Bacteriology Practice Questions

Q: A patient presents with a history of pastry intake causing food poisoning. What is the most likely causative agent?

Staphylococcus aureus. ***Staphylococcus aureus*** - *S. aureus* is a common cause of food poisoning linked to **creamy foods** like pastries, salads, and custards, as it produces **heat-stable enterotoxins** when allowed to proliferate. - The symptoms, typically rapid onset **nausea, vomiting**, and abdominal cramps, occur because of the **preformed toxins** in the food, not necessarily active infection. *Verotoxin-producing E. coli* - This strain, often **E. coli O157:H7**, is primarily associated with **undercooked beef** or contaminated produce, and typically causes **bloody diarrhea** and can lead to hemolytic uremic syndrome (HUS). - Its mechanism involves **verotoxins** directly damaging intestinal cells and blood vessels, which is different from the rapid, emetic-focused symptoms of *S. aureus* food poisoning. *Bacillus cereus* - *B. cereus* causes two main types of food poisoning: **emetic (vomiting)**, typically from **reheated rice**, and **diarrheal**, from meat products or vegetables. - While the emetic form can cause vomiting, it is most strongly associated with **rice dishes** and usually has a shorter incubation period than the diarrheal form, making *Staphylococcus aureus* a more classic cause for pastry-related outbreaks. *Enteroinvasive E. coli (EIEC)* - EIEC causes a disease similar to **shigellosis**, involving direct invasion of intestinal epithelial cells, leading to **bloody diarrhea** and fever. - It is typically spread through contaminated food and water and not specifically linked to pastry intake or characterized by the rapid onset emetic symptoms seen with preformed toxins.

Q: Which of the following is a gram-positive organism that shows the following appearance on Ziehl-Neelsen staining?

Nocardia. ***Nocardia*** * The image displays delicate, branching, **filamentous rods** that are stained **red/pink** against a blue background, which is characteristic of partially acid-fast organisms like *Nocardia* on a Ziehl-Neelsen stain. * *Nocardia* species are **gram-positive**, aerobic bacteria that can cause opportunistic infections, particularly in immunocompromised individuals. They are distinguished by their **partial acid-fastness** due to their mycolic acid content, similar to mycobacteria but to a lesser degree. * The characteristic **branching filamentous morphology** combined with partial acid-fastness on Ziehl-Neelsen staining is pathognomonic for *Nocardia*. *Incorrect: Mycobacterium tuberculosis* * While *M. tuberculosis* is **strongly acid-fast** on Ziehl-Neelsen staining (appearing red), it is **not truly gram-positive**—it is gram-variable or weakly gram-positive. * *Mycobacterium* appears as **straight or slightly curved rods**, NOT branching filaments like those shown in the image. *Incorrect: Actinomyces* * *Actinomyces* is a **gram-positive**, filamentous, branching organism that can morphologically resemble *Nocardia*. * However, *Actinomyces* is **NOT acid-fast** and would appear **blue** (not red/pink) on Ziehl-Neelsen staining as it takes up the counterstain. * *Actinomyces* is also anaerobic, whereas *Nocardia* is aerobic. *Incorrect: Rhodococcus* * *Rhodococcus* is a gram-positive organism that can show **partial acid-fastness**, similar to *Nocardia*. * However, *Rhodococcus* typically appears as **coccoid to short rods**, occasionally forming short chains, but does NOT show the extensive **branching filamentous** pattern characteristic of *Nocardia* seen in the image.

Q: Which of the following microorganisms will be resistant to meropenem and aminoglycosides but sensitive to piperacillin tazobactam and cotrimoxazole?

Stenotrophomonas. ***Stenotrophomonas maltophilia*** - *Stenotrophomonas maltophilia* exhibits **intrinsic resistance to carbapenems (like meropenem)** due to the presence of L1 and L2 metallo-beta-lactamases and chromosomally encoded beta-lactamases. - It is **resistant to aminoglycosides** via aminoglycoside-modifying enzymes and efflux pump mechanisms. - **Trimethoprim-sulfamethoxazole (cotrimoxazole) is the drug of choice** with consistent susceptibility, making it the first-line treatment. - **Susceptibility to piperacillin-tazobactam is variable** - while some isolates may show in vitro susceptibility, clinical efficacy is inconsistent and it is not considered a reliable first-line agent. Among the options given, this organism best fits the described pattern. *Pseudomonas aeruginosa* - **Generally susceptible to carbapenems (meropenem) and aminoglycosides**, which are important therapeutic options. - Does not match the resistance pattern described in the question. *Acinetobacter baumannii* - Shows **multidrug resistance including carbapenems and aminoglycosides** in most clinical isolates. - However, typically also **resistant to piperacillin-tazobactam and cotrimoxazole**, making it inconsistent with the described susceptibility pattern. *Burkholderia cepacia complex* - Exhibits **intrinsic resistance to multiple antibiotics** including aminoglycosides and often carbapenems. - **Variable and often resistant to piperacillin-tazobactam**, and susceptibility to cotrimoxazole is inconsistent. - Does not reliably match the described antibiotic profile.

Q: A peculiar organism has resistance to azithromycin, but is sensitive to ticarcillin, clavulanic acid, cotrimoxazole. Identify the organism?

Stenotrophomonas. ***Stenotrophomonas*** - *Stenotrophomonas maltophilia* is intrinsically **resistant to azithromycin** and other macrolides due to its efflux pumps. - It is known to be sensitive to **ticarcillin-clavulanic acid** (a beta-lactam/beta-lactamase inhibitor combination) and **cotrimoxazole** (trimethoprim-sulfamethoxazole), which are common treatment options. *Pseudomonas* - *Pseudomonas aeruginosa* is generally **resistant to macrolides like azithromycin** but can exhibit varying sensitivity to antibiotics. - However, it often shows resistance to cotrimoxazole and ticarcillin-clavulanic acid is not a first-line agent, and it would typically be sensitive to other antipseudomonal beta-lactams (e.g., piperacillin-tazobactam, carbapenems). *Staphylococcus* - Many *Staphylococcus* species, particularly **MRSA**, are resistant to azithromycin. - However, *Staphylococcus* species are typically sensitive to other antibiotics like **vancomycin**, and are generally not sensitive to ticarcillin-clavulanic acid, and their susceptibility to cotrimoxazole varies depending on the species and resistance mechanisms. *Mycoplasma* - *Mycoplasma* species are **intrinsically resistant to beta-lactam antibiotics like ticarcillin** because they lack a cell wall, which is the target of these drugs. - They are typically sensitive to macrolides like azithromycin and also to cotrimoxazole.

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.

Question 1

A patient presents with a history of pastry intake causing food poisoning. What is the most likely causative agent?

Accepted Answer

Staphylococcus aureus

Answer

Verotoxin-producing E. coli

Answer

Bacillus cereus

Answer

Enteroinvasive E. coli (EIEC)

Question 2

Which of the following is a gram-positive organism that shows the following appearance on Ziehl-Neelsen staining?

Accepted Answer

Nocardia

Answer

Mycobacterium tuberculosis

Answer

Actinomyces

Answer

Rhodococcus

Question 3

Which of the following microorganisms will be resistant to meropenem and aminoglycosides but sensitive to piperacillin tazobactam and cotrimoxazole?

Accepted Answer

Stenotrophomonas

Answer

Pseudomonas

Answer

Acinetobacter

Answer

Burkholderia cepacia

Question 4

A peculiar organism has resistance to azithromycin, but is sensitive to ticarcillin, clavulanic acid, cotrimoxazole. Identify the organism?

Accepted Answer

Stenotrophomonas

Answer

Pseudomonas

Answer

Staphylococcus

Answer

Mycoplasma

Question 5

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 6

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 7

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 8

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 9

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 10

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

Bacteriology — MCQs

Bacteriology — MCQs

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