Bacteriology — MCQs

A 34-year-old female visits her primary care physician because recently she has started to have painful, numb, and discolored toes. She is otherwise healthy and has no family history of similar conditions that she can recall. Occasionally during these episodes, her fingers and nose will also have similar symptoms. On examination, the patient's appearance is completely normal with warm and well perfused extremities. No evidence of discoloration is found. On closer questioning, she reveals that several months ago during the summer, she succumbed to a viral illness that caused her to feel fatigued and have a long bout of cold symptoms with sore throat and swollen lymph nodes. The bacterial species that is also associated with this patient's most likely condition has which of the following characteristics?

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 2101: A peculiar organism has resistance to azithromycin, but is sensitive to ticarcillin, clavulanic acid, cotrimoxazole. Identify the organism?

A. Pseudomonas
B. Staphylococcus
C. Stenotrophomonas (Correct Answer)
D. Mycoplasma

Explanation: ***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.

Question 2102: Which pathogen causes attachment - effacement lesion in the intestinal mucosa as shown in the image?

A. Enteropathogenic Escherichia coli (Correct Answer)
B. Enterotoxigenic Escherichia coli
C. Diffusely adherent Escherichia coli
D. Enteroinvasive Escherichia coli

Explanation: ***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.

Question 2103: 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?

A. Oxidase positive, catalase positive, ferments glucose and maltose (Correct Answer)
B. Catalase negative, optochin sensitive, alpha-hemolytic
C. Oxidase negative, catalase positive, coagulase positive
D. Catalase positive, urease positive, does not ferment glucose

Explanation: ***Oxidase positive, catalase positive, ferments glucose and maltose*** - The CSF findings show **Gram-negative diplococci**, characteristic of *Neisseria meningitidis*, a major cause of bacterial meningitis. - *N. meningitidis* is definitively identified by being **oxidase positive, catalase positive**, and able to **ferment both glucose and maltose**. *Catalase negative, optochin sensitive, alpha-hemolytic* - These are characteristic features of *Streptococcus pneumoniae*, which appears as **Gram-positive lancet-shaped diplococci**, not the Gram-negative diplococci seen in this case. - *S. pneumoniae* is **catalase negative** and shows **alpha-hemolysis** on blood agar, distinguishing it from Neisseria species. *Oxidase negative, catalase positive, coagulase positive* - These biochemical properties describe *Staphylococcus aureus*, which appears as **Gram-positive cocci in clusters** on microscopy. - *S. aureus* is **oxidase negative** and **coagulase positive**, completely different from the organism characteristics shown in the CSF sample. *Catalase positive, urease positive, does not ferment glucose* - This combination suggests organisms like **Enterobacteriaceae** or *Cryptococcus neoformans*, which have different morphological appearances. - The **urease positivity** and **lack of glucose fermentation** are inconsistent with *N. meningitidis*, which readily ferments glucose.

Question 2104: 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?

A. Pili
B. Lipopolysaccharide
C. Cytolethal distending toxin (Correct Answer)
D. Capsule

Explanation: ***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.

Question 2105: Which organism is the most common cause of non-gonococcal urethritis?

A. Trichomonas vaginalis
B. Chlamydia trachomatis (Correct Answer)
C. Mycoplasma genitalium
D. Ureaplasma urealyticum

Explanation: ***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.

Question 2106: How does Haemophilus ducreyi establish infection in the epidermis?

A. By spore formation
B. Through production of cytotoxin (Correct Answer)
C. By forming biofilms
D. Through capsular attachment

Explanation: ***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.

Question 2107: Which microorganism causes chancroid?

A. Chlamydia trachomatis
B. Haemophilus ducreyi (Correct Answer)
C. Klebsiella granulomatis
D. Treponema pallidum

Explanation: ***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.

Question 2108: Which cellular finding in urethral discharge is diagnostic of gonorrhea?

A. Intracellular gram-negative diplococci (Correct Answer)
B. Extracellular gram-negative cocci
C. Intracellular gram-positive diplococci
D. Gram-variable coccobacilli

Explanation: ***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 2109: How does Treponema pallidum evade host immune response during early infection?

A. By producing endotoxins
B. Through antigenic variation
C. By having few surface proteins (Correct Answer)
D. By forming biofilms

Explanation: ***By having few surface proteins*** - *Treponema pallidum* is characterized by its **reduced number of outer membrane proteins (OMPs)** visible to the host immune system. - This **"stealth" mechanism** limits the host's ability to recognize and target the bacterium during early infection. - The paucity of surface antigens (only 10-20 OMPs compared to hundreds in typical Gram-negative bacteria) makes *T. pallidum* nearly invisible to immune surveillance. *By producing endotoxins* - **Endotoxins** (lipopolysaccharide or LPS) are components of the outer membrane of Gram-negative bacteria. - While *T. pallidum* is technically Gram-negative, **endotoxin production is not its primary immune evasion mechanism** during early infection. - The organism's survival depends on immune evasion through reduced antigenicity, not endotoxin-mediated effects. *Through antigenic variation* - While **antigenic variation** is a common immune evasion strategy for many pathogens (e.g., *Neisseria gonorrhoeae*, *Borrelia recurrentis*), it is **not a primary mechanism** employed by *T. pallidum*. - *T. pallidum* lacks the extensive genetic machinery for significant antigenic switching of its surface proteins. - The organism relies on having few targets rather than changing targets. *By forming biofilms* - **Biofilm formation** allows some bacteria to evade host immunity and antibiotics by creating a protective matrix. - This is **not a well-documented or primary immune evasion strategy** for *T. pallidum* in the context of syphilis infection. - *T. pallidum* typically exists as planktonic cells or within host tissues rather than forming robust biofilms in vivo.

Question 2110: A 34-year-old female visits her primary care physician because recently she has started to have painful, numb, and discolored toes. She is otherwise healthy and has no family history of similar conditions that she can recall. Occasionally during these episodes, her fingers and nose will also have similar symptoms. On examination, the patient's appearance is completely normal with warm and well perfused extremities. No evidence of discoloration is found. On closer questioning, she reveals that several months ago during the summer, she succumbed to a viral illness that caused her to feel fatigued and have a long bout of cold symptoms with sore throat and swollen lymph nodes. The bacterial species that is also associated with this patient's most likely condition has which of the following characteristics?

A. Gram-positive
B. No cell wall (Correct Answer)
C. Acid-fast
D. Spirochete
E. Gram-negative

Explanation: ***No cell wall*** - The patient's symptoms (painful, numb, discolored toes, fingers, and nose that resolve, especially after a viral illness) suggest **Raynaud's phenomenon** secondary to an underlying condition. The history of a recent viral illness followed by the onset of these symptoms points towards **Mycoplasma pneumoniae** infection, which can trigger **cold agglutinin disease** leading to Raynaud-like features [1]. - *Mycoplasma pneumoniae* is unique among bacteria in that it **lacks a cell wall**, making it impervious to cell wall-targeting antibiotics like penicillins and cephalosporins [1]. *Gram-positive* - This characteristic applies to bacteria like *Staphylococcus* or *Streptococcus*, which have thick **peptidoglycan cell walls** that retain crystal violet stain. - While some gram-positive bacteria can cause infections, they are not typically associated with the development of cold agglutinin disease or Raynaud's phenomenon in the manner described. *Acid-fast* - This property is characteristic of mycobacteria (e.g., *Mycobacterium tuberculosis*), which have a waxy cell wall containing **mycolic acid**, making them resistant to decolorization by acid-alcohol. - These organisms do not cause the acute symptoms or sequelae described in the patient. *Spirochete* - Spirochetes, such as *Treponema pallidum* (syphilis) or *Borrelia burgdorferi* (Lyme disease), are spiral-shaped bacteria. - While some spirochetal infections can cause dermatologic and systemic symptoms, they are not typically linked to cold agglutinin disease or the specific presentation of Raynaud's phenomenon following a viral-like illness. *Gram-negative* - Gram-negative bacteria have a thin peptidoglycan layer and an **outer membrane containing lipopolysaccharide (LPS)**. They do not retain crystal violet stain. - While many gram-negative bacteria cause infections, they are not the typical causative agents of the cold agglutinin syndrome leading to Raynaud's as suggested by the patient's history.

Practice by Chapter

Staphylococci

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Streptococci and Enterococci

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Neisseria and Moraxella

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Corynebacterium and Listeria

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Bacillus and Clostridium

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Enterobacteriaceae

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Vibrio, Aeromonas, and Plesiomonas

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Pseudomonas and Related Bacteria

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Haemophilus and HACEK Group

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Bordetella and Brucella

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Mycobacteria

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Spirochetes

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