Bacteriology MCQs: Bacterial Skin and Eye Infections

You can find the answer to these multiple choice questions related to bacterial skin infections at the end of this blog post. Write down your answers and cross-check with the answer key provided.

1. A massive lesion caused by the spread of Staphylococcus aureus infection (often on the neck and upper back) is called:
   a. Folliculitis
   b. Abscess
   c. Furuncle
   d. Carbuncle
   e. Pustule
2. Scalded skin syndrome is caused by exotoxin (exfoliatins) produced by:
   a. Streptococcus pyogenes
   b. Pseudomonas aeruginosa
   c. Staphylococcus aureus
   d. Cutibacterium acnes (formerly Propionibacterium acnes)
   e. Demodex folliculorum
3. Which of the following microorganisms causes scarlet fever?
   a. Streptococcus pyogenes
   b. Pseudomonas aeruginosa
   c. Staphylococcus aureus
   d. Cutibacterium acnes
   e. Demodex folliculorum
4. The pus producing skin infections (pyoderma) can be caused by
   a. Staphylococci
   b. Streptococci
   c. Corynebacteria
   d. A combination of staphylococci, streptococci and corynebacteria
   e. All of these
5. Which of the following is not the characteristics of impetigo?
   a. Common in children
   b. Can be caused by Staphylococcus aureus
   c. Can be caused by Streptococcus pyogenes
   d. Is highly contagious
   e. Can be caused by pseudomonads
6. Burn patients often develop nosocomial infection caused by:
   a. Staphylococcus epidermidis
   b. Corynebacterium spp.
   c. Staphylococcus aureus
   d. Candida albicans
   e. Pseudomonas aeruginosa
7. The bacteria that multiply in blocked skin pores, metabolize sebum, and can lead to the development of acne are:
   a. Pseudomonas aeruginosa
   b. Propionibacterium spp.
   c. Serratia marcescens
   d. Streptococcus pyogenes
   e. Acne is not caused by bacterial infection
8. Bacterial conjunctivitis (inflammation of the eye conjunctiva) can be caused by:
   a. Staphylococcus aureus
   b. Streptococcus pneumoniae
   c. Neisseria gonorrhoeae
   d. Pseudomonas aeruginosa
   e. All of the above bacteria can cause conjunctivitis
9. The leading cause of preventable blindness in the world is caused by:
   a. Chlamydia trachomatis
   b. Haemophilus influenzae
   c. Neisseria gonorrhoeae
   d Staphylococcus aureus
   e. Streptococcus pneumoniae
10. Gas gangrene is most likely associated with infection with:
    a. Staphylococcus aureus
    b. Clostridium perfringens
    c. Streptococcus pneumoniae
    d. Neisseria gonorrhoeae
    e. Pseudomonas aeruginosa

Answer key

1. d. Carbuncle
2. c. Staphylococcus aureus
3. a. Streptococcus pyogenes
4. e. All of these
5. e. Can be caused by pseudomonads
6. e. Pseudomonas aeruginosa
7. b. Cutibacterium (formerly Propionibacterium) acnes
8. e. All of the above bacteria can cause conjunctivitis
9. a. Chlamydia trachomatis
10. b. Clostridium perfringens

Why these are the answers

1. Carbuncle. A carbuncle is a large lesion formed when several adjacent infected hair follicles (furuncles) coalesce into one deep, pus-draining mass, classically on the back of the neck and upper back, and caused by Staphylococcus aureus. That matches the stem exactly. The distractors sit on a progression worth memorizing: folliculitis (follicle inflammation) becomes a furuncle (a single-follicle boil), and multiple furuncles merging becomes a carbuncle. An abscess is a walled-off pus collection not tied to follicles, and a pustule is just a small pus-filled bump. Size and multiple follicles are what make this a carbuncle rather than a furuncle.
2. Staphylococcus aureus. Staphylococcal scalded skin syndrome is caused by exfoliative toxins (exfoliatins) from S. aureus. These toxins split the epidermis at the granular layer, so the top layer of skin peels away in sheets, most often in infants and young children. Streptococcus pyogenes causes other skin disease but not this toxin-mediated peeling.
3. Streptococcus pyogenes. Scarlet fever follows a group A streptococcal infection (usually strep throat) in strains that produce erythrogenic (pyrogenic) toxin, which causes the characteristic diffuse red rash and strawberry tongue. The toxin gene is carried by a lysogenic bacteriophage, which is why only certain strains cause scarlet fever.
4. All of these. Pyoderma simply means a pus-producing skin infection, and it is not limited to one genus. Staphylococci and streptococci are the usual causes, but corynebacteria and mixed flora can also produce pyoderma, so the inclusive answer is correct. The teaching point is that "pyoderma" is a descriptive term, not a single-organism diagnosis.
5. Can be caused by pseudomonads. This asks for the statement that is NOT true of impetigo. Impetigo is common in children, highly contagious, and caused by Staphylococcus aureus and Streptococcus pyogenes, so those statements are all true. It is not typically caused by pseudomonads, which makes that the false statement and the answer. Pseudomonas skin disease has a different profile (burns, hot-tub folliculitis), not the honey-crusted lesions of impetigo.
6. Pseudomonas aeruginosa. Burn patients are especially prone to Pseudomonas aeruginosa wound infection and sepsis, because the moist, protein-rich burn surface suits this environmental organism and the patient's skin barrier and immunity are compromised. Pseudomonas also resists many antibiotics and disinfectants, which is why it thrives in the hospital setting. The blue-green pigment and sweet, grape-like odor on the dressing are classic bedside clues.
7. Cutibacterium acnes (formerly Propionibacterium acnes). This slow-growing, anaerobic, gram-positive rod lives in hair follicles and sebaceous glands. When a pore becomes blocked, it metabolizes the trapped sebum and releases inflammatory products, contributing to acne. Its role means acne has a bacterial component (so the "not caused by bacteria" option is wrong), even though hormones and follicular blockage are also central.
8. All of the above bacteria can cause conjunctivitis. Staphylococcus aureus, Streptococcus pneumoniae, Neisseria gonorrhoeae, and Pseudomonas aeruginosa can all cause bacterial conjunctivitis, though with different profiles: N. gonorrhoeae causes a hyperacute, sight-threatening conjunctivitis, notably in newborns (ophthalmia neonatorum), and Pseudomonas is dangerous in contact-lens wearers because it can rapidly involve the cornea. Because every organism listed is a genuine cause, the inclusive answer is correct.
9. Chlamydia trachomatis. Trachoma, caused by repeated Chlamydia trachomatis infection (serovars A to C), is the leading infectious cause of preventable blindness worldwide. Recurrent infection scars the conjunctiva, turns the eyelashes inward (trichiasis), and abrades the cornea until it opacifies. It spreads in conditions of poor water and sanitation, which is why it is preventable. N. gonorrhoeae causes neonatal eye infection but not this chronic scarring blindness. See more about Chlamydia trachomatis
10. Clostridium perfringens. Gas gangrene (clostridial myonecrosis) is most often caused by Clostridium perfringens, an anaerobic, spore-forming, gram-positive rod. It grows in deep, poorly oxygenated wound tissue and produces alpha toxin (a lecithinase) that destroys muscle and releases gas, giving the crepitus (crackling under the skin) that names the disease. Its anaerobic, gas-forming biology is the whole picture: devitalized tissue, rapid spread, and gas in the tissues.