Bacteriology Practice Questions

Q: An elderly male presented with meningitis. The CSF sample was subjected to Gram staining, and the microscopy image is shown. Which of the following is a characteristic of the likely organism?

Motile at 22°C and non-motile at 37°C. ***Motile at 22°C and non-motile at 37°C*** - The image likely shows **Listeria monocytogenes**, a common cause of meningitis in the elderly, which exhibits **tumbling motility at 22-28°C** but is non-motile at body temperature (37°C). - This organism is characterized by **facultative intracellular growth** and the ability to move within host cells using **actin polymerization**, but its flagellar motility is temperature-dependent. *No motility at 22°C and motile at 37°C* - This characteristic temperature-dependent motility pattern is not typical for **Listeria monocytogenes**. - Most pathogenic bacteria that are motile use their flagella optimally at human body temperature (37°C), but they usually maintain some motility at lower temperatures, or are non-motile without such a switch. *Non motile at both 37°C and 22°C* - While some bacteria cause meningitis and are non-motile (e.g., **Streptococcus pneumoniae, Neisseria meningitidis**), **Listeria monocytogenes** is *not* one of them at 22°C. - Being non-motile at both temperatures would rule out **Listeria** as the likely cause given its known motility characteristics. *Motile at both 37°C and 22°C* - While some bacteria, like certain **enteric gram-negative rods**, are motile at both temperatures, this is not the characteristic motility pattern of **Listeria monocytogenes**. - The distinct temperature-dependent motility is a key identifier for **Listeria**.

Q: Which pathogen causes attachment - effacement lesion in the intestinal mucosa as shown in the image?

Enteropathogenic Escherichia coli. ***Enteropathogenic Escherichia coli*** - **Enteropathogenic E. coli (EPEC)** is characterized by its ability to cause **"attachment and effacement" (A/E) lesions** on intestinal epithelial cells, as depicted in the image. This involves the effacement of microvilli and pedestal formation. - EPEC utilizes the **Type III secretion system** to inject effector proteins into the host cell, leading to actin rearrangement and the characteristic A/E lesion. *Enterotoxigenic Escherichia coli* - **Enterotoxigenic E. coli (ETEC)** causes diarrhea by producing **heat-labile (LT) and/or heat-stable (ST) toxins**, which stimulate fluid and electrolyte secretion. - ETEC primarily mediates its effects through toxins that cause increased cAMP/cGMP, leading to secretory diarrhea without significant host cell damage. *Diffusely adherent Escherichia coli* - **Diffusely adherent E. coli (DAEC)** is known to adhere to the entire surface of epithelial cells in a diffuse pattern. - While it can cause diarrhea, its mechanism involves a different adhesion pattern and does not typically result in the dramatic attachment/effacement changes seen with EPEC. *Enteroinvasive Escherichia coli* - **Enteroinvasive E. coli (EIEC)** invades and destroys the epithelial cells of the colon, leading to symptoms similar to **shigellosis**, including dysentery (bloody, mucoid stools). - Its pathogenic mechanism involves intracellular replication and direct destruction of host cells, not the localized attachment and effacement seen in the image.

Q: A patient presented with meningitis, and the CSF sample was subjected to gram staining and microscopy and appears as shown in the microscopic image given below. Which of the following features / tests will be characteristic of the organism?

Catalase negative, optochin sensitive.. ***Catalase negative, optochin sensitive*** - The image shows **Gram-positive diplococci** (lancet-shaped, occurring in pairs), indicative of *Streptococcus pneumoniae*. - *S. pneumoniae* is known for being **catalase-negative** and **optochin-sensitive**, which are key diagnostic features that distinguish it from other α-hemolytic streptococci. - Additional characteristic: *S. pneumoniae* **ferments inulin** (bile soluble, quellung positive). *Ferments glucose and maltose but not inulin* - The phrase "does not ferment inulin" would suggest a streptococcus species OTHER than *S. pneumoniae*. - *S. pneumoniae* characteristically **DOES ferment inulin**, which is an important diagnostic feature. - This option is incorrect because it describes sugar fermentation that contradicts *S. pneumoniae* characteristics. *Oxidase and catalase negative, does not ferment inulin* - While *S. pneumoniae* is catalase-negative and oxidase-negative (oxidase test not routinely used for Gram-positive cocci), the statement "does not ferment inulin" is **incorrect** for *S. pneumoniae*. - *S. pneumoniae* **ferments inulin**, which is a key differentiating feature from other streptococci. - This combination of features does not match the organism shown in the microscopy. *Oxidase and catalase positive and does not ferment maltose or glucose* - This biochemical profile is typical for non-fermenting Gram-negative bacteria like *Pseudomonas aeruginosa*. - Such organisms would appear as **Gram-negative rods**, not the **Gram-positive diplococci** seen in the image. - This option is clearly incorrect based on the Gram stain morphology alone.

Q: A 28-year-old man presents with painful inguinal lymphadenopathy and penile ulcers. The ulcers are painful, soft, and have undermined edges with purulent base. Gram stain shows small, pleomorphic gram-negative rods. Which virulence factor is responsible for the tissue destruction in this infection?

Cytolethal distending toxin. ***Cytolethal distending toxin (CDT)*** - *Haemophilus ducreyi*, the causative agent of **chancroid**, produces **CDT** which induces **cell cycle arrest** and **apoptosis** in host cells. - This toxin contributes significantly to the **tissue destruction** and **ulcer formation** characteristic of chancroid, leading to the painful, purulent lesions and undermined edges described. *Pili* - **Pili** are primarily involved in **adherence** to host cells and tissues, facilitating bacterial colonization. - While important for initial infection, they are not directly responsible for the extensive **tissue destruction** and **necrosis** seen in chancroid ulcers. *Lipopolysaccharide* - **Lipopolysaccharide (LPS)** is a major component of the outer membrane of gram-negative bacteria and is a potent **endotoxin** that triggers a strong inflammatory response. - While LPS contributes to the overall immune response and systemic symptoms, it is not the primary virulence factor responsible for the **localized tissue destruction** and characteristic ulcer morphology of chancroid. *Capsule* - A **capsule** is an outer polysaccharide layer that helps bacteria evade phagocytosis and contributes to invasiveness. - While capsules can enhance bacterial survival and dissemination, they are not primarily responsible for the direct **cytotoxicity** and **tissue necrosis** that characterize chancroid ulcers.

Q: What is the mechanism by which Chlamydia trachomatis causes cellular damage in urethritis?

Through intracellular replication leading to cell lysis. ***Through intracellular replication leading to cell lysis*** - **Chlamydia trachomatis** is an **obligate intracellular bacterium** that replicates within the host cell's cytoplasm, forming **inclusion bodies** [1]. - As the bacteria multiply, they eventually overwhelm and lyse the host cell to release new elementary bodies, causing tissue damage and inflammation in the urethra [1]. *Through direct cytolysis* - This mechanism implies the bacterium directly ruptures the host cell without prior replication or involvement of a complex lifecycle. - **Chlamydia** does not directly lyse cells upon entry; instead, it has an **intracellular developmental cycle** that culminates in cell lysis [1]. *By toxin production* - While some bacteria cause damage through toxins, **Chlamydia trachomatis** is not known to produce potent exotoxins or endotoxins that are the primary drivers of cellular damage in urethritis. - Its pathogenicity is principally due to its **intracellular lifestyle** and subsequent cell destruction. *Through elementary body replication* - **Elementary bodies (EBs)** are the **infectious, metabolically inert** form of Chlamydia that attach to and enter host cells [1]. - EBs do not replicate; once inside the cell, they differentiate into **reticulate bodies (RBs)**, which are the metabolically active, replicative form [1]. **References:** [1] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Diseases Of The Urinary And Male Genital Tracts, pp. 507-508.

Q: Which organism is the most common cause of non-gonococcal urethritis?

Chlamydia trachomatis. ***Correct: Chlamydia trachomatis*** - **_Chlamydia trachomatis_** is the **most frequent cause** of sexually transmitted **non-gonococcal urethritis (NGU)** in men, accounting for **30-50%** of cases. - Infection presents with **dysuria**, **urethral discharge** (usually mucoid or mucopurulent), and **urethral itching**. - Diagnosed using **nucleic acid amplification tests (NAATs)** on urine or urethral swabs. *Incorrect: Mycoplasma genitalium* - **_Mycoplasma genitalium_** is recognized as a cause of NGU, accounting for **15-25%** of cases, but it is **less common** than **_Chlamydia trachomatis_**. - This bacterium is difficult to culture and requires specific **NAATs** for diagnosis. - Increasingly recognized as an important pathogen with potential for macrolide resistance. *Incorrect: Trichomonas vaginalis* - While **_Trichomonas vaginalis_** can cause urethritis, it is more commonly associated with **vaginitis** in women and is a **less frequent cause of NGU** in men. - Symptoms in men are often **mild or asymptomatic**, but can include **thin, whitish discharge** and **dysuria**. *Incorrect: Ureaplasma urealyticum* - **_Ureaplasma urealyticum_** can cause NGU, but its role is **less definitive** and it is isolated less frequently than **_Chlamydia trachomatis_** in NGU cases. - Considered a potential cause of NGU, particularly in cases where **_Chlamydia_** and **_Gonorrhea_** have been excluded.

Q: How does Haemophilus ducreyi establish infection in the epidermis?

Through production of cytotoxin. ***Through production of cytotoxin*** - *Haemophilus ducreyi* produces a **cytolethal distending toxin (CDT)** that is crucial for its pathogenesis, specifically in creating the epidermal lesions characteristic of **chancroid**. - This **CDT** is a tripartite toxin that induces **cell cycle arrest and apoptosis** in epithelial cells and keratinocytes, facilitating the bacterium's invasion and causing tissue destruction within the epidermis. - The toxin-mediated damage leads to the characteristic **painful genital ulcers** seen in chancroid. *By spore formation* - **Spore formation** is a survival mechanism seen in certain gram-positive bacteria (e.g., *Clostridium, Bacillus*) for enduring harsh conditions, not a mechanism for epidermal infection by *H. ducreyi*. - *Haemophilus ducreyi* is a **gram-negative bacterium** and does not form spores. *By forming biofilms* - While **biofilm formation** can contribute to bacterial persistence and evasion of host defenses, it is not the primary mechanism by which *H. ducreyi* initially invades and establishes infection in the epidermis to cause the acute lesions of chancroid. - Biofilms are more associated with chronic infections or colonization of indwelling medical devices. *Through capsular attachment* - Some bacteria use **capsules** for adherence and immune evasion, but for *Haemophilus ducreyi*, the production of **cytolethal distending toxin (CDT)** is the key factor in its ability to break down epidermal barriers and establish infection. - While some *Haemophilus* species can have capsules, it's not the primary virulence factor for *H. ducreyi*'s epidermal infection mechanism.

Q: Which microorganism causes chancroid?

Haemophilus ducreyi. ***Haemophilus ducreyi*** - This bacterium is the definitive causative agent of **chancroid**, a sexually transmitted infection. - Chancroid is characterized by **painful genital ulcers** with ragged edges and often associated with **buboes** (swollen, tender lymph nodes). - Diagnosis: Clinical presentation plus identification of gram-negative coccobacilli in "school of fish" pattern. *Chlamydia trachomatis* - This bacterium causes **chlamydia**, a common STI leading to urethritis, cervicitis, and lymphogranuloma venereum (LGV). - It does not cause the painful, ragged ulcers characteristic of chancroid. *Klebsiella granulomatis* - Previously known as **Calymmatobacterium granulomatis**, this is the causative agent of **donovanosis** (granuloma inguinale). - Donovanosis causes gradually enlarging, **painless**, beefy-red ulcers with rolled edges, distinct from chancroid's painful ulcers. *Treponema pallidum* - This spirochete causes **syphilis**, another sexually transmitted infection. - Primary syphilis presents with a **painless chancre** (indurated ulcer), which differs from the painful ulcers of chancroid.

Q: Which cellular finding in urethral discharge is diagnostic of gonorrhea?

Intracellular gram-negative diplococci. ***Intracellular gram-negative diplococci*** - The presence of **gram-negative diplococci within polymorphonuclear leukocytes (PMNs)** in urethral discharge is highly specific and diagnostic for **gonorrhea** in symptomatic males. - This finding indicates active infection by **_Neisseria gonorrhoeae_**, which are characteristic kidney bean-shaped bacteria that are phagocytosed by host immune cells. *Extracellular gram-negative cocci* - While _Neisseria gonorrhoeae_ are gram-negative cocci, their location **outside host cells** is less specific and can be observed with contamination or non-pathogenic species. - The key diagnostic feature for gonorrhea is their **intracellular presence** within phagocytes, indicating an active infection. *Intracellular gram-positive diplococci* - **Gram-positive diplococci** are characteristic of different bacterial species, such as **streptococci or staphylococci in clusters**, and are not associated with gonorrhea. - _Neisseria gonorrhoeae_ is definitively a **gram-negative bacterium**. *Gram-variable coccobacilli* - **Gram-variable coccobacilli** are a broad category of bacteria that show inconsistent staining or have shapes intermediate between cocci and bacilli, and are not specific for gonorrhea. - **_Neisseria gonorrhoeae_** are specifically characterized as **gram-negative diplococci**.

Question 1

An elderly male presented with meningitis. The CSF sample was subjected to Gram staining, and the microscopy image is shown. Which of the following is a characteristic of the likely organism?

Accepted Answer

Motile at 22°C and non-motile at 37°C

Answer

No motility at 22°C and motile at 37°C

Answer

Non motile at both 37°C and 22°C

Answer

Motile at both 37°C and 22°C

Question 2

Which pathogen causes attachment - effacement lesion in the intestinal mucosa as shown in the image?

Accepted Answer

Enteropathogenic Escherichia coli

Answer

Enterotoxigenic Escherichia coli

Answer

Diffusely adherent Escherichia coli

Answer

Enteroinvasive Escherichia coli

Question 3

A patient presented with meningitis, and the CSF sample was subjected to gram staining and microscopy and appears as shown in the microscopic image given below. Which of the following features / tests will be characteristic of the organism?

Accepted Answer

Catalase negative, optochin sensitive.

Answer

Ferments glucose and maltose but not inulin

Answer

Oxidase and catalase negative, does not ferment inulin

Answer

Oxidase and catalase positive and does not ferment maltose or glucose

Question 4

A patient presented with meningitis, and the CSF sample shows Gram-negative diplococci on Gram staining and microscopy. Which of the following features/tests will be characteristic of the organism?

Accepted Answer

Oxidase positive, catalase positive, ferments glucose and maltose

Answer

Catalase negative, optochin sensitive, alpha-hemolytic

Answer

Oxidase negative, catalase positive, coagulase positive

Answer

Catalase positive, urease positive, does not ferment glucose

Question 5

A 28-year-old man presents with painful inguinal lymphadenopathy and penile ulcers. The ulcers are painful, soft, and have undermined edges with purulent base. Gram stain shows small, pleomorphic gram-negative rods. Which virulence factor is responsible for the tissue destruction in this infection?

Accepted Answer

Cytolethal distending toxin

Answer

Pili

Answer

Lipopolysaccharide

Answer

Capsule

Question 6

What is the mechanism by which Chlamydia trachomatis causes cellular damage in urethritis?

Accepted Answer

Through intracellular replication leading to cell lysis

Answer

Through direct cytolysis

Answer

By toxin production

Answer

Through elementary body replication

Question 7

Which organism is the most common cause of non-gonococcal urethritis?

Accepted Answer

Chlamydia trachomatis

Answer

Trichomonas vaginalis

Answer

Mycoplasma genitalium

Answer

Ureaplasma urealyticum

Question 8

How does Haemophilus ducreyi establish infection in the epidermis?

Accepted Answer

Through production of cytotoxin

Answer

By spore formation

Answer

By forming biofilms

Answer

Through capsular attachment

Question 9

Which microorganism causes chancroid?

Accepted Answer

Haemophilus ducreyi

Answer

Chlamydia trachomatis

Answer

Klebsiella granulomatis

Answer

Treponema pallidum

Question 10

Which cellular finding in urethral discharge is diagnostic of gonorrhea?

Accepted Answer

Intracellular gram-negative diplococci

Answer

Extracellular gram-negative cocci

Answer

Intracellular gram-positive diplococci

Answer

Gram-variable coccobacilli

Bacteriology — MCQs

Bacteriology — MCQs

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