Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 1001: Which of the following is NOT true of Streptococcus?

A. It is a group of lactic acid producing bacteria. (Correct Answer)
B. It shows adherence to dental enamel.
C. It is most common in humans.
D. It is most common in animals.

Explanation: This question focuses on the classification and ecological distribution of the genus *Streptococcus*. ### **Explanation of the Correct Answer** **Option A** is the correct answer because it is a **true** statement, making it the "incorrect" choice in the context of a "NOT true" question format. *Streptococcus* species are indeed a major group of **lactic acid-producing bacteria**. They are Gram-positive, aerotolerant anaerobes that perform fermentation, converting sugars into lactic acid. This process is clinically significant in the pathogenesis of dental caries (especially by *S. mutans*). *Note: In some versions of this specific MCQ, the intended "NOT true" answer depends on the specific species being discussed (e.g., S. pyogenes vs. S. mutans). However, based on general microbiology:* ### **Analysis of Other Options** * **Option B (Adherence to dental enamel):** This is **true**. Viridans group streptococci (like *S. mutans* and *S. sanguinis*) produce extracellular polysaccharides (glucans) from sucrose, allowing them to adhere strongly to the dental pellicle. * **Option C & D (Commonality in humans vs. animals):** These are **true** statements depending on the species. *Streptococcus* is a ubiquitous genus. While many species are primary human pathogens (*S. pyogenes, S. pneumoniae*), the genus is equally prevalent as commensals and pathogens in the animal kingdom (e.g., *S. agalactiae* in bovine mastitis or *S. equi* in horses). ### **NEET-PG High-Yield Pearls** * **Catalase Test:** All Streptococci are **Catalase negative**, which distinguishes them from Staphylococci. * **M Protein:** The chief virulence factor of *S. pyogenes* (Group A Strep); it is anti-phagocytic and shares antigenicity with human myocardial tissue (molecular mimicry leading to Rheumatic Fever). * **Quellung Reaction:** Positive in *S. pneumoniae* due to its polysaccharide capsule. * **Optochin Sensitivity:** Used to differentiate *S. pneumoniae* (Sensitive) from Viridans streptococci (Resistant).

Question 1002: Which organism is responsible for drug-induced pseudomembranous colitis?

A. Clostridium perfringens
B. Clostridium difficile (Correct Answer)
C. Clostridium tetani
D. Fusobacterium

Explanation: **Explanation:** **Clostridium difficile** (now reclassified as *Clostridioides difficile*) is the primary causative agent of antibiotic-associated pseudomembranous colitis. The underlying medical concept involves the disruption of normal colonic flora by broad-spectrum antibiotics (most classically **Clindamycin**, but also fluoroquinolones and cephalosporins). This allows *C. difficile* to overgrow and release two potent exotoxins: **Toxin A (Enterotoxin)**, which causes fluid secretion and inflammation, and **Toxin B (Cytotoxin)**, which causes mucosal damage and the formation of characteristic yellowish-white "pseudomembranes" composed of fibrin, mucus, and inflammatory cells. **Analysis of Incorrect Options:** * **Clostridium perfringens:** Primarily responsible for gas gangrene (myonecrosis) and food poisoning; it does not cause pseudomembranous colitis. * **Clostridium tetani:** Produces tetanospasmin, leading to spastic paralysis (Tetanus) by inhibiting GABA/glycine release; it has no role in enteric disease. * **Fusobacterium:** A Gram-negative anaerobe associated with Vincent’s angina and Lemierre’s syndrome, but not drug-induced colitis. **High-Yield Clinical Pearls for NEET-PG:** * **Diagnosis:** The gold standard is the **Tissue Culture Cytotoxicity Assay**, but the most common rapid test is detecting toxins in stool via **ELISA** or **PCR** (GDH antigen is used for screening). * **Treatment:** The current first-line drug of choice is **Oral Vancomycin** or **Fidaxomicin**. Metronidazole is now reserved for mild cases if others are unavailable. * **Morphology:** It is a Gram-positive, spore-forming anaerobic rod. Spores are resistant to alcohol-based hand rubs; handwashing with soap and water is mandatory.

Question 1003: What is the primary cause of Group A hemolytic pharyngitis?

A. Local infection (Correct Answer)
B. Systemic toxicity
C. Invasion of mucosa
D. Local toxins

Explanation: **Explanation:** The primary cause of Group A Streptococcal (GAS) pharyngitis, commonly known as "strep throat," is a **local infection** of the pharyngeal mucosa. 1. **Why "Local Infection" is correct:** *Streptococcus pyogenes* (Group A Strep) colonizes the throat and attaches to the pharyngeal epithelium using adhesins like M-protein and lipoteichoic acid. The resulting inflammation, exudate, and sore throat are direct consequences of the bacteria multiplying locally and triggering a host immune response at the site of entry. 2. **Why other options are incorrect:** * **Systemic toxicity:** While GAS can produce toxins that enter the bloodstream (e.g., Pyrogenic exotoxins causing Scarlet Fever or Streptococcal Toxic Shock Syndrome), the initial pharyngitis itself is a localized inflammatory process, not a systemic toxic state. * **Invasion of mucosa:** Although GAS is capable of invasion, the hallmark of pharyngitis is superficial colonization and inflammation. Deep invasion usually leads to complications like peritonsillar abscess (Quinsy) or cellulitis, rather than simple pharyngitis. * **Local toxins:** While GAS produces local enzymes (Streptolysins O and S), these facilitate the infection rather than being the primary "cause" of the disease entity. The disease is defined by the presence and multiplication of the bacteria (infection) rather than a pure toxemia. **High-Yield Clinical Pearls for NEET-PG:** * **Centor Criteria:** Used to clinically diagnose GAS pharyngitis (Fever, Tonsillar exudates, Tender cervical lymphadenopathy, and Absence of cough). * **Post-Streptococcal Sequelae:** Important complications include **Acute Rheumatic Fever** (follows only pharyngeal infection) and **Acute Glomerulonephritis** (can follow either pharyngeal or skin infections). * **Drug of Choice:** Penicillin remains the treatment of choice for GAS pharyngitis to prevent Rheumatic Fever.

Question 1004: What is true about mycobacteria other than tuberculosis?

A. Causes disseminated infection
B. Occurs in persons with normal immunity
C. Causes decreased efficacy of BCG due to cross-immunity (Correct Answer)
D. Person-to-person transmission is seen

Explanation: **Explanation:** **Mycobacteria Other Than Tuberculosis (MOTT)**, also known as Nontuberculous Mycobacteria (NTM), are environmental saprophytes found in soil and water. **Why Option C is Correct:** The most significant epidemiological impact of MOTT is their interference with the **BCG vaccine**. Exposure to environmental mycobacteria induces a degree of **cross-reactivity/cross-immunity** to antigens shared with *M. tuberculosis*. This "masking" or "blocking" effect is a primary reason why BCG efficacy varies geographically, showing significantly lower protection in tropical regions where MOTT exposure is high. **Analysis of Incorrect Options:** * **A & B:** MOTT are generally **opportunistic pathogens**. While they can cause localized disease (like Buruli ulcer or swimming pool granuloma) in healthy individuals, **disseminated infection** is almost exclusively seen in severely immunocompromised patients (e.g., advanced HIV/AIDS with CD4 <50 cells/µL). * **D:** Unlike *M. tuberculosis*, MOTT are acquired from environmental sources (inhalation or ingestion). There is **no documented person-to-person transmission** (with the rare exception of specific *M. abscessus* strains in Cystic Fibrosis patients). **High-Yield Clinical Pearls for NEET-PG:** * **Runyon Classification:** Categorizes MOTT based on growth rate and pigment production (Photochromogens, Scotochromogens, Non-photochromogens, and Rapid growers). * **M. avium-intracellulare (MAC):** The most common MOTT causing systemic disease in HIV patients; requires prophylaxis when CD4 counts are very low. * **M. marinum:** Causes "Fish tank granuloma." * **M. kansasii:** Most closely resembles TB clinically and radiologically. * **Diagnosis:** MOTT are typically resistant to standard anti-tubercular drugs (RNTCP regimens).

Question 1005: Which of the following microorganisms grows in cell culture media?

A. Chlamydia
B. Ureaplasma urealyticum
C. Pseudomonas (Correct Answer)
D. Tropheryma whippelii

Explanation: **Explanation:** The core concept tested here is the distinction between **obligate intracellular** pathogens and **extracellular** bacteria regarding their growth requirements. **Why Pseudomonas is Correct:** *Pseudomonas aeruginosa* is a non-fastidious, aerobic, extracellular Gram-negative bacillus. It grows readily on standard laboratory media (like Blood Agar or MacConkey Agar) and can also grow in **cell culture media** (the nutrient-rich fluids used to maintain mammalian cells, such as DMEM or RPMI). In research and clinical virology, *Pseudomonas* is a common contaminant of cell cultures because it thrives on the amino acids and glucose present in these media. **Analysis of Incorrect Options:** * **Chlamydia (Option A):** These are **obligate intracellular** bacteria. They cannot grow in cell culture *media* (the fluid) alone; they require living host cells (cell lines like McCoy or HeLa) to replicate. * **Ureaplasma urealyticum (Option B):** These are the smallest free-living organisms. While they lack a cell wall, they are extracellular and are typically grown on specialized enriched media (like PPLO agar) containing sterols, rather than standard cell culture media. * **Tropheryma whippelii (Option D):** The causative agent of Whipple’s disease is extremely fastidious. While it was first isolated using specialized cell culture techniques, it does not grow in standard cell culture media and is generally considered "unculturable" in routine diagnostic labs. **NEET-PG High-Yield Pearls:** * **Pseudomonas** is famous for producing **Pyocyanin** (blue-green pigment) and having a characteristic **fruity/grape-like odor**. * **Obligate Intracellular Organisms:** (Mnemonic: **R**eally **C**old) **R**ickettsia and **C**hlamydia. These require living cells (egg yolk sac or cell lines), not just media. * **Cell Culture Contaminants:** *Pseudomonas* and *Mycoplasma* are the most frequent culprits that ruin laboratory cell cultures.

Question 1006: Which of the following is NOT true regarding Chlamydia psittaci?

A. It is endemic in birds.
B. It causes non-gonococcal urethritis. (Correct Answer)
C. It can be grown in a specified laboratory medium.
D. Tetracycline is the treatment of choice.

Explanation: ### Explanation **Why Option B is the Correct Answer:** *Chlamydia psittaci* is primarily a respiratory pathogen and does **not** cause non-gonococcal urethritis (NGU). NGU is caused by **_Chlamydia trachomatis_** (Serotypes D-K), *Ureaplasma urealyticum*, and *Mycoplasma genitalium*. *C. psittaci* causes Psittacosis (Ornithosis), a zoonotic atypical pneumonia characterized by fever, headache, and a dry cough. **Analysis of Incorrect Options:** * **Option A:** *C. psittaci* is indeed endemic in birds (parrots, pigeons, poultry). Humans get infected by inhaling dried bird excreta or dust from feathers. * **Option C:** Like all Chlamydiae, *C. psittaci* is an **obligate intracellular parasite**. It cannot grow on artificial agar but can be grown in "specified laboratory media" such as **cell cultures** (e.g., McCoy or HeLa cells), yolk sacs of embryonated eggs, or experimental animals. * **Option D:** **Tetracyclines (Doxycycline)** are the first-line treatment for all Chlamydial infections, including Psittacosis. Macrolides (Azithromycin) are an alternative. **High-Yield Clinical Pearls for NEET-PG:** * **Horder’s Spots:** A rare clinical sign of Psittacosis consisting of rose-colored spots on the face. * **Splenomegaly:** Unlike most atypical pneumonias, Psittacosis is frequently associated with splenomegaly. * **Diagnosis:** Serology (Complement Fixation Test or Microimmunofluorescence) is the gold standard. * **Inclusion Bodies:** *C. psittaci* produces diffuse, irregular inclusion bodies (LCL bodies) that **do not contain glycogen** (unlike *C. trachomatis*) and are therefore **iodine-negative**.

Question 1007: Which of the following are synergohymenotropic toxins of Staphylococcus?

A. Alpha toxin
B. Gamma toxin (Correct Answer)
C. Alpha toxin and Gamma toxin
D. Alpha toxin and Delta toxin

Explanation: ### Explanation **Concept Overview** **Synergohymenotropic toxins** are a unique class of bicomponent exotoxins produced by *Staphylococcus aureus*. The term refers to their mechanism: two distinct protein components (S and F) act **synergistically** to form pores in the **hymen** (membrane) of host cells, particularly leukocytes and erythrocytes. **Why Option B is Correct** * **Gamma toxin ($\gamma$-hemolysin):** This is a classic bicomponent toxin consisting of two polypeptide chains (HlgA/HlgC and HlgB). It is produced by almost all strains of *S. aureus*. * **Panton-Valentine Leukocidin (PVL):** Another critical synergohymenotropic toxin, often associated with community-acquired MRSA (CA-MRSA) and necrotizing pneumonia. **Why Other Options are Incorrect** * **Alpha toxin ($\alpha$-hemolysin):** While it is a potent pore-forming toxin and the most important virulence factor for skin infections, it is a **monocomponent** toxin (secreted as a monomer that heptamerizes). It does not require the synergistic action of two different protein subunits. * **Delta toxin ($\delta$-hemolysin):** This is a small, surfactant-like peptide (amphipathic) that acts like a detergent to disrupt host membranes. It is not a bicomponent synergohymenotropic toxin. **High-Yield NEET-PG Pearls** * **Bicomponent Structure:** Synergohymenotropic toxins require an **S-component** (Slow-eluting) and an **F-component** (Fast-eluting). * **PVL Association:** PVL is specifically leukocidal (kills WBCs) but lacks hemolytic activity, whereas Gamma toxin is both leukocidal and hemolytic. * **Clinical Link:** High levels of PVL are a marker for **CA-MRSA** and are linked to severe, recurrent furuncles and necrotic skin lesions. * **Exfoliative Toxins:** Do not confuse these with membrane-disrupting toxins; ET-A and ET-B are proteases responsible for Staphylococcal Scalded Skin Syndrome (SSSS).

Question 1008: Which of the following tests is most useful to differentiate Micrococci from Staphylococci?

A. Catalase test
B. Hugh Leifson test (Correct Answer)
C. Oxidase test
D. All of the above

Explanation: **Explanation:** The differentiation between *Micrococcus* and *Staphylococcus* is a common high-yield topic in bacteriology, as both are Gram-positive, catalase-positive cocci. **1. Why Hugh Leifson (O/F) Test is correct:** The **Hugh Leifson Oxidation-Fermentation (O/F) test** is the gold standard for differentiating these genera based on their glucose metabolism. * **Staphylococci** are **facultative anaerobes**; they can utilize glucose both oxidatively and fermentatively (O+/F+). * **Micrococci** are **obligate aerobes**; they utilize glucose only oxidatively or are non-saccharolytic (O+/F-). **2. Why other options are incorrect:** * **Catalase test:** Both *Micrococcus* and *Staphylococcus* are **Catalase positive**. This test is used to differentiate them from *Streptococci* (Catalase negative), not from each other. * **Oxidase test:** While most Staphylococci are oxidase negative and many Micrococci are oxidase positive (Modified Oxidase/Microdase test), the Hugh Leifson test remains the definitive biochemical differentiator in standard diagnostic algorithms. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Bacitracin Sensitivity:** *Micrococcus* is sensitive (0.04 U), while *Staphylococcus* is resistant. * **Furazolidone Sensitivity:** *Staphylococcus* is sensitive, while *Micrococcus* is resistant. * **Lysostaphin:** *Staphylococci* are susceptible to lysis by lysostaphin, whereas *Micrococci* are resistant. * **Arrangement:** *Micrococci* often appear in tetrads or sarcina (packets of 8), while *Staphylococci* typically appear in irregular grape-like clusters.

Question 1009: Primary atypical pneumonia is caused by which organism?

A. Legionella
B. Streptococcus
C. Mycoplasma (Correct Answer)
D. Listeria

Explanation: ### Explanation **Correct Answer: C. Mycoplasma** **Primary Atypical Pneumonia** is most commonly caused by ***Mycoplasma pneumoniae***. The term "atypical" refers to the clinical presentation where the systemic symptoms (headache, low-grade fever, malaise) and radiological findings (patchy infiltrates) are more severe than the physical signs (minimal cough, absence of consolidation). * **Mechanism:** *Mycoplasma* lacks a cell wall (making it resistant to Beta-lactams) and attaches to the respiratory epithelium via the **P1 adhesion protein**, causing "walking pneumonia." --- ### Why the other options are incorrect: * **A. Legionella:** While *Legionella pneumophila* causes atypical pneumonia (Legionnaires' disease), it is typically associated with contaminated water systems, gastrointestinal symptoms (diarrhea), and hyponatremia. It is not the "primary" or most common cause. * **B. Streptococcus:** *Streptococcus pneumoniae* is the most common cause of **Typical (Lobar) Pneumonia**, characterized by high fever, productive cough (rusty sputum), and clear signs of consolidation on X-ray. * **C. Listeria:** *Listeria monocytogenes* primarily causes neonatal sepsis, meningitis in immunocompromised adults, and food poisoning; it is not a standard cause of pneumonia. --- ### NEET-PG High-Yield Pearls: 1. **Cold Agglutinins:** *Mycoplasma* infection is associated with the development of **Cold Agglutinin antibodies (IgM)** directed against I-antigens on RBCs, which can lead to autoimmune hemolytic anemia. 2. **Diagnosis:** The "Gold Standard" is PCR. On culture (Eaton’s agar), it produces characteristic **"Fried Egg" colonies**. 3. **Treatment:** Since it lacks a cell wall, it is inherently resistant to Penicillins. **Macrolides** (Azithromycin) or Tetracyclines (Doxycycline) are the drugs of choice. 4. **Radiology:** Characterized by "dissociation"—the X-ray looks much worse than the patient’s clinical appearance.

Question 1010: Which disease is caused by Rickettsia prowazekii?

A. Epidemic typhus (Correct Answer)
B. Endemic typhus
C. Scrub typhus
D. Trench fever

Explanation: **Explanation:** **Rickettsia prowazekii** is the causative agent of **Epidemic typhus**. It is an obligate intracellular bacterium transmitted to humans primarily by the **human body louse** (*Pediculus humanus corporis*). The transmission occurs when louse feces containing the bacteria are rubbed into bite wounds or mucous membranes. Historically, this disease is associated with overcrowded conditions, war, and famine. A unique feature of *R. prowazekii* is its ability to remain latent in the lymphoid tissues, leading to a recrudescent form of the disease years later known as **Brill-Zinsser disease**. **Analysis of Incorrect Options:** * **Endemic (Murine) typhus:** Caused by ***Rickettsia typhi*** and transmitted by the rat flea (*Xenopsylla cheopis*). It is generally milder than the epidemic form. * **Scrub typhus:** Caused by ***Orientia tsutsugamushi***. It is transmitted by the bite of larval mites (**chiggers**) and is characterized by a necrotic skin lesion called an **eschar**. * **Trench fever:** Caused by ***Bartonella quintana***. Like epidemic typhus, it is transmitted by the human body louse, but the pathogen is not a Rickettsia species. **High-Yield Clinical Pearls for NEET-PG:** * **Vector:** Body louse (Epidemic typhus) vs. Rat flea (Endemic typhus). * **Weil-Felix Test:** A heterophile agglutination test using *Proteus* antigens. Epidemic typhus shows a positive reaction with **OX-19**. * **Drug of Choice:** **Doxycycline** is the gold standard treatment for all rickettsial infections. * **Rash:** In typhus, the rash typically starts on the trunk and spreads peripherally to the extremities (sparing the palms and soles), which is the opposite of Rocky Mountain Spotted Fever.

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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