cocci in pairs and short chains. Bile esculin positive, growth in 6.5% NaCl, PYR positive. Cefotaxime was started empirically. What does the laboratory advise?

    1. AViridans streptococci; cefotaxime is adequate
    2. BS. gallolyticus; colonoscopy and continue cefotaxime
    3. CS. agalactiae; penicillin is the drug of choice
    4. DEnterococcus; cephalosporins are inactive, test ampicillin

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20. Question 20 · Medium

    Blood cultures from a 64-year-old man with endocarditis grow Streptococcus gallolyticus (formerly S. bovis biotype I). What further investigation is advised?

    1. AUrine culture for enterococci
    2. BColonoscopy for colorectal tumour
    3. CBone marrow biopsy for acute leukaemia
    4. DThroat swab for group A carriage

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21. Question 21 · Medium

    Botulinum toxin causes flaccid paralysis by:

    1. AInhibiting cholinesterase at the synapse
    2. BBlocking acetylcholine release at nerve endings
    3. CBlocking glycine release from inhibitory spinal neurons
    4. DDestroying acetylcholine receptors on muscle

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22. Question 22 · Hard

    A blood isolate of S. aureus gives a cefoxitin (30 µg) zone of 17 mm. Interpretive criteria: resistant 21 mm or less, susceptible 22 mm or more. The isolate is also penicillin resistant. How should the oxacillin and other beta-lactam results be reported?

    1. AMRSA; report beta-lactams as resistant
    2. BReport only penicillin resistant; others susceptible
    3. CReport oxacillin susceptible as cefoxitin is only a screen
    4. DReport as borderline and repeat with a methicillin disc

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23. Question 23 · Medium

    When the catalase test is done on a colony taken from blood agar, a streptococcus may appear catalase-positive. Why?

    1. AHaemolysins release oxygen from haemoglobin
    2. BRed cells in the medium contain catalase
    3. CHydrogen peroxide reacts with the agar
    4. DBlood agar induces catalase in streptococci

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24. Question 24 · Easy

    In McFadyean's reaction, blood smears from an animal dead of anthrax are stained with polychrome methylene blue. The capsule appears:

    1. APink around red-stained bacilli
    2. BGolden fluorescent outline around bacilli
    3. CGreen around black bacilli
    4. DPurple amorphous material around blue bacilli

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25. Question 25 · Medium

    The metachromatic (volutin) granules of Corynebacterium diphtheriae, seen with Albert's stain, are composed of:

    1. APoly-beta-hydroxybutyrate
    2. BPolymetaphosphate
    3. CSulphur
    4. DGlycogen

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