Bacteriology — MCQs
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Which of the following stimulate adenylate cyclase with G-protein coupled action ?
Which of the following bacteria does not exhibit bipolar staining?
Who discovered the bacterium Treponema pallidum?
Which of the following is a superantigen ?
Boutonneuse fever is caused by which of the following?
Which of the following is called Preisz-Nocard bacillus?
Which of the following Chlamydia species is primarily associated with respiratory infections?
Pseudomonas is which type of bacteria?
All are non-sporing anaerobes of medical importance except which of the following?
Which bacterium is known for forming irregular, rough colonies on culture media?
Bacteriology Indian Medical PG Practice Questions and MCQs
Question 2411: Which of the following stimulate adenylate cyclase with G-protein coupled action ?
- A. Shiga toxin
- B. Cholera toxin (Correct Answer)
- C. Diphtheria toxin
- D. Pseudomonas toxin
Explanation: ***Cholera toxin*** - Cholera toxin is a **G-protein-activating toxin** that irreversibly activates **adenylate cyclase**. - This leads to increased intracellular levels of **cAMP**, causing excessive fluid secretion into the intestinal lumen and severe diarrhea. *Shiga toxin* - Shiga toxin acts by inactivating the **60S ribosomal subunit**, thereby inhibiting protein synthesis in eukaryotic cells. - Its primary effect is **cytotoxicity**, not direct stimulation of adenylate cyclase. *Diphtheria toxin* - Diphtheria toxin inhibits **protein synthesis** by inactivating **elongation factor-2 (EF-2)** through ADP-ribosylation. - This toxin specifically targets host cells, leading to cellular death and tissue damage. *Pseudomonas toxin* - **Exotoxin A** produced by *Pseudomonas aeruginosa* also inhibits **protein synthesis** by ADP-ribosylating and inactivating **EF-2**, similar to diphtheria toxin. - It does not directly affect adenylate cyclase activity.
Question 2412: Which of the following bacteria does not exhibit bipolar staining?
- A. Haemophilus influenzae (Correct Answer)
- B. Yersinia pestis
- C. Calymmatobacterium granulomatis
- D. Francisella tularensis
Explanation: ***Haemophilus influenzae*** - *Haemophilus influenzae* is a **pleomorphic coccobacillus** that typically stains uniformly and does not exhibit characteristic **bipolar staining**. - Its presence is often identified by Gram stain showing small, Gram-negative rods, but without the distinctive safety pin appearance. *Calymmatobacterium granulomatis* - *Calymmatobacterium granulomatis* (now *Klebsiella granulomatis*) is known to exhibit **bipolar staining**, often described as a **"safety pin" appearance**, especially in tissue smears from granuloma inguinale lesions. - This characteristic staining is due to the concentration of stain at the ends of the rod-shaped bacteria. *Yersinia pestis* - *Yersinia pestis*, the causative agent of plague, is classically described as having **bipolar staining**, giving it a distinctive **"safety pin" appearance** under a microscope. - This finding is a key diagnostic feature, particularly when observed in Gram-stained smears of clinical samples. *Francisella tularensis* - *Francisella tularensis* is a small, Gram-negative coccobacillus that is known to exhibit **bipolar staining**, though it may be less prominent than in *Yersinia pestis*. - This characteristic can assist in the microscopic identification of the bacterium, which causes tularemia.
Question 2413: Who discovered the bacterium Treponema pallidum?
- A. Robert Koch
- B. Twort
- C. Ellerman
- D. Fritz Schaudinn and Erich Hoffmann (Correct Answer)
Explanation: ***Fritz Schaudinn and Erich Hoffmann*** - **Fritz Schaudinn** was a German zoologist, and **Erich Hoffmann** was a German dermatologist; they jointly discovered **Treponema pallidum** in **1905**. - Their discovery of the spirochete was a crucial step in understanding the etiology of **syphilis**. *Robert Koch* - **Robert Koch** is renowned for identifying the causative agents of **tuberculosis**, **cholera**, and **anthrax**. - He developed Koch's postulates, a fundamental set of criteria for establishing the causal relationship between a microbe and a disease. *Twort* - **Frederick Twort** was a British bacteriologist who is credited with the discovery of **bacteriophages** in **1915**. - His work involved examining transparent areas in bacterial cultures, leading to the identification of lytic viruses that infect bacteria. *Ellerman* - **Vilhelm Ellerman** was a Danish pathologist known for his work in **hematology** and **virology**. - Alongside **Olaf Bang**, he demonstrated that avian leukemia (erythroleukemia) could be transmitted by a filterable agent, indicating a viral etiology for some cancers.
Question 2414: Which of the following is a superantigen ?
- A. Cholera toxin
- B. Diphtheria toxin
- C. TSST (Correct Answer)
- D. Vero-cytoxin
Explanation: ***TSST*** - **Toxic Shock Syndrome Toxin-1 (TSST-1)** is a classic example of a superantigen produced by *Staphylococcus aureus*. - Superantigens **bind directly to MHC class II molecules and T-cell receptors (TCRs)** outside of the antigen-binding groove, leading to non-specific activation of a large percentage of T cells and a massive release of cytokines. *Cholera toxin* - **Cholera toxin** is an exotoxin produced by *Vibrio cholerae* that causes massive fluid secretion in the intestine by **activating adenylate cyclase** in enterocytes. - It functions by **ADP-ribosylating the Gs alpha subunit**, leading to constitutive activation of cyclic AMP production, but it is not a superantigen. *Diphtheria toxin* - **Diphtheria toxin**, produced by *Corynebacterium diphtheriae*, inhibits protein synthesis in eukaryotic cells by **ADP-ribosylating elongation factor-2 (EF-2)**. - This action leads to cell death and the characteristic pseudomembrane formation in diphtheria, but it does not act as a superantigen. *Vero-cytoxin* - **Vero-cytoxin** (also known as Shiga toxin or Shiga-like toxin) is produced by *E. coli* O157:H7 and other Shiga toxin-producing *E. coli* (STEC). - It inhibits protein synthesis by **cleaving ribosomal RNA**, primarily causing damage to intestinal cells and renal endothelial cells, but it is not a superantigen.
Question 2415: Boutonneuse fever is caused by which of the following?
- A. A. Rickettsia japonica
- B. B. Rickettsia conorii (Correct Answer)
- C. C. Rickettsia sibirica
- D. D. Rickettsia australis
Explanation: ***Rickettsia conorii*** - **Boutonneuse fever**, also known as Mediterranean spotted fever, is specifically caused by the bacterium **Rickettsia conorii**. - This zoonotic disease is transmitted to humans through the bite of infected ticks, primarily the **dog tick** (Rhipicephalus sanguineus). *Rickettsia japonica* - **Rickettsia japonica** is the causative agent of **oriental spotted fever**, a tick-borne illness primarily found in Japan. - This Rickettsial disease is distinct from Boutonneuse fever, having a different geographical distribution and primary vector. *Rickettsia sibirica* - **Rickettsia sibirica** is responsible for **Siberian tick typhus** (North Asian tick typhus), which is prevalent in Siberia, Mongolia, and Northern China. - While also a tick-borne Rickettsial infection, its clinical presentation and geographic range differ from Boutonneuse fever. *Rickettsia australis* - **Rickettsia australis** causes **Queensland tick typhus**, an indigenous Rickettsial disease found in Australia. - This pathogen is distinct from Rickettsia conorii and causes a localized form of tick typhus.
Question 2416: Which of the following is called Preisz-Nocard bacillus?
- A. C. diphtheriae
- B. C. pseudotuberculosis (Correct Answer)
- C. M. tuberculosis
- D. Mycoplasma
Explanation: ***C. pseudotuberculosis*** - This bacterium is historically known as the **Preisz-Nocard bacillus**, named after its discoverers. - It is an important pathogen in animals, causing conditions like **caseous lymphadenitis** in sheep and goats. *C. diphtheriae* - This bacterium causes **diphtheria** and is known for producing a potent **exotoxin** that can lead to systemic complications. - It forms a characteristic **pseudomembrane** in the throat and is not associated with the Preisz-Nocard designation. *M. tuberculosis* - This is the causative agent of **tuberculosis** in humans, primarily affecting the lungs. - It is known for its **acid-fast staining** property and a complex pathogenesis involving granuloma formation. *Mycoplasma* - This genus includes bacteria that lack a **cell wall**, making them resistant to many common antibiotics. - They are known for causing various infections, including **atypical pneumonia** and genitourinary tract infections, but are not referred to as the Preisz-Nocard bacillus.
Question 2417: Which of the following Chlamydia species is primarily associated with respiratory infections?
- A. C psittaci
- B. C pneumoniae (Correct Answer)
- C. C trachomatis
- D. None of the options
Explanation: ***C pneumoniae*** - *C. pneumoniae* is the **primary Chlamydia species** associated with **respiratory infections** in humans - It is a common cause of **community-acquired pneumonia** (5-10% of cases), **atypical pneumonia**, **bronchitis**, **pharyngitis**, and **sinusitis** - Transmission occurs via **respiratory droplets** from person to person - Causes both **acute and chronic respiratory infections** and has been linked to exacerbations of asthma and COPD - It is the most frequently encountered Chlamydia species in respiratory tract infections *C psittaci* - Causes **psittacosis** (ornithosis), a specific type of **atypical pneumonia** transmitted from **birds** (parrots, pigeons, poultry) - While it does cause respiratory infection, it is much **less common** than C. pneumoniae and is associated with **occupational or recreational bird exposure** - Not the primary Chlamydia species for general respiratory infections *C trachomatis* - Primarily causes **genital tract infections** (urethritis, cervicitis, PID) and **ocular infections** (trachoma, inclusion conjunctivitis) - Can cause **neonatal pneumonia** in infants exposed during birth, presenting at 1-3 months of age with a staccato cough - NOT a primary respiratory pathogen in adults *None of the options* - This is incorrect because **C. pneumoniae** is definitively the Chlamydia species primarily associated with respiratory infections
Question 2418: Pseudomonas is which type of bacteria?
- A. Anaerobic
- B. Aerobic (Correct Answer)
- C. Microaerophilic
- D. Obligate anaerobe
Explanation: ***Aerobic*** - **Pseudomonas aeruginosa** is a classic example of an **aerobic bacterium**, meaning it requires oxygen for growth and metabolism. - Its ability to thrive in aerobic environments contributes to its prevalence in hospital settings and its capacity to cause a wide range of infections. *Anaerobic* - Anaerobic bacteria grow in the absence of oxygen and often die in its presence. **Pseudomonas** does not exhibit this characteristic. - Examples of anaerobic bacteria include *Clostridium* species. *Microaerophilic* - Microaerophilic bacteria require oxygen but only in low concentrations (5-10%), higher concentrations are inhibitory. - **Pseudomonas** grows optimally in atmospheric oxygen levels, not restricted to low concentrations. *Obligate anaerobe* - Obligate anaerobes are severely inhibited or killed by oxygen. - This is the opposite of **Pseudomonas**, which strictly requires oxygen for survival.
Question 2419: All are non-sporing anaerobes of medical importance except which of the following?
- A. Bacteroides
- B. Fusobacterium
- C. Clostridia (Correct Answer)
- D. Actinomyces
Explanation: ***Clostridia*** - This genus is characterized by its ability to form **spores**, which are highly resistant structures allowing survival in adverse conditions. - Examples include *Clostridium difficile*, *Clostridium tetani*, and *Clostridium botulinum*, all of which are important human pathogens. *Actinomyces* - *Actinomyces* are **non-sporing, Gram-positive, anaerobic rods** that cause chronic, granulomatous infections, often forming sinus tracts. - They are known for producing characteristic **"sulfur granules"** in pus. *Bacteroides* - *Bacteroides* are **non-sporing, Gram-negative, anaerobic rods** that are a major component of the normal human gut flora. - They are frequently involved in **intra-abdominal infections** and abscess formation. *Fusobacterium* - *Fusobacterium* species are **non-sporing, Gram-negative, fusiform-shaped anaerobic rods**. - They are common in the **oral cavity** and can cause infections such as Vincent's angina and Lemierre's syndrome.
Question 2420: Which bacterium is known for forming irregular, rough colonies on culture media?
- A. Streptococcus pyogenes
- B. Bacillus anthracis
- C. Staphylococcus aureus
- D. Corynebacterium diphtheriae (Correct Answer)
Explanation: ***Corynebacterium diphtheriae*** - This bacterium is known for forming **irregular**, **rough**, and **dry colonies** on culture media. - On **Loeffler's serum agar**, colonies appear grayish-white with irregular edges and a rough, granular surface. - On **cysteine-tellurite blood agar**, colonies are black or gray with a characteristic irregular, rough morphology often described as resembling "daisy heads." - The combination of irregular shape and rough texture is a key diagnostic feature. *Staphylococcus aureus* - Typically forms **circular**, **smooth**, **shiny**, and often **golden-yellow colonies** on blood agar. - Colonies are regular in shape with a creamy, opaque appearance—distinctly different from the irregular, rough colonies of *C. diphtheriae*. *Streptococcus pyogenes* - Characteristically produces **small**, **translucent**, **smooth**, **beta-hemolytic colonies** on blood agar. - Colonies are regular, round, and smooth-textured, lacking both the irregular shape and rough texture of *C. diphtheriae*. *Bacillus anthracis* - Forms **large**, **flat**, **gray-white colonies** with a **rough, ground-glass texture** and the distinctive **"Medusa head" appearance** (comma-shaped projections at the colony edge). - While colonies do have a rough texture, they are typically **large and flat with relatively regular circular shape** when young, unlike the characteristically **irregular-shaped** colonies of *C. diphtheriae*. - The "Medusa head" pattern is the most distinctive colonial feature, differentiating it from the irregular, rough colonial morphology of *C. diphtheriae*.
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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