Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 2221: Hebra nose is caused by:

A. C. diphtheriae
B. Pseudomonas
C. Staph aureus
D. Frisch bacillus (Correct Answer)

Explanation: ***Frisch bacillus*** - **Frisch bacillus**, also known as *Klebsiella rhinoscleromatis*, is the causative agent of **rhinoscleroma**, which often presents as a condition causing tumor-like growths in the nose, leading to the characteristic "Hebra nose." - **Hebra nose** refers to the chronic, deforming nasal lesion associated with late stages of **rhinoscleroma**, characterized by hardening and widening of the nose. *C. diphtheriae* - *Corynebacterium diphtheriae* causes **diphtheria**, primarily affecting the respiratory tract and skin. - It forms a **pseudomembrane** in the throat and can lead to systemic complications due to toxin production, but it does not cause Hebra nose. *Pseudomonas* - *Pseudomonas aeruginosa* is an opportunistic pathogen commonly causing **nosocomial infections**, such as pneumonia, urinary tract infections, and wound infections. - While it can cause various infections, it is not associated with the specific nasal deformity known as Hebra nose. *Staph aureus* - *Staphylococcus aureus* is a common bacterium causing a wide range of infections, including **skin infections** (e.g., boils, cellulitis), **pneumonia**, and **sepsis**. - Although it can cause nasal colonization and local infections, it does not cause the chronic, deforming condition known as Hebra nose.

Question 2222: Chancroid is caused by ?

A. N. Gonorrhoeae
B. T. Pallidum
C. H. ducreyi (Correct Answer)
D. H. influenzae

Explanation: ***H. ducreyi*** - **Chancroid** is a sexually transmitted infection characterized by painful genital ulcers and is specifically caused by the bacterium **Haemophilus ducreyi**. - This bacterium requires specialized culture media for isolation, contributing to diagnostic challenges. *N. Gonorrhoeae* - This bacterium is responsible for **gonorrhea**, a common STI causing urethritis, cervicitis, and other inflammatory conditions. - Unlike chancroid, gonorrhea typically presents with purulent discharge and can lead to complications like pelvic inflammatory disease, but not painful genital ulcers as the primary manifestation. *T. Pallidum* - This spirochete is the causative agent of **syphilis**, an STI that progresses through distinct stages. - Syphilis initially presents with a painless chancre, which is a key differentiator from the painful ulcers of chancroid. *H. influenzae* - **Haemophilus influenzae** is a common respiratory pathogen known to cause infections such as otitis media, epiglottitis, and pneumonia. - It is not associated with sexually transmitted infections or genital ulcers.

Question 2223: A sexually active 25-year-old man develops epididymitis and orchitis. Needle biopsy demonstrates a prominent leukocytic infiltrate with numerous neutrophils. Which of the following organisms is the most likely cause of this man's infection?

A. Pseudomonas sp
B. Neisseria gonorrhoeae (Correct Answer)
C. Escherichia coli
D. Mycobacterium tuberculosis

Explanation: ***Neisseria gonorrhoeae*** - In a sexually active man under 35, **Neisseria gonorrhoeae** is a common cause of epididymitis and orchitis, often manifesting with a prominent **neutrophilic infiltrate**. - This organism is a **Gram-negative diplococcus** that causes gonococcal urethritis, which can ascend to cause epididymitis. *Pseudomonas sp* - **Pseudomonas** infections of the genitourinary tract are typically associated with **nosocomial infections**, indwelling catheters, or compromised immune systems. - While it can cause severe inflammation, it is less common in a healthy, sexually active young man without predisposing factors. *Escherichia coli* - **E. coli** is a common cause of epididymitis, particularly in men over 35 or those engaging in **anal intercourse**, often due to retrograde flow of infected urine. - While it can cause an inflammatory response, *Neisseria gonorrhoeae* is more prevalent in the described demographic for sexually transmitted infections. *Mycobacterium tuberculosis* - **Mycobacterium tuberculosis** causes granulomatous inflammation and typically presents with **caseating necrosis** and **giant cells** rather than a prominent neutrophilic infiltrate. - While it can cause chronic epididymitis, it causes a different histological picture and is less acute than described.

Question 2224: Which of the following is the most common cause of subacute endocarditis?

A. Staphylococcus epidermidis
B. Streptococcus pyogenes
C. Streptococcus viridans (Correct Answer)
D. Staphylococcus aureus

Explanation: ***Streptococcus viridans*** - This group of bacteria is the most frequent cause of **subacute bacterial endocarditis**, particularly in individuals with pre-existing valvular heart disease. - They typically cause a **slowly progressive infection** on damaged heart valves, often following dental procedures. *Staphylococcus epidermidis* - This bacterium is a common cause of **nosocomial infections** and endocarditis associated with **prosthetic heart valves** or intravascular devices. - While it can cause endocarditis, it is less common than *S. viridans* for native valve subacute endocarditis. *Streptococcus pyogenes* - *S. pyogenes* is known for causing pharyngitis ("strep throat"), cellulitis, and **rheumatic fever**, which can lead to valvular damage. - However, it is a rare direct cause of infective endocarditis. *Staphylococcus aureus* - This bacterium is a common cause of **acute infective endocarditis**, particularly in intravenous drug users and those with prosthetic valves or catheters. - It typically causes a **rapidly destructive infection** and is less commonly associated with the subacute form.

Question 2225: True statement about diphtheria toxin is -

A. Toxin acts by inhibiting synthesis of capsule
B. Endotoxaemia causes systemic manifestation
C. Toxin is phage mediated (Correct Answer)
D. Toxin is required for local infection

Explanation: ***Toxin is phage mediated*** - The diphtheria toxin gene (tox gene) is carried by a **bacteriophage (a bacterial virus)**, specifically a lysogenic bacteriophage called corynephage beta. - This means that *Corynebacterium diphtheriae* only produces the toxin if it has been infected by and integrated the DNA of this specific bacteriophage into its genome, a process known as **lysogenic conversion**. *Toxin acts by inhibiting synthesis of capsule* - The diphtheria toxin specifically inhibits **protein synthesis** in eukaryotic cells by inactivating **Elongation Factor 2 (EF-2)**. - It does not interfere with the synthesis of a bacterial capsule, which *Corynebacterium diphtheriae* typically does not possess. *Endotoxaemia causes systemic manifestation* - Diphtheria toxin is an **exotoxin**, meaning it is secreted by the live bacterium and causes systemic effects by entering the bloodstream. - **Endotoxins** are components of the outer membrane of gram-negative bacteria (specifically **Lipopolysaccharide or LPS**) that are released upon bacterial cell lysis, and *Corynebacterium diphtheriae* is a gram-positive bacterium. *Toxin is required for local infection* - While the toxin is critical for the **virulence and systemic manifestations** of diphtheria, the initial local infection (e.g., pseudomembrane formation in the throat) is primarily due to bacterial growth and inflammatory response. - The toxin's main role is to cause **tissue necrosis** at the infection site and lead to serious systemic effects like **myocarditis** and **neuropathy**.

Question 2226: The most virulent pathogenic Bacillus species is

A. Bacillus cereus
B. Bacillus subtilis
C. Bacillus megaterium
D. Bacillus anthracis (Correct Answer)

Explanation: ***Bacillus anthracis*** - This species is the **causative agent of anthrax**, a severe and often fatal disease in humans and animals, making it the most virulent. - Its virulence is attributed to its **capsule** and the production of **anthrax toxins** (edema factor, lethal factor, and protective antigen). *Bacillus cereus* - Primarily known for causing **food poisoning** (emetic and diarrheal types) due to preformed toxins in contaminated food. - While it can cause opportunistic infections, it is generally considered **less virulent** than *B. anthracis*. *Bacillus subtilis* - This is a common **soil bacterium** and is often used as a **probiotic** and in industrial applications. - It is generally considered **non-pathogenic** to humans, though rare opportunistic infections have been reported in immunocompromised individuals. *Bacillus megaterium* - A large, **spore-forming bacterium** found in soil and various environments. - It is largely **non-pathogenic** and is commonly used in industrial microbiology for enzyme production.

Question 2227: A clinically depressed farmer complains of extreme weakness, a daily rise and fall in fever, and night sweats. Small gram-negative rods are isolated from blood cultures after a 2-week incubation period. Which of the following organisms is the most likely etiologic agent?

A. Campylobacter jejuni
B. Francisella tularensis
C. Brucella melitensis (Correct Answer)
D. Salmonella enteritidis

Explanation: ***Brucella melitensis*** - This organism is known to cause **brucellosis**, which presents with **undulant fever** (daily rise and fall), night sweats, and fatigue, consistent with the patient's symptoms. The profession of a **farmer** puts him at higher risk due to exposure to infected livestock. - **Neuropsychiatric manifestations** including depression, fatigue, and malaise are well-recognized features of chronic brucellosis, explaining the patient's clinical depression. - **_Brucella_ species** are characteristic for their slow growth, often requiring **extended incubation periods** (up to 2 weeks) in blood cultures, and appear as small gram-negative rods. *Campylobacter jejuni* - This bacterium is a common cause of **gastroenteritis**, leading to **diarrhea**, abdominal cramps, and fever. While it can cause bacteremia, it typically presents with more prominent gastrointestinal symptoms. - _C. jejuni_ is a **curved or spiral-shaped** gram-negative rod, distinct from the small rods described, and does not typically cause an undulant fever pattern. *Francisella tularensis* - This agent causes **tularemia**, which can manifest with fever, chills, and fatigue, but often includes a characteristic **skin lesion (ulceroglandular)** and prominent lymphadenopathy. - Although it is a small gram-negative rod, the specific **undulant fever pattern** and the farmer's exposure history are more indicative of brucellosis. *Salmonella enteritidis* - This bacterium commonly causes **food poisoning** with symptoms like diarrhea, fever, and vomiting. While it can lead to bacteremia, it is less likely to present with the prolonged, **undulant fever** seen in brucellosis. - **_Salmonella_ species** are typically readily isolated from blood cultures within a few days, unlike the prolonged incubation needed for _Brucella_.

Question 2228: The animal model frequently used for M. leprae is -

A. Rabbits
B. Guinea pig
C. Mice
D. Nine-banded armadillos (Correct Answer)

Explanation: ***Nine-banded armadillos*** - **Nine-banded armadillos** are particularly susceptible to *Mycobacterium leprae* due to their unusually low body temperature (around 30-35°C), which is ideal for the growth of the bacterium. - They are the only other species known to naturally develop **leprosy**, making them an invaluable model for studying the disease and testing vaccines and treatments. *Rabbits* - Rabbits are generally not susceptible to *Mycobacterium leprae* infection and are not used as a primary animal model for leprosy research. - While some mycobacterial infections can be studied in rabbits, they do not mimic the human disease progression of leprosy. *Guinea pig* - Guinea pigs are known for their susceptibility to certain mycobacterial infections, particularly tuberculosis, but they are not a suitable model for *Mycobacterium leprae*. - The unique growth requirements and disease presentation of leprosy are not replicated in guinea pigs. *Mice* - While mice can be experimentally infected with *Mycobacterium leprae*, especially immunodeficient strains (e.g., nude mice, SCID mice), the infection in mice is often limited and does not fully mimic the natural human disease. - The **footpad infection model** in mice is used for drug screening but does not allow for widespread systemic infection characteristic of natural leprosy.

Question 2229: Infective dose of S.typhi is

A. 10⁷-10¹² bacilli
B. 10 bacilli
C. 10² bacilli
D. 10³-10⁶ bacilli (Correct Answer)

Explanation: ***10³-10⁶ bacilli*** - The **infective dose** for *Salmonella typhi* is generally considered to be in the range of **10³ to 10⁶ bacilli**, making it a relatively high dose required for infection. - This relatively high infective dose contributes to the epidemiology of typhoid fever, often requiring significant contamination for transmission. *10⁷-10¹² bacilli* - This range represents an **extremely high infective dose**, far exceeding the typical amount needed for *S. typhi* to cause infection. - Such a quantity would imply a much greater resistance to host defenses or a much lower virulence for the pathogen than observed. *10 bacilli* - An infective dose of **10 bacilli** is considered very low, characteristic of highly virulent pathogens like *Shigella* species. - *Salmonella typhi* is not typically associated with such a small infective dose, as it requires a larger inoculum to overcome host defenses. *10² bacilli* - An infective dose of **10² (100) bacilli** is still considered a relatively low dose for bacterial pathogens. - While some pathogens can cause infection with this number, *S. typhi* generally requires a larger inoculum to establish an infection and cause typhoid fever.

Question 2230: Legionnaires' disease is caused by?

A. Motile gram positive
B. Non-motile gram negative
C. Motile gram negative (Correct Answer)
D. Non-motile gram positive

Explanation: ***Motile gram negative*** - **Legionella pneumophila**, the causative agent of Legionnaire's disease, is a **motile gram-negative** rod. - Its **motility** is due to the presence of flagella, allowing it to move in aqueous environments. *Motile gram positive* - This option is incorrect because Legionella pneumophila is a **gram-negative** bacterium, not gram-positive. - Gram-positive bacteria have a thick peptidoglycan layer that stains purple with gram stain, which is not characteristic of Legionella. *Non-motile gram negative* - While Legionella pneumophila is gram-negative, it is in fact **motile**, making this option incorrect. - The organism uses flagella for movement, which is a key characteristic. *Non-motile gram positive* - This option is incorrect as Legionella pneumophila is both **motile** and **gram-negative**. - Combining these two incorrect characteristics makes this choice definitively false.

Practice by Chapter

Staphylococci

Practice Questions

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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