Bacteriology Practice Questions

Q: A 12 year old boy presents with vomiting within 3 hours of consumption of food at a party. What is the likely causative organism responsible for the symptoms?

Staphylococcus aureus. ***Staphylococcus aureus*** - **Vomiting occurring within 1-6 hours of food consumption** is highly characteristic of preformed toxin ingestion, commonly associated with *S. aureus*. - *S. aureus* produces **heat-stable enterotoxins** that cause rapid onset nausea, vomiting, and abdominal cramps. *Clostridium perfringens* - This bacterium typically causes symptoms like **abdominal cramps and diarrhea**, with vomiting being less common. - The onset of illness from *C. perfringens* is usually **8-16 hours** after consumption, which is longer than the 3-hour window in this case. *Clostridium botulinum* - *C. botulinum* causes **botulism**, a neuroparalytic illness, which may include nausea and vomiting in its early stages but is primarily characterized by **flaccid paralysis**. - Symptoms usually appear **12-36 hours** after exposure, which is much longer than the presentation described. *Salmonella* - **Salmonellosis** typically has an incubation period of **6-72 hours**, with symptoms including diarrhea, fever, and abdominal cramps; vomiting may occur but is not as immediate or predominant as with *S. aureus* toxin. - The illness is caused by bacterial colonization and invasion, not preformed toxins, leading to a longer onset compared to the 3-hour presentation.

Q: A 29 year old male was brought to ED with complaints of nausea, vomiting and abdominal cramps 3-4 hours after eating meat at dinner. What is the likely cause of this condition?

Staphylococcus aureus. ***Staphylococcus aureus*** - The rapid onset of symptoms (3-4 hours) after eating meat, characterized by **nausea**, **vomiting**, and **abdominal cramps**, is highly suggestive of preformed toxin-mediated food poisoning, with **Staphylococcus aureus** being a common culprit. - *S. aureus* produces heat-stable enterotoxins that, when ingested, cause prompt gastrointestinal symptoms due to their direct irritant effect on the gut. *Clostridium perfringens* - This bacterium typically causes food poisoning with an incubation period of **8-16 hours** and symptoms primarily include **abdominal cramps** and **diarrhea**, with vomiting being less common. - Symptoms arise from toxins produced *in vivo* after spore ingestion, not preformed toxins, which leads to a longer incubation time. *Clostridium botulinum* - While *Clostridium botulinum* produces a potent neurotoxin, its food poisoning primarily manifests as **neurological symptoms** (e.g., flaccid paralysis, diplopia, dysphagia), not acute gastrointestinal upset like vomiting and cramps. - The incubation period is typically **12-36 hours**, longer than the reported 3-4 hours, and it is usually associated with improperly canned foods. *Salmonella Enteritidis* - *Salmonella* infections usually have a longer incubation period of **6 hours to 6 days**, and while they can cause nausea, vomiting, cramps, and diarrhea, the rapid onset in this case makes *S. aureus* more likely. - Infection typically leads to **inflammatory gastroenteritis**, sometimes with fever, which is not mentioned as a prominent symptom here.

Q: Superantigen is produced by which of the following?

Streptococcus pyogenes. ***Streptococcus pyogenes*** - *Streptococcus pyogenes* produces **superantigens** such as **streptococcal pyrogenic exotoxins (SPE-A, SPE-B, SPE-C)**. - These superantigens cause massive non-specific T-cell activation, leading to **toxic shock syndrome** and **scarlet fever**. - Superantigens bypass normal antigen processing by directly binding MHC class II molecules to T-cell receptors. *Clostridium perfringens* - *Clostridium perfringens* primarily produces **alpha-toxin** (phospholipase C) and other toxins causing **gas gangrene** and **food poisoning**. - It does not produce superantigens. *Vibrio cholerae* - *Vibrio cholerae* produces **cholera toxin**, an **enterotoxin** that causes massive fluid secretion in the intestine, leading to profuse watery diarrhea. - Cholera toxin is not a superantigen; it acts by activating adenylate cyclase through ADP-ribosylation. *Staphylococcus epidermidis* - *Staphylococcus epidermidis* is a common skin commensal that can cause infections related to **medical devices** and **biofilm formation**. - Unlike *Staphylococcus aureus* (which produces TSST-1 and enterotoxins), *S. epidermidis* does not produce superantigens.

Q: Urethritis in males is not caused by:

H. ducreyi. ***H. ducreyi*** - *Haemophilus ducreyi* is the causative agent of **chancroid**, a sexually transmitted infection characterized by **genital ulcers** [1] with painful regional lymphadenopathy. - It does not typically cause urethritis as its primary site of infection is the **skin and mucous membranes** of the external genitalia, forming ulcers rather than urethral inflammation. *Chlamydia* - **Chlamydia trachomatis** is a common cause of **non-gonococcal urethritis** in males [1]. - It often presents with **dysuria**, **urethral discharge**, and sometimes can be asymptomatic [1]. *Gonococcus* - **Neisseria gonorrhoeae** (gonococcus) is a well-known cause of urethritis, often referred to as **gonococcal urethritis** [1]. - It typically causes a **purulent urethral discharge** and **dysuria** [1]. *Trichomonas* - **Trichomonas vaginalis** can cause **urethritis** in males [1], although it is less common than in females. - Males may experience **dysuria**, **urethral discharge**, or can be asymptomatic carriers.

Q: Gram stain of CSF shows gram-negative diplococci. Culture grows on Thayer-Martin medium. Most likely organism?

N. meningitidis. ***N. meningitidis*** - The presence of **gram-negative diplococci** in CSF is a hallmark feature of *Neisseria meningitidis*. - **Thayer-Martin medium** is a selective medium specifically designed for the isolation and growth of *Neisseria* species, inhibiting other bacterial and fungal contaminants. *L. monocytogenes* - *Listeria monocytogenes* is a **gram-positive rod**, which contradicts the gram stain finding of gram-negative diplococci. - It typically causes meningitis in neonates, elderly, and immunocompromised individuals, but its morphology is distinct. *S. pneumoniae* - *Streptococcus pneumoniae* is a **gram-positive coccus**, often appearing in pairs (diplococci) or short chains, but it is gram-positive, not gram-negative. - It is a common cause of bacterial meningitis but does not grow readily on Thayer-Martin medium. *H. influenzae* - *Haemophilus influenzae* is a **pleomorphic gram-negative coccobacillus**, not a diplococcus, and it requires specific growth factors (X and V factors) found in chocolate agar. - While it can cause meningitis, its characteristic morphology and growth requirements differ from the description.

Q: Which microorganism causes pseudomembranous colitis?

C. difficile. ***C. difficile*** - **Clostridioides difficile** (formerly Clostridium difficile) infection is the most common cause of **pseudomembranous colitis**, characterized by inflammation of the colon and formation of pseudomembranes. - This infection often occurs after **antibiotic use**, which disrupts the normal gut flora, allowing C. difficile to proliferate and produce toxins. *Shigella* - **Shigella** species cause **shigellosis**, a type of dysentery characterized by bloody diarrhea, fever, and abdominal cramps. - While it can cause severe colitis, it typically does not lead to the distinct pseudomembranous lesions seen in C. difficile infection. *E. coli* - Various strains of **Escherichia coli** can cause different gastrointestinal illnesses, such as traveler's diarrhea (enterotoxigenic E. coli) or hemorrhagic colitis (enterohemorrhagic E. coli O157:H7). - However, E. coli is not the causative agent of **pseudomembranous colitis**. *Salmonella* - **Salmonella** species cause salmonellosis, which can manifest as **gastroenteritis** (food poisoning) or **typhoid fever**. - While it causes enterocolitis, it does not typically result in the formation of **pseudomembranes** that characterize C. difficile infection.

Q: Most commonly associated organism in bacterial vaginosis?

Gardnerella vaginalis. ***Gardnerella vaginalis*** - **Gardnerella vaginalis** is the predominant bacterium associated with the polymicrobial condition known as **bacterial vaginosis (BV)**. - While BV involves a shift in the vaginal microbiome with an increase in various anaerobic bacteria, **Gardnerella vaginalis** is consistently implicated in its pathogenesis and is often present in high concentrations. *Candida albicans* - **Candida albicans** is the most common cause of **vulvovaginal candidiasis** (yeast infection), not bacterial vaginosis. - Symptoms of candidiasis typically include intense itching, burning, and thick, white, "cottage cheese-like" discharge, distinct from BV. *E. coli* - **E. coli** is a common bacterium found in the gastrointestinal tract and is a frequent cause of **urinary tract infections (UTIs)**. - It is not a primary cause of bacterial vaginosis in the vagina. *Trichomonas vaginalis* - **Trichomonas vaginalis** is a **protozoan parasite** that causes **trichomoniasis**, a sexually transmitted infection. - While it causes vaginitis, its presentation often includes frothy, greenish-yellow discharge and a "strawberry cervix," differentiating it from BV.

Q: A neonate develops sepsis with organism showing CAMP test positive. Likely organism?

Group B Streptococcus. ***Group B Streptococcus*** - **Group B Streptococcus (GBS)**, or *Streptococcus agalactiae*, is the **most common cause of neonatal sepsis** and is the **classic organism** associated with a **positive CAMP test**. - The **CAMP test** (Christie-Atkins-Munch-Petersen) detects synergistic hemolysis between the CAMP factor produced by GBS and *Staphylococcus aureus* beta-lysin, resulting in an **arrowhead-shaped zone of enhanced hemolysis**. - GBS is strongly associated with **early-onset neonatal sepsis** (within first 7 days), transmitted vertically during delivery. - When the CAMP test is mentioned in the context of neonatal sepsis, **GBS is the intended answer** due to its classical association and epidemiological importance. *S. aureus* - *Staphylococcus aureus* can cause **sepsis** in neonates but is **CAMP test negative**. - It provides the beta-lysin used in the CAMP test to detect other organisms but does not produce the CAMP factor itself. *E. coli* - *Escherichia coli* is a **Gram-negative rod** and a frequent cause of **neonatal sepsis** and meningitis. - As a Gram-negative bacterium, *E. coli* is **CAMP test negative**. The CAMP test is specific for certain Gram-positive bacteria. *Listeria* - *Listeria monocytogenes* is **also CAMP test positive**, which can cause diagnostic confusion. - However, it causes a distinct clinical pattern: **granulomatosis infantiseptica**, meningoencephalitis, and is associated with **maternal ingestion of contaminated food**. - Listeria is **less common** than GBS as a cause of neonatal sepsis and is not the classic teaching association for CAMP positivity. - The CAMP positivity of Listeria is **weaker** and shows a different pattern (reverse CAMP) compared to the strong, characteristic arrowhead pattern of GBS.

Q: A sex worker presents with purulent urethral discharge and gram-negative diplococci. Diagnosis?

N. gonorrhoeae. ***N. gonorrhoeae*** - The presence of **purulent urethral discharge** and **Gram-negative diplococci** on microscopy is highly characteristic of **gonorrhea**. - **Neisseria gonorrhoeae** is a common cause of sexually transmitted infections, particularly in individuals with a higher risk profile like sex workers. *T. pallidum* - **Treponema pallidum** causes syphilis, which typically presents with a **chancre** (primary stage) or a rash (secondary stage), not purulent urethral discharge. - It is a **spirochete** and cannot be visualized as Gram-negative diplococci. *C. trachomatis* - **Chlamydia trachomatis** causes **nongonococcal urethritis**, which often presents with a less purulent or mucoid discharge, and may be asymptomatic. - It is an **obligate intracellular bacterium** and does not appear as Gram-negative diplococci on Gram stain. *H. ducreyi* - **Haemophilus ducreyi** causes **chancroid**, characterized by painful genital ulcers with regional lymphadenopathy. - It would not typically present with purulent urethral discharge and is a **Gram-negative rod**, not diplococci.

Q: A sex worker presents with purulent urethral discharge and gram-negative diplococci in culture. What is the most likely diagnosis?

Neisseria gonorrhoeae. ***Neisseria gonorrhoeae*** - **Purulent urethral discharge** and identification of **Gram-negative diplococci** on microscopy are classic diagnostic features of gonococcal urethritis [1]. - This sexually transmitted infection is common among sexually active individuals, including **sex workers** [1]. *Treponema pallidum* - This bacterium causes **syphilis**, which is characterized by **chancres** in the primary stage, and widespread rashes or lesions in later stages [2]. - It would not typically present with Gram-negative diplococci or purulent urethral discharge [2]. *Haemophilus ducreyi* - This organism is responsible for **chancroid**, a sexually transmitted infection that causes painful **genital ulcers** with ragged borders and often associated with lymphadenopathy [3]. - It would not lead to purulent urethral discharge, and while Gram-negative, it is typically seen as pleomorphic rods in chains. *Chlamydia trachomatis* - **Chlamydia** infection often presents with mucopurulent urethral discharge, but it is typically less purulent than gonorrhea and may be **asymptomatic** [1]. - **Chlamydia** is an **obligate intracellular bacterium** and would not be visualized as Gram-negative diplococci on a Gram stain [1].

Question 1

A 12 year old boy presents with vomiting within 3 hours of consumption of food at a party. What is the likely causative organism responsible for the symptoms?

Accepted Answer

Staphylococcus aureus

Answer

Clostridium perfringens

Answer

Clostridium botulinum

Answer

Salmonella

Question 2

A 29 year old male was brought to ED with complaints of nausea, vomiting and abdominal cramps 3-4 hours after eating meat at dinner. What is the likely cause of this condition?

Accepted Answer

Staphylococcus aureus

Answer

Clostridium perfringens

Answer

Clostridium botulinum

Answer

Salmonella Enteritidis

Question 3

Superantigen is produced by which of the following?

Accepted Answer

Streptococcus pyogenes

Answer

Clostridium perfringens

Answer

Vibrio cholerae

Answer

Staphylococcus epidermidis

Question 4

Urethritis in males is not caused by:

Accepted Answer

H. ducreyi

Answer

Chlamydia

Answer

Gonococcus

Answer

Trichomonas

Question 5

Gram stain of CSF shows gram-negative diplococci. Culture grows on Thayer-Martin medium. Most likely organism?

Accepted Answer

N. meningitidis

Answer

L. monocytogenes

Answer

S. pneumoniae

Answer

H. influenzae

Question 6

Which microorganism causes pseudomembranous colitis?

Accepted Answer

C. difficile

Answer

Shigella

Answer

E. coli

Answer

Salmonella

Question 7

Most commonly associated organism in bacterial vaginosis?

Accepted Answer

Gardnerella vaginalis

Answer

Candida albicans

Answer

E. coli

Answer

Trichomonas vaginalis

Question 8

A neonate develops sepsis with organism showing CAMP test positive. Likely organism?

Accepted Answer

Group B Streptococcus

Answer

S. aureus

Answer

E. coli

Answer

Listeria

Question 9

A sex worker presents with purulent urethral discharge and gram-negative diplococci. Diagnosis?

Accepted Answer

N. gonorrhoeae

Answer

T. pallidum

Answer

C. trachomatis

Answer

H. ducreyi

Question 10

A sex worker presents with purulent urethral discharge and gram-negative diplococci in culture. What is the most likely diagnosis?

Accepted Answer

Neisseria gonorrhoeae

Answer

Treponema pallidum

Answer

Haemophilus ducreyi

Answer

Chlamydia trachomatis

Bacteriology — MCQs

Bacteriology — MCQs

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