Medical Bacteriology MCQs

Question 1

A gram-positive coccus isolated from a wound shows golden-yellow colonies on blood agar but is coagulase negative. The most likely organism is:

Accepted Answer

Staphylococcus saprophyticus — S. saprophyticus can produce yellow pigment but is coagulase negative, unlike S. aureus which is coagulase positive. This organism is commonly associated with urinary tract infections in young women.

Question 2

A patient with "rice water" stool most likely has infection with:

Accepted Answer

Vibrio cholerae — Rice water stool (watery with flecks of mucus resembling water in which rice has been washed) is characteristic of cholera caused by V. cholerae.

Question 3

Which organism can grow at refrigeration temperature (4°C)?

Accepted Answer

Listeria monocytogenes — Listeria is psychrophilic and can multiply at refrigeration temperatures (4°C), making it a significant food safety concern.

Question 4

A 25-year-old sexually active female presents with urinary tract infection. The organism that is coagulase-negative and novobiocin resistant is:

Accepted Answer

Staphylococcus saprophyticus — S. saprophyticus is novobiocin resistant and causes UTI in young sexually active women. S. epidermidis is novobiocin sensitive.

Question 5

The "string of pearls" appearance on Loeffler serum medium is characteristic of:

Accepted Answer

Bacillus anthracis — B. anthracis shows string of pearls appearance when grown in the presence of penicillin on Loeffler medium due to the formation of round bodies along the bacilli chain.

Question 6

A neonate develops meningitis. CSF culture grows gram-negative bacilli with K1 capsular antigen. The organism is:

Accepted Answer

Escherichia coli — E. coli with K1 capsular antigen is the most common cause of neonatal meningitis.

Question 7

"Medusa head" colonies on blood agar are produced by:

Accepted Answer

Bacillus anthracis — B. anthracis produces characteristic medusa head colonies due to chaining of bacilli with irregular edges and comma-shaped projections.

Question 8

Which test differentiates Staphylococcus from Streptococcus?

Accepted Answer

Catalase test — Staphylococcus is catalase positive while Streptococcus is catalase negative.

Question 9

A patient develops pseudomembranous colitis after prolonged antibiotic therapy. The causative organism is:

Accepted Answer

Clostridium difficile — C. difficile produces toxins A and B causing antibiotic-associated pseudomembranous colitis when normal gut flora is disrupted.

Question 10

"Chinese letter" or "V and L shaped" arrangement is seen in:

Accepted Answer

Corynebacterium diphtheriae — C. diphtheriae shows characteristic angular arrangement resembling Chinese letters due to snapping division.

Question 11

A burn patient develops green-blue pus with a fruity odor. The organism is:

Accepted Answer

Pseudomonas aeruginosa — Pseudomonas produces pyocyanin (blue-green pigment) and has a characteristic grape-like or fruity odor.

Question 12

The CAMP test is positive for:

Accepted Answer

Streptococcus agalactiae — Group B Streptococcus (S. agalactiae) shows enhanced hemolysis (arrowhead-shaped zone) when grown near S. aureus on blood agar.

Question 13

Which organism produces black colonies on bismuth sulfite agar?

Accepted Answer

Salmonella typhi — S. typhi produces hydrogen sulfide (H2S) which reacts with bismuth sulfite to form black colonies.

Question 14

A patient presents with flaccid paralysis and descending weakness. The organism that causes this by blocking acetylcholine release is:

Accepted Answer

Clostridium botulinum — C. botulinum toxin blocks acetylcholine release at neuromuscular junctions causing flaccid paralysis.

Question 15

"Swarming" motility on blood agar is characteristic of:

Accepted Answer

Proteus species — Proteus shows swarming motility creating concentric waves on agar due to highly active flagella.

Question 16

A gram-negative coccus that ferments maltose but not glucose is:

Accepted Answer

None of the above — Neisseria species ferment glucose first. N. gonorrhoeae ferments only glucose; N. meningitidis ferments both glucose and maltose.

Question 17

"Elek test" is used to detect toxigenicity of:

Accepted Answer

Corynebacterium diphtheriae — Elek immunoprecipitation test detects diphtheria toxin production.

Question 18

Which organism produces "boxcar" shaped gram-positive rods?

Accepted Answer

Clostridium perfringens — C. perfringens appears as large, rectangular, boxcar-shaped gram-positive rods.

Question 19

A patient develops hemolytic uremic syndrome (HUS) after eating undercooked hamburger. The organism is:

Accepted Answer

Escherichia coli O157:H7 — EHEC O157:H7 produces Shiga toxin causing hemorrhagic colitis and HUS.

Question 20

"Drumstick" or "tennis racket" appearance of spores is seen in:

Accepted Answer

Clostridium tetani — C. tetani has terminal round spores giving a drumstick appearance.

Question 21

The most common cause of acute bacterial meningitis in adults is:

Accepted Answer

Streptococcus pneumoniae — S. pneumoniae is the leading cause of bacterial meningitis in adults.

Question 22

A stool sample shows gram-negative curved rods with "gull wing" or "S" shape. The organism is:

Accepted Answer

Campylobacter jejuni — Campylobacter has characteristic S-shaped or gull wing morphology and darting motility.

Question 23

Which organism is urease positive and causes "alkaline tide" in urine?

Accepted Answer

Proteus mirabilis — Proteus produces urease which splits urea to ammonia, raising urine pH and promoting struvite stone formation.

Question 24

"Nagler reaction" is positive for:

Accepted Answer

Clostridium perfringens — Nagler test detects lecithinase (alpha toxin) production by C. perfringens.

Question 25

A patient with infective endocarditis has blood culture showing catalase negative, gram-positive cocci resistant to optochin and not lysed by bile. The organism is:

Accepted Answer

Enterococcus faecalis — Enterococcus is optochin resistant and bile insoluble, distinguishing it from S. pneumoniae.

Question 26

"Quellung reaction" is used to identify capsule of:

Accepted Answer

All of the above — Quellung reaction (capsular swelling test) can identify capsules of any encapsulated bacteria using specific antisera.

Question 27

Which organism causes "woolsorter's disease"?

Accepted Answer

Bacillus anthracis — Woolsorter's disease is inhalational anthrax from B. anthracis spores.

Question 28

A comma-shaped gram-negative rod showing "shooting star" motility in hanging drop is:

Accepted Answer

Vibrio cholerae — V. cholerae shows rapid darting or shooting star motility due to its single polar flagellum.

Question 29

"Widal test" detects antibodies against:

Accepted Answer

Salmonella typhi — Widal test detects antibodies against O and H antigens of S. typhi in typhoid fever.

Question 30

Which organism produces "currant jelly" sputum?

Accepted Answer

Klebsiella pneumoniae — Klebsiella pneumonia produces thick, bloody, mucoid sputum resembling currant jelly.

Question 31

"McFadyean reaction" showing pink-staining capsule is diagnostic for:

Accepted Answer

Bacillus anthracis — McFadyean staining shows a pink/purple capsule around blue B. anthracis bacilli.

Question 32

A patient develops gas gangrene after a crushing injury. The organism most likely responsible is:

Accepted Answer

Clostridium perfringens — C. perfringens produces alpha toxin causing myonecrosis with gas production.

Question 33

Which organism shows "tumbling motility" at 25°C but not at 37°C?

Accepted Answer

Listeria monocytogenes — Listeria exhibits tumbling motility at room temperature (20-25°C) but motility is reduced at 37°C.

Question 34

"Satellite phenomenon" on blood agar indicates:

Accepted Answer

Staphylococcus aureus providing factors for Haemophilus — H. influenzae grows as satellite colonies around S. aureus, which provides V factor (NAD).

Question 35

A patient with facial nerve palsy, meningitis, and rash has CSF showing gram-negative diplococci. The organism is:

Accepted Answer

Neisseria meningitidis — N. meningitidis causes meningitis with petechial rash and appears as gram-negative kidney bean-shaped diplococci.

Question 36

"Double zone hemolysis" on blood agar is characteristic of:

Accepted Answer

Clostridium perfringens — C. perfringens shows an inner zone of complete beta-hemolysis and an outer zone of incomplete hemolysis.

Question 37

A diabetic patient develops malignant external otitis. The organism is:

Accepted Answer

Pseudomonas aeruginosa — P. aeruginosa causes invasive external otitis in diabetic patients, which can extend to the temporal bone.

Question 38

Which organism is both oxidase and catalase positive?

Accepted Answer

Pseudomonas aeruginosa — P. aeruginosa is both oxidase and catalase positive. Enterobacteriaceae are typically oxidase negative.

Medical Bacteriology MCQs

Questions, Answers & Explanations