Bacteriology — MCQs

A 47-year-old man with chronic asthma who has been treated with steroids for many years develops a productive cough, weight loss, and night sweats. Imaging studies reveal the presence of abscesses in the lungs and brain. Cultures show gram-positive filaments that are weakly acid-fast. Which of the following organisms is responsible for this patient's condition?

Q2127

Chlamydia causes:

Q2128

Acute infectious diarrhea can be caused by all EXCEPT:

Q2129

A 45 year-old HIV-positive male presented with multiple red hemangioma-like lesions. Biopsy specimens show clusters of bacilli that stain positively with the Warthin-Starry stain. Which of the following could be the causative organism?

Q2130

The primary site of multiplication of rickettsial organisms is in the

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 2121: Quellung reaction is seen with:

A. Pneumococcus (Correct Answer)
B. Gonococcus
C. Staphylococcus
D. Streptococcus

Explanation: ***Pneumococcus*** - The Quellung reaction, also known as the **Neufeld reaction**, is a classic immunological test used to identify bacteria based on their **capsular polysaccharides**. - In the presence of specific antisera, the capsule of **_Streptococcus pneumoniae_** (Pneumococcus) swells visibly when viewed under a microscope, appearing larger and more refractile. - **Pneumococcus is the CLASSIC organism** associated with the Quellung reaction in clinical microbiology and is the standard answer in medical examinations. - While other encapsulated bacteria (_Haemophilus influenzae_, _Klebsiella pneumoniae_, _N. meningitidis_) can theoretically show capsular swelling, the test is primarily used for and associated with pneumococcal identification. *Gonococcus* - **_Neisseria gonorrhoeae_** (Gonococcus) is a Gram-negative diplococcus that causes sexually transmitted infections. - It does not possess a prominent polysaccharide capsule that exhibits the Quellung reaction in routine clinical practice. *Staphylococcus* - **_Staphylococcus_** species are Gram-positive cocci that form grape-like clusters. - While some staphylococci produce capsules or slime layers, these are not characterized using the Quellung reaction. *Streptococcus* - While _S. pneumoniae_ is technically a species of Streptococcus, in clinical terminology **"Pneumococcus" is the specific term** that denotes this particular organism. - When asked about the Quellung reaction, **"Pneumococcus" is the preferred and correct answer** rather than the broader genus term "Streptococcus." - The Quellung test is not routinely used for other Streptococcus species (like _S. pyogenes_, _S. agalactiae_) in standard clinical practice, making "Pneumococcus" the most accurate answer.

Question 2122: Bacillus anthracis is:

A. Gram negative cocci in cluster
B. Gram positive rods with square ends (Correct Answer)
C. Gram positive cocci in cluster
D. Gram positive bacilli with spherical ends

Explanation: ***Gram positive rods with square ends*** - *Bacillus anthracis* is a **gram-positive bacterium**, meaning it retains the crystal violet stain due to its thick peptidoglycan layer. - Microscopically, it appears as **large, rod-shaped bacteria** (bacilli) with characteristic **square-cut ends**, often arranged in chains. *Gram negative cocci in cluster* - This describes organisms like **Neisseria gonorrhoeae** or **meningitidis**, which are spherical (cocci) and appear pink/red after Gram staining. - *Bacillus anthracis* is a **rod-shaped bacterium** that stains Gram-positive. *Gram positive cocci in cluster* - This morphology is characteristic of **Staphylococcus species**, which are spherical bacteria that stain purple and arrange in grape-like clusters. - *Bacillus anthracis* is a **rod-shaped bacterium**, not a coccus. *Gram positive bacilli with spherical ends* - While *Bacillus anthracis* is a **gram-positive bacillus**, its ends are distinctly **square-cut**, not spherical. - Bacteria with spherical or rounded ends would include some other *Bacillus* species or members of the **Clostridium** genus.

Question 2123: The causative agent for duodenal ulcer is:

A. Lactobacilli
B. E. coli
C. H. Pylori (Correct Answer)
D. Shigella

Explanation: ***H. pylori*** - ***H. pylori*** is a **Gram-negative spiral-shaped bacterium** that colonizes the stomach and duodenum, causing inflammation and increasing the risk of peptic ulcers, including duodenal ulcers. - It produces **urease**, which neutralizes stomach acid by converting urea to ammonia, protecting the bacteria and contributing to mucosal damage. *Lactobacilli* - **Lactobacilli** are **Gram-positive, rod-shaped bacteria** commonly found in the digestive tract and fermented foods. - They are generally considered **beneficial probiotics** and do not cause duodenal ulcers. *E. coli* - ***E. coli*** is a **Gram-negative, rod-shaped bacterium** typically found in the intestines of humans and animals. - While some strains can cause **gastrointestinal infections** (e.g., traveler's diarrhea), *E. coli* is not a known cause of duodenal ulcers. *Shigella* - **Shigella** is a **Gram-negative, rod-shaped bacterium** that causes **shigellosis**, an intestinal infection characterized by severe diarrhea (dysentery). - It primarily affects the **large intestine** and does not cause duodenal ulcers.

Question 2124: Which of the following is anaerobic non-acid fast bacilli:

A. Mycobacterium tuberculosis
B. Listeria
C. Nocardia
D. Actinomyces (Correct Answer)

Explanation: ***Actinomyces*** - *Actinomyces* species are **obligate anaerobes** to **facultative anaerobes** that appear as **Gram-positive, non-acid-fast, filamentous rods** (bacilli). - They are classically described as **anaerobic bacteria** and are the prototypical example of anaerobic non-acid-fast bacilli. - They cause chronic infections (actinomycosis) often involving abscess formation with sulfur granules and sinus tracts. *Mycobacterium tuberculosis* - *Mycobacterium tuberculosis* is an **obligate aerobe**, not anaerobic. - It is characterized by its **acid-fastness** due to high mycolic acid content in its cell wall. - Requires oxygen for growth and is the causative agent of tuberculosis. *Listeria* - *Listeria monocytogenes* is a **facultative anaerobe**, meaning it preferentially grows in **aerobic conditions** but can survive anaerobically. - While it is a non-acid-fast Gram-positive bacillus, it is **not classified as an anaerobic organism** in standard microbiology terminology. - *Actinomyces* is the classic answer for anaerobic non-acid-fast bacilli. *Nocardia* - *Nocardia* species are **strict aerobes**, not anaerobic. - They are **weakly acid-fast** filamentous bacteria, distinguishing them from the non-acid-fast *Actinomyces*. - Require oxygen for growth and are found in soil.

Question 2125: Eaton agent is:

A. Pseudomonas
B. Mycoplasma (Correct Answer)
C. Gonococcus
D. Corynebacterium

Explanation: ***Correct: Mycoplasma*** - The term **"Eaton agent"** refers to *Mycoplasma pneumoniae*, a common cause of atypical pneumonia. - This organism was initially identified by Monroe D. Eaton in the 1940s while studying respiratory infections. - *M. pneumoniae* is a cell wall-deficient bacterium that causes "walking pneumonia" and is resistant to beta-lactam antibiotics. *Incorrect: Pseudomonas* - **Pseudomonas** species, particularly *Pseudomonas aeruginosa*, are opportunistic pathogens often associated with nosocomial infections and infections in immunocompromised individuals. - They are not related to the "Eaton agent" or atypical pneumonia caused by *Mycoplasma pneumoniae*. *Incorrect: Gonococcus* - **Gonococcus** refers to *Neisseria gonorrhoeae*, a bacterium responsible for the sexually transmitted infection gonorrhea. - It causes distinct symptoms in the genitourinary tract and is unrelated to respiratory infections or the "Eaton agent." *Incorrect: Corynebacterium* - **Corynebacterium** species include *Corynebacterium diphtheriae*, the causative agent of diphtheria, which primarily affects the upper respiratory tract and skin. - While it can cause respiratory issues, it is not the "Eaton agent" and has a different clinical presentation and pathogenicity from *Mycoplasma pneumoniae*.

Question 2126: A 47-year-old man with chronic asthma who has been treated with steroids for many years develops a productive cough, weight loss, and night sweats. Imaging studies reveal the presence of abscesses in the lungs and brain. Cultures show gram-positive filaments that are weakly acid-fast. Which of the following organisms is responsible for this patient's condition?

A. Actinomyces israelii
B. Nocardia asteroides (Correct Answer)
C. Bacillus anthracis
D. Mycobacterium tuberculosis

Explanation: ***Nocardia asteroides*** - The presence of **productive cough, weight loss, night sweats**, and **abscesses in the lungs and brain** in an immunocompromised patient (due to chronic steroid use for asthma) is highly suggestive of Nocardiosis. - Identification of **gram-positive, branching filaments** that are **weakly acid-fast** in cultures confirms *Nocardia* as the causative organism. *Actinomyces israelii* - While *Actinomyces* causes chronic abscesses and is a **gram-positive filamentous bacterium**, it is **not acid-fast** and typically causes cervicofacial, thoracic, or abdominal infections often associated with trauma or surgery, not disseminated brain abscesses in this context. - *Actinomyces* infections are characterized by **"sulfur granules"** in pus, which are not mentioned here. *Bacillus anthracis* - *Bacillus anthracis* is a **gram-positive rod** that causes **anthrax**, typically presenting as cutaneous, inhalational, or gastrointestinal forms. - It does **not form filamentous structures** and is not acid-fast, and its clinical presentation of rapid-onset severe pneumonia or skin lesions does not match the described chronic progressive illness with multiple abscesses. *Mycobacterium tuberculosis* - *Mycobacterium tuberculosis* causes **tuberculosis**, characterized by cough, weight loss, and night sweats, and can cause lung and brain abscesses (tuberculomas). - However, *Mycobacterium* species are **strongly acid-fast** due to their mycolic acid cell wall and are not described as gram-positive filamentous organisms.

Question 2127: Chlamydia causes:

A. Trachoma
B. Trachoma and Conjunctivitis
C. Trachoma and Lymphogranuloma venereum (Correct Answer)
D. Conjunctivitis

Explanation: **Trachoma and Lymphogranuloma venereum** - This is the **most comprehensive correct answer** as it lists two major diseases caused by different serovars of *Chlamydia trachomatis* - **Trachoma** (serovars A, B, Ba, C) - chronic keratoconjunctivitis and the leading infectious cause of blindness worldwide - **Lymphogranuloma venereum (LGV)** (serovars L1, L2, L3) - systemic sexually transmitted infection with inguinal lymphadenopathy - This answer captures both the **ocular** and **genital** manifestations of *C. trachomatis*, making it the best option *Trachoma* - While medically accurate, this is **incomplete** as it only mentions one disease - *C. trachomatis* causes multiple clinically significant diseases beyond trachoma - This option ignores LGV and other important chlamydial syndromes *Trachoma and Conjunctivitis* - This option is **redundant and misleading** - trachoma IS a specific type of chronic bacterial conjunctivitis - Listing both together suggests they are separate entities when trachoma is actually a subset of conjunctivitis - Misses other major non-ocular diseases like LGV, making it inferior to the correct answer *Conjunctivitis* - While *C. trachomatis* does cause conjunctivitis (trachoma, inclusion conjunctivitis), this is **too broad and non-specific** - Conjunctivitis has numerous causes (viral, bacterial, allergic) and this answer lacks specificity - Completely omits systemic/genital manifestations like LGV, urethritis, and PID

Question 2128: Acute infectious diarrhea can be caused by all EXCEPT:

A. Enterococcus faecalis (Correct Answer)
B. Staphylococcus
C. Clostridium
D. Vibrio

Explanation: ***Enterococcus faecalis*** - While *Enterococcus faecalis* is a significant hospital-acquired pathogen, it is **not a common cause of acute infectious diarrhea**. - It primarily causes infections like **urinary tract infections**, **endocarditis**, and **bacteremia**, especially in immunocompromised individuals. *Staphylococcus* - *Staphylococcus aureus* can cause **food poisoning** due to preformed toxins, leading to acute onset vomiting and diarrhea. - This is an example of an **acute infectious diarrheal illness**, though often toxin-mediated rather than direct infection. *Clostridium* - *Clostridium difficile* is a well-known cause of **antibiotic-associated diarrhea** and pseudomembranous colitis, which is an acute infectious diarrhea. - Other *Clostridium* species, like *Clostridium perfringens*, can cause **food poisoning** with diarrheal symptoms. *Vibrio* - *Vibrio cholerae* is the classic cause of **cholera**, an acute, severe watery diarrhea that can lead to rapid dehydration. - Other *Vibrio* species, such as *Vibrio parahaemolyticus*, frequently cause **foodborne illness** from contaminated seafood, presenting with acute diarrhea.

Question 2129: A 45 year-old HIV-positive male presented with multiple red hemangioma-like lesions. Biopsy specimens show clusters of bacilli that stain positively with the Warthin-Starry stain. Which of the following could be the causative organism?

A. Bartonella quintana
B. HHV8
C. Bartonella henselae (Correct Answer)
D. EBV

Explanation: ***Bartonella henselae*** - This clinical presentation of **red hemangioma-like lesions** (bacillary angiomatosis) in an **HIV-positive** individual, coupled with **Warthin-Starry stain positive bacilli** on biopsy, is highly characteristic of infection with *Bartonella henselae*. - *Bartonella henselae* is associated with **cat scratch disease** and bacillary angiomatosis, particularly in immunocompromised patients. *Bartonella quintana* - While *Bartonella quintana* can also cause **bacillary angiomatosis**, it is classically associated with **trench fever** and **endocarditis** more frequently than *Bartonella henselae*. - The clinical picture of typical "cat-scratch-like" lesions (hemangioma-like) more strongly points to *B. henselae*. *HHV8* - **Human Herpesvirus 8 (HHV8)** is the causative agent of **Kaposi's sarcoma**, which can also present with purple-red skin lesions in HIV-positive patients. - However, Kaposi's sarcoma lesions are typically **spindle cell proliferations** and would not show **clusters of bacilli** on Warthin-Starry stain. *EBV* - **Epstein-Barr Virus (EBV)** is associated with various lymphoproliferative disorders and **oral hairy leukoplakia** in HIV-positive individuals. - It does not cause **bacillary angiomatosis** or present with Warthin-Starry positive bacilli.

Question 2130: The primary site of multiplication of rickettsial organisms is in the

A. Media of arteries
B. Parenchymal cells of the liver
C. Adventitia of all blood vessels
D. Endothelial cells of small vessels (Correct Answer)

Explanation: ***Endothelial cells of small vessels*** - Rickettsial organisms are **obligate intracellular bacteria** and have a strong tropism for the **endothelial cells** lining small blood vessels. - This tropism leads to **vasculitis**, which causes the characteristic rash and systemic symptoms seen in rickettsial infections. *Media of arteries* - While rickettsiae infect blood vessels, their primary site of multiplication is not the **media of arteries**, which mainly consists of smooth muscle cells. - The **endothelium**, the innermost layer, is the critical target for replication and subsequent host cell damage. *Parenchymal cells of the liver* - Although some rickettsial infections can cause liver dysfunction, the **parenchymal cells (hepatocytes)** of the liver are not their primary site of multiplication. - Liver involvement is often secondary to systemic vasculitis rather than direct hepatocyte infection. *Adventitia of all blood vessels* - The **adventitia** is the outermost layer of blood vessels and is composed mainly of connective tissue. - Rickettsial organisms primarily target the **endothelial cells** lining the lumen, not the adventitial layer.

Practice by Chapter

Staphylococci

Practice Questions

Streptococci and Enterococci

Practice Questions

Neisseria and Moraxella

Practice Questions

Corynebacterium and Listeria

Practice Questions

Bacillus and Clostridium

Practice Questions

Enterobacteriaceae

Practice Questions

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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