Bacteriology Practice Questions

Q: A surgeon told the patient’s attendant that his patient has “flesh eating bug” which is the cause of such severe infection in his dirty wound on his lower limb. Which organism was the surgeon referring to?

Streptococcus pyogenes. ***Streptococcus pyogenes*** - *Streptococcus pyogenes* (Group A Streptococcus) is the **primary and most common cause** of **necrotizing fasciitis**, which is classically referred to as the "flesh-eating bug" due to its rapid and extensive tissue destruction. - It produces powerful **exotoxins (streptolysins O and S) and enzymes (streptokinase, hyaluronidase)** that lead to widespread tissue necrosis, systemic toxicity, and a severe inflammatory response. - The term **"flesh-eating bug"** specifically refers to this organism in medical parlance. *Clostridium difficile* - *Clostridium difficile* primarily causes **pseudomembranous colitis**, an infection of the colon typically associated with antibiotic use and disruption of normal gut flora. - While it can be life-threatening, it does not cause necrotizing soft tissue infections in open wounds on limbs. *Pseudomonas aeruginosa* - *Pseudomonas aeruginosa* is a common cause of wound infections, especially in **burn patients**, immunocompromised individuals, and nosocomial settings, characterized by **blue-green pus** and a **fruity/grape-like odor**. - While it can cause severe wound infections, it is not typically referred to as the "flesh-eating bug" and causes a different clinical picture than the rapidly spreading necrotizing fasciitis caused by *S. pyogenes*. *Staphylococcus aureus* - *Staphylococcus aureus* (including MRSA) causes various skin and soft tissue infections including cellulitis, abscesses, and surgical site infections. - While it can occasionally cause necrotizing fasciitis (especially in polymicrobial infections), the colloquial term **"flesh-eating bug"** specifically refers to *Streptococcus pyogenes*, which is the most common single pathogen causing this condition.

Q: Throat swab positive and Schick test negative indicate that the person is:

Immune to diphtheria. ***Immune to diphtheria*** - A **negative Schick test** indicates the presence of sufficient **antitoxin antibodies** in the individual's blood, conferring **immunity** against diphtheria toxin. - A positive throat swab combined with a negative Schick test indicates the person is a **healthy carrier** (or **immune carrier**) – harboring *Corynebacterium diphtheriae* but protected from developing disease due to existing immunity. - Such carriers pose a **public health concern** as they can transmit the organism to susceptible individuals, despite being personally protected. *Hypersensitive to diphtheria* - **Hypersensitivity** to diphtheria is not assessed by the Schick test; a negative result indicates protective immunity, not an allergic or hypersensitivity reaction. - The Schick test specifically measures the ability to neutralize diphtheria toxin through antitoxin antibodies, not immune hypersensitivity. *Susceptible to diphtheria* - **Susceptibility to diphtheria** would be indicated by a **positive Schick test**, meaning there is insufficient antitoxin to neutralize the injected toxin, resulting in a local inflammatory reaction at the test site. - The given scenario states a **negative Schick test**, which definitively rules out susceptibility. *Suffering from diphtheria* - While a **positive throat swab** indicates the presence of *Corynebacterium diphtheriae*, a **negative Schick test** means the individual has protective immunity and is **not suffering from clinical disease**. - Active diphtheria presents with characteristic symptoms (pseudomembrane, bull neck, etc.), which would not occur in an immune individual despite bacterial colonization.

Q: The most common organism causing Acute Otitis media in children is:

Streptococcus pneumoniae. ***Streptococcus pneumoniae*** - **_Streptococcus pneumoniae_** is the most frequent bacterial cause of **acute otitis media (AOM)** in children, accounting for a significant percentage of cases. - Its presence often leads to **inflammation** and **fluid accumulation** in the middle ear, characteristic of AOM. *Staphylococcus epidermidis* - While **_Staphylococcus epidermidis_** is a common **skin commensal** and a frequent cause of device-related infections, it is rarely implicated in acute otitis media. - Its role in AOM is usually limited to **contaminant cultures** rather than true infection. *Klebsiella pneumoniae* - **_Klebsiella pneumoniae_** is more commonly associated with **pneumonia**, **urinary tract infections**, and **healthcare-associated infections**, particularly in immunocompromised individuals. - It is an **uncommon cause** of typical acute otitis media in healthy children. *Escherichia coli* - **_Escherichia coli_** is a common cause of **urinary tract infections** and **gastrointestinal infections**. - It is **rarely a primary cause** of acute otitis media, especially in otherwise healthy children, unless there are underlying risk factors or specific infection pathways.

Q: A 27-year-old man presents with clear urethral discharge and dysuria for 3 days. Gram stain shows numerous polymorphonuclear leukocytes without intracellular diplococci. What is the most likely pathogen?

Chlamydia trachomatis. ***Chlamydia trachomatis*** - The presentation of **clear urethral discharge** and **dysuria** in a sexually active young man, coupled with polymorphonuclear leukocytes on Gram stain but **no intracellular diplococci**, is highly characteristic of **chlamydial urethritis**. - *Chlamydia trachomatis* causes **non-gonococcal urethritis** and is a common cause of sexually transmitted infections (STIs). *Neisseria gonorrhoeae* - This pathogen typically causes **purulent urethral discharge** and would show **intracellular Gram-negative diplococci** on microscopy, which were absent in this case. - Gonococcal urethritis is usually associated with more severe symptoms and a different Gram stain finding. *Trichomonas vaginalis* - While *Trichomonas vaginalis* can cause urethritis in men, it is less common than in women and typically presents with a **foamy, greenish discharge** and may be associated with **pruritus**. - *Trichomonas* is not visible on Gram stain; diagnosis requires **wet mount microscopy** showing motile trophozoites, making this less likely given the laboratory findings. *Ureaplasma urealyticum* - *Ureaplasma urealyticum* can cause **non-gonococcal urethritis**, but it is generally associated with **milder symptoms** or can even be asymptomatic. - While it's a possibility, *Chlamydia trachomatis* is a more common and prominent cause of the described clinical picture.

Q: What mechanism accounts for the neuroinvasive potential of Treponema pallidum?

Penetration through endothelial tight junctions. ***Penetration through endothelial tight junctions*** - *Treponema pallidum* has the ability to directly cross the **blood-brain barrier** by disrupting and passing through the tight junctions between endothelial cells. - This direct penetration allows the spirochete to invade the **central nervous system** early in the infection course, leading to neurosyphilis. - This mechanism of paracellular penetration through compromised tight junctions is the primary route of CNS invasion. *Perivascular infiltration* - While **inflammatory cells** may infiltrate the perivascular spaces in neurosyphilis, this is a consequence of the infection rather than the primary mechanism of *T. pallidum*'s entry into the CNS. - Simply infiltrating around blood vessels does not explain how the bacteria traverse the **endothelial barrier**. *Transport within infected macrophages* - Although some pathogens use a "Trojan horse" mechanism by being carried within **macrophages**, there is limited evidence that this is the primary or significant mechanism for *T. pallidum* to cross the **blood-brain barrier**. - The direct invasiveness of *T. pallidum* through endothelial cells is considered the main route. *Production of neurotoxins* - *Treponema pallidum* is not known to produce potent **neurotoxins** that directly cause its neuroinvasive potential or neurological damage, unlike some other bacteria (e.g., *Clostridium botulinum*). - The pathology in neurosyphilis is primarily due to the **inflammatory response** to the presence of the spirochetes within the CNS.

Q: Which structure of Treponema pallidum is responsible for its characteristic motility?

Endoflagella. ***Endoflagella*** - **Endoflagella** (also known as axial filaments) are located in the **periplasmic space** between the cell membrane and outer membrane of *Treponema pallidum*. - Their rotation causes the organism to move in a characteristic **corkscrew-like fashion**, allowing it to penetrate tissues. *External flagella* - *Treponema pallidum* does **not** possess external flagella. - **External flagella** are whip-like appendages that extend outward from the cell surface in many other motile bacteria, but spirochetes like *Treponema pallidum* use internal endoflagella instead. *Pili* - **Pili** (or fimbriae) are hair-like appendages primarily involved in **adhesion to host cells** and biofilm formation, as well as genetic exchange (sex pili). - They do not contribute to the type of **corkscrew motility** seen in *Treponema pallidum*. *Fimbriae* - **Fimbriae** are synonymous with pili and function mainly in **adhesion** to surfaces and host cells. - They are not responsible for the **motility** of spirochetes such as *Treponema pallidum*.

Q: Which organism causes Granuloma inguinale (Donovanosis)?

Klebsiella granulomatis. ***Klebsiella granulomatis*** - This bacterium, formerly known as *Calymmatobacterium granulomatis*, is the causative agent of **Granuloma inguinale**, also known as **Donovanosis**. - It leads to chronic, progressive ulcerative lesions in the genital and perineal areas, characterized by the presence of **Donovan bodies** within macrophages. *Haemophilus ducreyi* - This organism is responsible for **chancroid**, a sexually transmitted infection (STI) characterized by painful genital ulcers and suppurative regional lymphadenopathy. - Its clinical presentation differs significantly from the slowly progressive, painless ulcers of Donovanosis. *Chlamydia trachomatis L1-L3* - These serovars of *Chlamydia trachomatis* cause **Lymphogranuloma venereum (LGV)**, which typically presents with transient small genital lesions followed by significant regional lymphadenopathy, often referred to as buboes. - This is distinct from the extensive, destructive ulcerations seen in Donovanosis. *Treponema pallidum* - This spirochete is the causative agent of **syphilis**, a multi-stage STI. - Primary syphilis presents as a single, painless chancre, which is different from the chronic, beefy red, friable ulcers of Granuloma inguinale.

Q: Why does Chlamydia trachomatis infection often remain asymptomatic in women?

Due to infection of columnar epithelium in endocervix. ***Due to infection of columnar epithelium in endocervix*** - Chlamydia trachomatis primarily infects the **columnar epithelial cells** of the endocervix. - These cells often do not elicit a significant **inflammatory response** or nerve stimulation, leading to a lack of noticeable symptoms. *Due to rapid clearance of organisms* - **Chlamydia infections** are not typically cleared rapidly; they can persist for months or even years if untreated. - The persistence of the organism, even in the absence of symptoms, is what makes it a common cause of **pelvic inflammatory disease (PID)** and infertility. *Due to inhibition of inflammatory response* - While Chlamydia does not always trigger a robust symptomatic inflammatory response in the early stages, it does not actively **inhibit** the immune system's general ability to respond. - Chronic or ascending infections eventually lead to **inflammation** and tissue damage, often resulting in silent but progressive pathology. *Due to hormonal influence on epithelial cells* - While **hormonal changes** can influence the susceptibility of epithelial cells to certain infections, they are not the primary reason for the asymptomatic nature of Chlamydia in women. - The main factor is the **type of cells** infected and the initial lack of a strong symptomatic host response.

Q: What structure of Neisseria gonorrhoeae is primarily responsible for its pathogenicity?

Pili. ***Pili*** - **Pili** enable *Neisseria gonorrhoeae* to adhere to host epithelial cells, which is crucial for colonization and initiation of infection. - They also facilitate evasion of the host immune response by undergoing **antigenic variation**, making it difficult for the immune system to recognize and eliminate the bacteria. *Capsule* - While some bacterial species use a **capsule** for immune evasion and adherence, *Neisseria gonorrhoeae* does not typically possess a prominent capsule. - Its pathogenicity is primarily attributed to other factors like pili and outer membrane proteins. *Flagella* - **Flagella** provide motility to bacteria, but *Neisseria gonorrhoeae* is a non-motile bacterium and does not possess flagella. - Therefore, flagella do not contribute to its pathogenicity. *Endotoxin* - **Endotoxin** (specifically **lipooligosaccharide, LOS**, in *Neisseria*) does contribute to the pathology of gonococcal infections by causing inflammation and tissue damage. - However, the primary factor for initial adherence and colonization, which is essential for establishing infection, is the **pili**.

Q: Which spirochete is the causative agent of syphilis?

Treponema pallidum. ***Correct: Treponema pallidum*** - **_Treponema pallidum_** is the specific spirochete responsible for causing **syphilis**, a sexually transmitted infection. - This bacterium is characterized by its **helical shape** and corkscrew-like motility, which helps it penetrate tissues. *Incorrect: Borrelia burgdorferi* - **_Borrelia burgdorferi_** is the causative agent of **Lyme disease**, transmitted by ticks. - Its clinical presentation involves **erythema migrans**, arthritis, and neurological symptoms, distinct from syphilis. *Incorrect: Leptospira interrogans* - **_Leptospira interrogans_** causes **leptospirosis**, a zoonotic disease typically acquired through contact with contaminated water or soil. - Symptoms can range from mild flu-like illness to severe forms like **Weil's disease**, involving liver and kidney failure. *Incorrect: Borrelia recurrentis* - **_Borrelia recurrentis_** is the bacterium responsible for **louse-borne relapsing fever**. - It is transmitted by the human body louse and causes recurrent febrile episodes due to **antigenic variation**.

Question 1

A surgeon told the patient’s attendant that his patient has “flesh eating bug” which is the cause of such severe infection in his dirty wound on his lower limb. Which organism was the surgeon referring to?

Accepted Answer

Streptococcus pyogenes

Answer

Clostridium difficile

Answer

Pseudomonas aeruginosa

Answer

Staphylococcus aureus

Question 2

Throat swab positive and Schick test negative indicate that the person is:

Accepted Answer

Immune to diphtheria

Answer

Hypersensitive to diphtheria

Answer

Susceptible to diphtheria

Answer

Suffering from diphtheria

Question 3

The most common organism causing Acute Otitis media in children is:

Accepted Answer

Streptococcus pneumoniae

Answer

Staphylococcus epidermidis

Answer

Klebsiella pneumoniae

Answer

Escherichia coli

Question 4

A 27-year-old man presents with clear urethral discharge and dysuria for 3 days. Gram stain shows numerous polymorphonuclear leukocytes without intracellular diplococci. What is the most likely pathogen?

Accepted Answer

Chlamydia trachomatis

Answer

Neisseria gonorrhoeae

Answer

Trichomonas vaginalis

Answer

Ureaplasma urealyticum

Question 5

What mechanism accounts for the neuroinvasive potential of Treponema pallidum?

Accepted Answer

Penetration through endothelial tight junctions

Answer

Perivascular infiltration

Answer

Transport within infected macrophages

Answer

Production of neurotoxins

Question 6

Which structure of Treponema pallidum is responsible for its characteristic motility?

Accepted Answer

Endoflagella

Answer

External flagella

Answer

Pili

Answer

Fimbriae

Question 7

Which organism causes Granuloma inguinale (Donovanosis)?

Accepted Answer

Klebsiella granulomatis

Answer

Haemophilus ducreyi

Answer

Chlamydia trachomatis L1-L3

Answer

Treponema pallidum

Question 8

Why does Chlamydia trachomatis infection often remain asymptomatic in women?

Accepted Answer

Due to infection of columnar epithelium in endocervix

Answer

Due to rapid clearance of organisms

Answer

Due to inhibition of inflammatory response

Answer

Due to hormonal influence on epithelial cells

Question 9

What structure of Neisseria gonorrhoeae is primarily responsible for its pathogenicity?

Accepted Answer

Pili

Answer

Capsule

Answer

Flagella

Answer

Endotoxin

Question 10

Which spirochete is the causative agent of syphilis?

Accepted Answer

Treponema pallidum

Answer

Borrelia burgdorferi

Answer

Leptospira interrogans

Answer

Borrelia recurrentis

Bacteriology — MCQs

Bacteriology — MCQs

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