Bacteriology — MCQs
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Which of the following are Gram-negative cocci?
Which bacterial species is commonly found on the skin?
Endemic typhus is caused by which bacterium?
Which organism can resist a temperature of 50°C for minutes and grows in the presence of >6.5% NaCl?
Enterohemorrhagic, enterotoxigenic, and enterotoxigenic are types of which bacterium?
A 9-year-old girl presents with a 2-day history of sore throat. Physical examination reveals pharyngeal erythema with yellowish exudates over swollen palatine tonsils. A Gram stain of the exudate shows gram-positive cocci in chains. She is treated with penicillin. What is the most likely complication prevented by prompt treatment?
Which of the following is another name for C. diphtheriae?
A 54-year-old diabetic patient presents with an unresolved skin lesion on his foot. The lesion began several weeks ago as a blister and has since become a painful, erosive, expanding sore. On examination, the affected site is now 5 cm in diameter, with a black necrotic center and raised red edges. Which toxin has a mechanism of action most similar to the toxin responsible for tissue damage in this patient?
Which rickettsial disease shows a positive Weil-Felix reaction with OX-19 only?
E. coli can be classified by their characteristic virulence properties and different mechanisms that cause disease. To which group does the verotoxin producing E. coli O157:H7 serotype belong?
Bacteriology Indian Medical PG Practice Questions and MCQs
Question 1211: Which of the following are Gram-negative cocci?
- A. Neisseria gonorrhoeae and Neisseria meningitidis (Correct Answer)
- B. Bacillus
- C. Staphylococcus
- D. Streptococcus
Explanation: **Explanation:** The correct answer is **A. *Neisseria gonorrhoeae* and *Neisseria meningitidis***. In bacteriology, organisms are classified based on their Gram stain reaction (color) and morphology (shape). **Gram-negative** bacteria appear pink/red under a microscope due to a thin peptidoglycan layer and an outer lipopolysaccharide membrane. **Cocci** are spherical or oval-shaped bacteria. The genus *Neisseria* consists of Gram-negative, kidney-bean-shaped **diplococci** (occurring in pairs). **Analysis of Incorrect Options:** * **B. *Bacillus*:** These are Gram-positive rods (bacilli). They are known for being spore-formers (e.g., *B. anthracis*). * **C. *Staphylococcus*:** These are Gram-positive cocci that typically arrange themselves in grape-like clusters. * **D. *Streptococcus*:** These are Gram-positive cocci that typically arrange themselves in chains or pairs. **High-Yield NEET-PG Clinical Pearls:** * **The "Big Three" Gram-negative cocci:** *Neisseria meningitidis*, *Neisseria gonorrhoeae*, and *Moraxella catarrhalis*. * **Culture Media:** *Neisseria* species are fastidious and grow best on **Thayer-Martin medium** (a selective Chocolate agar). * **Biochemical Test:** All *Neisseria* species are **Oxidase positive** and **Catalase positive**. * **Differentiation:** *N. meningitidis* ferments both **M**altose and **G**lucose (**M**eningitidis = **M**altose), whereas *N. gonorrhoeae* ferments only **G**lucose (**G**onorrhoeae = **G**lucose). * **Virulence:** *N. meningitidis* is encapsulated (polysaccharide capsule), while *N. gonorrhoeae* is non-encapsulated but possesses highly antigenic pili.
Question 1212: Which bacterial species is commonly found on the skin?
- A. Lactobacillus
- B. Streptococcus pneumoniae
- C. Bacteroides fragilis
- D. Propionibacterium (Correct Answer)
Explanation: **Explanation:** The human skin is a complex ecosystem dominated by specific microbial flora adapted to its acidic, salty, and nutrient-poor environment. **Why Propionibacterium is correct:** *Propionibacterium* (now often reclassified as *Cutibacterium*, e.g., *C. acnes*) is a Gram-positive, anaerobic/microaerophilic rod that is a primary inhabitant of the skin, particularly in areas rich in sebaceous glands (face, back, and chest). It thrives by breaking down lipids in sebum into fatty acids. It is clinically significant as a major factor in the pathogenesis of **Acne vulgaris** and can cause opportunistic infections in prosthetic joints or heart valves. **Analysis of Incorrect Options:** * **Lactobacillus:** These are the predominant normal flora of the **female genitourinary tract** (Vagina). They maintain an acidic pH by producing lactic acid, protecting against pathogens. * **Streptococcus pneumoniae:** This is a common inhabitant of the **nasopharynx** and upper respiratory tract. It is a leading cause of community-acquired pneumonia and meningitis, but it is not normal skin flora. * **Bacteroides fragilis:** This is the most abundant **anaerobe in the colon** (large intestine). It is a frequent cause of intra-abdominal abscesses following bowel perforation. **High-Yield Clinical Pearls for NEET-PG:** * **Most common skin commensal:** *Staphylococcus epidermidis* (Coagulase-negative Staph). * **Dominant flora in oily (sebaceous) areas:** *Propionibacterium*. * **Dominant flora in moist areas (axilla/groin):** *Corynebacterium* and *Staphylococcus aureus*. * **Resident vs. Transient:** Resident flora (like *S. epidermidis*) are permanent, while transient flora (like *S. aureus*) are temporarily colonizing the skin surface.
Question 1213: Endemic typhus is caused by which bacterium?
- A. Rickettsia prowazekii
- B. Rickettsia typhi (Correct Answer)
- C. Rickettsia rickettsiae
- D. Rickettsia akari
Explanation: **Explanation:** **Endemic typhus** (also known as Murine typhus) is caused by ***Rickettsia typhi***. The primary reservoir for this bacterium is the rat, and it is transmitted to humans via the **rat flea** (*Xenopsylla cheopis*). Clinically, it presents with fever, headache, and a maculopapular rash, though it is generally milder than epidemic typhus. **Analysis of Options:** * **Rickettsia prowazekii (Option A):** Causes **Epidemic typhus**. It is transmitted by the **human body louse** (*Pediculus humanus corporis*). It is more severe and can recur years later as Brill-Zinsser disease. * **Rickettsia rickettsiae (Option C):** Causes **Rocky Mountain Spotted Fever (RMSF)**, primarily in the Western Hemisphere. It is transmitted by **hard ticks** (*Dermacentor* species). * **Rickettsia akari (Option D):** Causes **Rickettsialpox**. It is transmitted by **mites** (*Liponyssoides sanguineus*) and is characterized by a distinctive initial eschar at the bite site. **High-Yield Clinical Pearls for NEET-PG:** * **Weil-Felix Test:** A heterophile agglutination test used for diagnosis. *R. typhi* reacts with **OX-19**. * **Drug of Choice:** **Doxycycline** is the gold standard treatment for all rickettsial infections, including in children. * **Rash Progression:** In typhus group rickettsiae, the rash typically starts on the trunk and spreads peripherally to the extremities (sparing palms and soles), whereas in RMSF, it starts on the wrists/ankles and spreads centrally.
Question 1214: Which organism can resist a temperature of 50°C for minutes and grows in the presence of >6.5% NaCl?
- A. S. pyogenes
- B. S. aureus
- C. Enterococcus (Correct Answer)
- D. Pneumococci
Explanation: **Explanation:** The correct answer is **Enterococcus**. This question tests the ability to differentiate between various Gram-positive cocci based on their biochemical and physiological resilience. **1. Why Enterococcus is correct:** Enterococci (formerly classified as Group D Streptococci) are known for their remarkable "hardiness." They are uniquely characterized by their ability to survive extreme conditions that would inhibit most other non-spore-forming bacteria. Key physiological markers include: * **Heat Resistance:** They can withstand temperatures of **60°C for 30 minutes** (thermal resistance). * **Salt Tolerance:** They grow in media containing **6.5% NaCl**. * **Bile Tolerance:** They grow in the presence of 40% bile and hydrolyze **esculin** (Bile Esculin positive). * **pH Range:** They can grow at a high pH of 9.6. **2. Why the other options are incorrect:** * **S. pyogenes (Group A):** These are delicate organisms that are highly sensitive to heat and high salt concentrations. They are typically identified by Bacitracin sensitivity. * **S. aureus:** While *S. aureus* is salt-tolerant (growing on Mannitol Salt Agar which contains 7.5% NaCl), it does not typically share the specific thermal resistance profile (60°C for 30 mins) or the Group D carbohydrate antigen characteristic of Enterococci. * **Pneumococci (S. pneumoniae):** These are extremely fastidious and fragile organisms. They are bile-soluble and sensitive to Optochin, making them unable to survive the harsh conditions described. **NEET-PG High-Yield Pearls:** * **PYR Test:** Enterococci are **PYR positive** (similar to *S. pyogenes*). * **VRE:** Vancomycin-Resistant Enterococci (especially *E. faecium*) are a major cause of nosocomial infections. * **Clinical Presentation:** Commonly cause UTIs, biliary tract infections, and subacute bacterial endocarditis (often following GI/GU procedures).
Question 1215: Enterohemorrhagic, enterotoxigenic, and enterotoxigenic are types of which bacterium?
- A. E. coli (Correct Answer)
- B. Klebsiella
- C. Shigella
- D. Streptococcus pneumoniae
Explanation: **Explanation:** The correct answer is **A. E. coli**. *Escherichia coli* (E. coli) is a versatile Gram-negative rod classified into various "pathotypes" based on specific virulence factors and clinical presentations. The terms mentioned in the question refer to these distinct groups: * **Enterohemorrhagic E. coli (EHEC):** Produces Shiga-like toxins (Verotoxins); associated with Hemolytic Uremic Syndrome (HUS) and bloody diarrhea (O157:H7). * **Enterotoxigenic E. coli (ETEC):** Produces heat-labile (LT) and heat-stable (ST) toxins; the leading cause of **Traveler’s diarrhea**. * **Enteropathogenic E. coli (EPEC):** Causes infantile diarrhea by attaching to intestinal mucosa via "attaching and effacing" (A/E) lesions. * Other types include Enteroinvasive (EIEC) and Enteroaggregative (EAEC). **Why incorrect options are wrong:** * **Klebsiella:** Known for causing pneumonia (currant jelly sputum) and UTIs, but does not use these specific "entero-" classifications. * **Shigella:** While EHEC produces a Shiga-like toxin, *Shigella* species are distinct organisms categorized by Serogroups (A-D), not these pathotypes. * **Streptococcus pneumoniae:** This is a Gram-positive coccus primarily causing respiratory infections and meningitis; it is not an enteric pathogen. **High-Yield NEET-PG Pearls:** * **ETEC:** Think "T" for Traveler's and Toxins (LT/ST). * **EHEC:** Does **not** ferment sorbitol (MacConkey Sorbitol Agar is used for screening O157:H7). * **EIEC:** Clinically indistinguishable from Shigellosis (dysentery). * **EPEC:** Associated with "pediatric" outbreaks in nurseries.
Question 1216: A 9-year-old girl presents with a 2-day history of sore throat. Physical examination reveals pharyngeal erythema with yellowish exudates over swollen palatine tonsils. A Gram stain of the exudate shows gram-positive cocci in chains. She is treated with penicillin. What is the most likely complication prevented by prompt treatment?
- A. Carditis (Correct Answer)
- B. Hepatitis
- C. Meningitis
- D. Otitis
Explanation: **Explanation:** The clinical presentation of sore throat, pharyngeal exudates, and Gram-positive cocci in chains is diagnostic of **Acute Pharyngitis** caused by **Group A Streptococcus (GAS)**, specifically *Streptococcus pyogenes*. **Why Carditis is correct:** Prompt antibiotic treatment (within 9 days of symptom onset) is primarily aimed at preventing **Acute Rheumatic Fever (ARF)**. ARF is a nonsuppurative, immune-mediated complication caused by molecular mimicry between streptococcal M-proteins and human cardiac tissue. **Carditis** is the most serious manifestation of ARF and can lead to permanent valvular damage. Notably, while antibiotics prevent ARF, they do **not** prevent Post-Streptococcal Glomerulonephritis (PSGN). **Why other options are incorrect:** * **Hepatitis:** This is not a recognized complication of *S. pyogenes* infection. * **Meningitis:** While GAS can rarely cause meningitis via hematogenous spread or local extension, it is not the primary complication targeted by routine penicillin therapy for pharyngitis. * **Otitis:** Otitis media is a common *suppurative* complication of streptococcal pharyngitis. While penicillin reduces its incidence, the most critical "preventable" systemic sequela emphasized in medical exams is the life-altering carditis associated with ARF. **High-Yield NEET-PG Pearls:** * **Jones Criteria:** Used for diagnosing ARF (Major: Carditis, Polyarthritis, Chorea, Erythema marginatum, Subcutaneous nodules). * **ASO Titre:** Elevated Anti-Streptolysin O (ASO) titers indicate recent streptococcal infection. * **Drug of Choice:** Penicillin remains the gold standard for GAS pharyngitis due to 100% sensitivity. * **PSGN vs. ARF:** Antibiotics prevent ARF but **not** PSGN. PSGN can follow both skin (impetigo) and throat infections, whereas ARF follows **only** pharyngeal infections.
Question 1217: Which of the following is another name for C. diphtheriae?
- A. Koch's bacillus
- B. Roux bacillus
- C. Klebs-Löffler bacillus (Correct Answer)
- D. Yersin bacillus
Explanation: **Explanation:** The correct answer is **C. Klebs-Löffler bacillus**. *Corynebacterium diphtheriae* is historically known as the **Klebs-Löffler bacillus** because it was first described by Edwin Klebs in 1883 (in diphtheritic membranes) and subsequently cultivated in pure culture by Friedrich Löffler in 1884. Understanding these eponymous names is high-yield for NEET-PG, as they frequently appear in clinical vignettes. **Analysis of Incorrect Options:** * **A. Koch's bacillus:** This refers to *Mycobacterium tuberculosis*, discovered by Robert Koch in 1882. * **B. Roux bacillus:** While Émile Roux was a collaborator of Pasteur who discovered the diphtheria toxin, the bacterium itself is not named after him. * **D. Yersin bacillus:** This refers to *Yersinia pestis* (formerly *Pasteurella pestis*), the causative agent of plague, discovered by Alexandre Yersin. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** Gram-positive, non-motile, club-shaped bacilli arranged in "Chinese letter" or cuneiform patterns due to snapping division. * **Granules:** Contains **Volutin/Metachromatic (Babes-Ernst) granules** which stain red with Albert’s or Neisser’s stain. * **Culture Media:** **Löffler's Serum Slope** (rapid growth) and **Potassium Tellurite Agar** (black colonies due to tellurite reduction). * **Pathogenesis:** Disease is mediated by an exotoxin (encoded by the *tox* gene via a **beta-corynephage**) which inhibits protein synthesis by inactivating **EF-2**. * **Virulence Test:** **Elek’s gel precipitation test** is the gold standard for detecting toxigenicity.
Question 1218: A 54-year-old diabetic patient presents with an unresolved skin lesion on his foot. The lesion began several weeks ago as a blister and has since become a painful, erosive, expanding sore. On examination, the affected site is now 5 cm in diameter, with a black necrotic center and raised red edges. Which toxin has a mechanism of action most similar to the toxin responsible for tissue damage in this patient?
- A. Anthrax toxin
- B. Botulinum toxin
- C. Cholera toxin
- D. Diphtheria toxin (Correct Answer)
Explanation: **Explanation:** The clinical presentation describes **Ecthyma gangrenosum**, a characteristic skin lesion caused by ***Pseudomonas aeruginosa***, typically seen in immunocompromised or diabetic patients. The hallmark of this condition is a hemorrhagic bulla that evolves into a necrotic ulcer with a black eschar, caused by perivascular bacterial invasion and toxin-mediated tissue destruction. The primary virulence factor responsible for tissue necrosis in *Pseudomonas* is **Exotoxin A**. Its mechanism of action is the **inactivation of Elongation Factor-2 (EF-2)** via ADP-ribosylation, which halts protein synthesis and leads to cell death. **Why Diphtheria Toxin is the correct answer:** * **Diphtheria toxin** (produced by *Corynebacterium diphtheriae*) shares the **exact same mechanism**: it ADP-ribosylates EF-2, inhibiting protein synthesis. This is a high-yield "mirror" mechanism frequently tested in NEET-PG. **Analysis of Incorrect Options:** * **Anthrax toxin:** Consists of Edema Factor (increases cAMP) and Lethal Factor (a zinc metalloproteinase that cleaves MAP kinase). * **Botulinum toxin:** A protease that cleaves SNARE proteins, preventing the release of acetylcholine at the neuromuscular junction. * **Cholera toxin:** Permanently activates Gs alpha subunits, leading to increased adenylate cyclase activity and high cAMP levels in intestinal cells. **Clinical Pearls for NEET-PG:** * **Pseudomonas vs. Diphtheria:** Both use EF-2 ADP-ribosylation, but *Pseudomonas* Exotoxin A acts systemically/locally in wounds, while Diphtheria toxin primarily targets the heart and nerves. * **Ecthyma gangrenosum** is a sign of *Pseudomonas* septicemia; it is **not** the same as Ecthyma (which is a deep form of impetigo caused by *S. pyogenes*). * **Other cAMP activators:** *Vibrio cholerae*, *ETEC* (Heat-labile toxin), and *Bordetella pertussis* (via Gi inhibition).
Question 1219: Which rickettsial disease shows a positive Weil-Felix reaction with OX-19 only?
- A. Epidemic typhus (Correct Answer)
- B. Scrub typhus
- C. Trench fever
- D. Q fever
Explanation: The **Weil-Felix reaction** is a heterophile agglutination test used to diagnose rickettsial infections. It relies on the cross-reactivity between antibodies produced against Rickettsial antigens and the somatic (O) antigens of certain **Proteus** strains (*P. vulgaris* OX-19, OX-2 and *P. mirabilis* OX-K). ### **Explanation of Options** * **Epidemic Typhus (Correct):** Caused by *Rickettsia prowazekii*, this disease (along with Endemic/Murine typhus) typically shows a strong positive reaction with **OX-19 only**. The antibodies cross-react specifically with the OX-19 strain of *Proteus vulgaris*. * **Scrub Typhus (Incorrect):** Caused by *Orientia tsutsugamushi*, it shows a positive reaction with **OX-K only**. It does not react with OX-19 or OX-2. * **Trench Fever (Incorrect):** Caused by *Bartonella quintana*. This organism does not belong to the Rickettsia genus and is **Weil-Felix negative**. * **Q Fever (Incorrect):** Caused by *Coxiella burnetii*. This is a classic "high-yield" negative; Q fever is **Weil-Felix negative** because it does not share antigens with Proteus. ### **High-Yield Clinical Pearls for NEET-PG** | Disease | Agent | Weil-Felix Pattern | | :--- | :--- | :--- | | **Epidemic/Endemic Typhus** | *R. prowazekii / R. typhi* | **OX-19 (+++)** | | **Rocky Mountain Spotted Fever** | *R. rickettsii* | OX-19 (+) and OX-2 (+) | | **Scrub Typhus** | *O. tsutsugamushi* | **OX-K (+++)** | | **Q Fever / Trench Fever** | *Coxiella / Bartonella* | **Negative** | * **Note:** The Weil-Felix test is now largely replaced by more specific tests like IFA (Indirect Fluorescent Antibody), but it remains a favorite for NEET-PG examiners due to these distinct patterns.
Question 1220: E. coli can be classified by their characteristic virulence properties and different mechanisms that cause disease. To which group does the verotoxin producing E. coli O157:H7 serotype belong?
- A. Enteroaggregative E. coli (EAEC)
- B. Enterohemorrhagic E. coli (EHEC) (Correct Answer)
- C. Enteroinvasive E. coli (EIEC)
- D. Enteropathogenic E. coli (EPEC)
Explanation: **Explanation:** **Enterohemorrhagic *E. coli* (EHEC)** is the correct answer because the **O157:H7 serotype** is the most clinically significant member of this group. Its primary virulence factor is the production of **Verotoxins (VT1 and VT2)**, also known as **Shiga-like toxins (SLT)**. These toxins inhibit protein synthesis by damaging the 28S rRNA of the 60S ribosomal subunit, leading to cell death. **Analysis of Options:** * **Enteroaggregative *E. coli* (EAEC):** Characterized by a "stacked-brick" adhesion pattern on HEp-2 cells. It causes persistent diarrhea in children and HIV patients. * **Enteroinvasive *E. coli* (EIEC):** Pathogenically similar to *Shigella*. It invades the colonic epithelium using actin tails, causing inflammatory diarrhea with blood and mucus (dysentery). * **Enteropathogenic *E. coli* (EPEC):** Known for causing infantile diarrhea in developing countries. It produces **Attaching and Effacing (A/E) lesions** but does not produce toxins. **NEET-PG High-Yield Pearls:** 1. **Clinical Presentation:** EHEC causes a spectrum of disease from mild diarrhea to **Hemorrhagic Colitis** (grossly bloody stools without fever). 2. **Complication:** It is the leading cause of **Hemolytic Uremic Syndrome (HUS)**, characterized by the triad of Microangiopathic Hemolytic Anemia, Thrombocytopenia, and Acute Renal Failure. 3. **Diagnosis:** Unlike most *E. coli*, O157:H7 does **not ferment Sorbitol**. It is identified on **Sorbitol MacConkey (SMAC) agar** as colorless colonies. 4. **Management:** Antibiotics are generally avoided as they may increase toxin release and the risk of HUS.
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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