Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 2341: Which organism is partially acid-fast?

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

Explanation: ***Nocardia asteroides*** - *Nocardia* species are distinguished by their **Gram-positive, beaded, branching filamentous** morphology and their unique cell wall structure. - They are considered **partially acid-fast** because their cell walls contain **mycolic acid**, which retains carbolfuchsin stain but is decolorized by weak acid alcohol, unlike the stronger acid-fastness of Mycobacteria. *Mycobacterium bovis* - *Mycobacterium bovis* is a **fully acid-fast** organism. - Its cell wall contains a high concentration of **mycolic acid**, making it resistant to decolorization by strong acid-alcohol solutions. *Actinomyces israelii* - *Actinomyces* species are **Gram-positive, branching filamentous bacteria** similar in morphology to Nocardia. - However, they are **NOT acid-fast** at all, as their cell walls lack mycolic acid. - This is a key distinguishing feature from Nocardia. *Mycobacterium tuberculosis* - This is a classic example of a **fully acid-fast** bacterium. - The presence of a thick, waxy layer of **mycolic acid** in its cell wall prevents decolorization even with strong acid-alcohol.

Question 2342: The Griffith classification is based on which of the following?

A. M, T, R antigens
B. Types of hemolysis
C. Oxygen requirement
D. Capsular polysaccharides (Correct Answer)

Explanation: ***Capsular polysaccharides*** - The **Griffith classification** system categorizes *Streptococcus pneumoniae* strains based on the biochemical and antigenic differences in their **polysaccharide capsules**. - These capsules are crucial **virulence factors** that protect bacteria from phagocytosis and are targeted by vaccines. *M, T, R antigens* - These antigens are associated with the **Lancefield grouping system** for beta-hemolytic streptococci, primarily *Streptococcus pyogenes*, and distinguish different serotypes based on cell wall carbohydrate antigens. - They are not the basis for the Griffith classification, which focuses specifically on *Streptococcus pneumoniae*. *Types of hemolysis* - Hemolysis patterns (alpha, beta, gamma) are used as a general preliminary classification for many streptococcal species cultured on **blood agar**, indicating their ability to lyse red blood cells. - While *Streptococcus pneumoniae* typically exhibits **alpha-hemolysis**, this characteristic is not specific enough to differentiate between the numerous serotypes defined by the Griffith system. *Oxygen requirement* - Oxygen requirement defines a microorganism's growth conditions (e.g., aerobic, anaerobic, facultative anaerobic) and is a fundamental characteristic for bacterial identification but is **not used for serotyping**. - *Streptococcus pneumoniae* is a **facultative anaerobe**, but this physiological trait does not differentiate its serotypes.

Question 2343: What is a characteristic feature of Chlamydia?

A. Has a unique cell wall structure
B. Requires host cells for growth
C. Contains inclusion bodies (Correct Answer)
D. None of the options

Explanation: ***Contains inclusion bodies*** - *Chlamydia* species form characteristic **intracytoplasmic inclusion bodies** within infected host cells during their replication cycle. - These inclusion bodies contain both **elementary bodies (EBs)** - the infectious, metabolically inactive form - and **reticulate bodies (RBs)** - the non-infectious, metabolically active replicating form. - The presence of these inclusion bodies is a classic diagnostic feature visible on cytological examination (e.g., Giemsa staining) and is highly characteristic of *Chlamydia* infections. - *Chlamydia* exhibits a unique **biphasic developmental cycle** alternating between EB and RB forms within these inclusions. *Has a unique cell wall structure* - While *Chlamydia* does lack **peptidoglycan** in its cell wall (making beta-lactam antibiotics ineffective), this is a distinguishing feature but the question asks for a characteristic feature, not necessarily unique to *Chlamydia* alone. - The cell wall contains lipopolysaccharide (LPS) and uses disulfide bond cross-linking instead of peptidoglycan. *Requires host cells for growth* - *Chlamydia* species are **obligate intracellular bacteria** that must replicate inside host cells. - However, this characteristic is shared by other bacterial pathogens such as **Rickettsia**, **Coxiella burnetii**, and **Ehrlichia**, making it not specific to *Chlamydia*. - Their dependence on host ATP ("energy parasites") is due to their inability to synthesize their own ATP. *None of the options* - The formation of inclusion bodies is indeed a characteristic feature of *Chlamydia*, making this option incorrect.

Question 2344: Which organism uses the Iron-regulated surface determinant (Isd) system for acquiring iron from hemoglobin as a key virulence mechanism?

A. Staphylococcus aureus (Correct Answer)
B. Streptococcus pneumoniae
C. Haemophilus influenzae
D. Escherichia coli

Explanation: ***Staphylococcus aureus*** - *Staphylococcus aureus* possesses a unique **iron-regulated surface determinant (Isd) system** that allows it to extract iron directly from **hemoglobin and heme-containing proteins**. - The Isd system includes cell wall-anchored proteins (IsdA, IsdB, IsdC) that capture hemoglobin, extract heme, and transport iron across the cell wall and membrane. - This specialized iron acquisition system is a **critical virulence factor** enabling *S. aureus* to thrive in iron-limited host environments, promoting bacterial growth, biofilm formation, and resistance to oxidative stress. - The Isd system is particularly important during deep tissue infections where hemoglobin from damaged red blood cells is the primary iron source. *Streptococcus pneumoniae* - *S. pneumoniae* uses different iron acquisition mechanisms, primarily **ABC transporters** (PiaA, PiuA) for ferric iron uptake from transferrin and lactoferrin. - Does not possess the specific Isd system characteristic of *S. aureus*. - Its virulence relies more on **capsular polysaccharides** and **pneumolysin** rather than specialized hemoglobin-based iron acquisition. *Haemophilus influenzae* - Acquires iron through **transferrin and lactoferrin binding proteins** but lacks the Isd system. - Cannot utilize hemoglobin directly without the Isd system machinery. - Virulence factors include **capsule** (type b strains) and **IgA protease**, with iron obtained from iron-binding glycoproteins rather than hemoglobin. *Escherichia coli* - Uses **siderophores** (enterobactin, aerobactin) to chelate ferric iron from the environment. - Does not possess the Isd system and cannot efficiently extract iron from hemoglobin. - Pathogenic strains have diverse virulence mechanisms (adhesins, toxins) with iron acquisition via siderophore-mediated pathways, not hemoglobin extraction.

Question 2345: What is the causative agent of erythrasma?

A. Staphylococcus
B. Streptococcus
C. Herpes Virus
D. Corynebacterium (Correct Answer)

Explanation: ***Corynebacterium*** - **Erythrasma** is a superficial skin infection caused by a specific species of bacteria, **Corynebacterium minutissimum**. - This bacterium produces **porphyrins** that fluoresce a distinctive coral-red color under a Wood's lamp, aiding in diagnosis. *Staphylococcus* - **Staphylococcus aureus** is a common cause of various skin infections such as **impetigo**, **folliculitis**, and **boils**, but not erythrasma. - These infections typically present with **pus-filled lesions** or crusting. *Streptococcus* - **Streptococcus pyogenes** is known to cause **erysipelas** and **cellulitis**, which are characterized by rapidly spreading red, warm, and tender skin. - It is also a causative agent of **impetigo**, but not erythrasma. *Herpes Virus* - Herpes viruses, such as **Herpes Simplex Virus (HSV)**, cause **cold sores** and **genital herpes**, manifesting as painful blisters. - They are **viruses**, not bacteria, and do not cause erythrasma.

Question 2346: Which of the following Staphylococcal infections is not primarily toxin-mediated?

A. Toxic shock syndrome
B. Scalded skin syndrome
C. Food poisoning
D. Septic shock (Correct Answer)

Explanation: ***Septic shock*** - Septic shock is primarily mediated by the **host's inflammatory response** to bacteria (or their components like **LPS** in Gram-negative bacteria, or **peptidoglycan** and **teichoic acid** in Gram-positive bacteria, like *Staphylococcus*), rather than by a specific bacterial toxin. - The systemic inflammatory response can lead to widespread **vasodilation**, capillary leak, and **organ dysfunction**, commonly seen in severe infections like **bacteremia** with *Staphylococcus aureus*. *Toxic shock syndrome* - This condition is caused by **superantigen toxins** like **TSST-1** (Toxic Shock Syndrome Toxin-1) produced by certain strains of *Staphylococcus aureus*, leading to massive **T cell activation** and cytokine release. - It presents with sudden onset of **fever**, **hypotension**, diffuse erythematous rash, and **multisystem organ involvement**, distinguishing it from septic shock primarily driven by host immune response. *Scalded skin syndrome* - This syndrome is caused by **exfoliative toxins (ETA and ETB)** produced by *Staphylococcus aureus*, which target **desmoglein 1** in the skin, leading to widespread **epidermal exfoliation** and Nikolsky's sign. - The toxins cause the distinct blistering and peeling of the skin, making it a classic example of a **toxin-mediated dermopathy**. *Food poisoning* - *Staphylococcal* food poisoning is mediated by **heat-stable enterotoxins (e.g., SEA, SEB)** produced by *Staphylococcus aureus* growing in contaminated food. - Symptoms like **nausea**, **vomiting**, and **diarrhea** occur rapidly after ingestion, as preformed toxins directly stimulate the **vagal nerves** in the gastrointestinal tract.

Question 2347: Pseudo-hemoptysis is seen mostly with

A. Escherichia coli
B. Respiratory Syncytial Virus (RSV)
C. Streptococcus pneumoniae
D. Serratia marcescens (Correct Answer)

Explanation: ***Serratia marcescens*** - This bacterium is well-known for producing a distinctive **red pigment**, **prodigiosin**, which can cause sputum to appear blood-tinged. - This pigment can lead to a false impression of **hemoptysis (pseudo-hemoptysis)**, particularly in patients with respiratory infections caused by this organism. *Streptococcus pneumoniae* - While a common cause of pneumonia, it typically causes **rust-colored sputum** due to the presence of blood and inflammatory cells, but not a vivid red pigment that mimics fresh blood. - This organism is more commonly associated with **frank hemoptysis** or purulent sputum, rather than pseudo-hemoptysis caused by pigment. *Escherichia coli* - Primarily a **gastrointestinal pathogen**, *E. coli* rarely causes respiratory infections, and when it does, it does not produce a red pigment in sputum. - It is not known to cause pseudo-hemoptysis; its presence in sputum would indicate a severe systemic infection or aspiration. *Respiratory Syncytial Virus (RSV)* - RSV is a common cause of **viral respiratory infections**, especially in children, and is associated with clear or whitish nasal secretions and cough. - It does not cause the production of pigmented sputum, nor does it typically lead to hemoptysis or pseudo-hemoptysis.

Question 2348: Which of the following organisms is known to cause atrophic rhinitis?

A. Streptococcus foetidus
B. Streptococcus pneumoniae
C. Klebsiella ozaenae (Correct Answer)
D. Klebsiella pneumoniae

Explanation: ***Klebsiella ozaenae*** - *Klebsiella ozaenae* is the organism most commonly associated with **primary atrophic rhinitis**, also known as **ozena**. - It produces characteristic **foul-smelling crusts** in the nasal cavity due to its metabolic byproducts. *Streptococcus pneumoniae* - *Streptococcus pneumoniae* is a common cause of **bacterial sinusitis** and **otitis media**, but not atrophic rhinitis. - It typically causes acute infections rather than chronic atrophic changes. *Streptococcus foetidus* - *Streptococcus foetidus* is a less common organism and is not a known primary cause of **atrophic rhinitis**. - It is more often associated with infections like **abscesses** or **postoperative complications**. *Klebsiella pneumoniae* - *Klebsiella pneumoniae* is a significant pathogen causing **pneumonia**, **urinary tract infections**, and **hospital-acquired infections**. - While it belongs to the same genus as *K. ozaenae*, it is not specifically implicated in **atrophic rhinitis**.

Question 2349: Which of the following statements about endotoxins is true?

A. Endotoxins are proteins.
B. Endotoxins are highly antigenic.
C. Endotoxins have no enzymatic activity. (Correct Answer)
D. Endotoxins are produced by Gram-positive bacteria.

Explanation: ***Endotoxins have no enzymatic activity.*** - Endotoxins are **lipopolysaccharides (LPS)** with potent biological effects, but they do not possess intrinsic enzymatic capabilities. - Their toxic effects are mediated by their ability to stimulate the host's immune system, not by catalytic reactions. *Endotoxins are proteins.* - Endotoxins are primarily composed of **lipopolysaccharide (LPS)**, which contains lipid and carbohydrate components, not protein. - The protein components associated with bacteria that contribute to disease are often **exotoxins**, which are distinct from endotoxins. *Endotoxins are highly antigenic.* - While endotoxins can elicit an immune response, particularly against the **O-antigen polysaccharide chain**, they are generally considered **poorly antigenic** compared to exotoxins or other complex protein antigens. - The **lipid A component**, responsible for most of the endotoxin's toxicity, is highly conserved among gram-negative bacteria and is often less immunogenic than the variable O-antigen. *Endotoxins are produced by Gram-positive bacteria.* - Endotoxins are characteristic components of the **outer membrane of Gram-negative bacteria**, not Gram-positive bacteria. - Gram-positive bacteria produce various **exotoxins** and have a thick **peptidoglycan layer**, but they lack LPS.

Question 2350: In which part of Mycoplasma are sterols found?

A. Cell wall of Rickettsia
B. Cell membrane of Rickettsia
C. Cell wall of Mycoplasma
D. Cell membrane of Mycoplasma (Correct Answer)

Explanation: ***Cell membrane of Mycoplasma*** - **Mycoplasma** species are unique among bacteria because they lack a **cell wall**. - Instead, their **cell membrane** contains **sterols**, which are acquired from the host, providing stability and strength to the membrane. *Cell wall of Rickettsia* - **Rickettsia** are obligate intracellular bacteria that possess a typical **gram-negative cell wall**, but it does not contain **sterols**. - **Sterols** are generally not found in bacterial cell walls, and most bacteria cannot synthesize them. *Cell membrane of Rickettsia* - **Rickettsia** have a typical bacterial **cell membrane** that lacks **sterols**. - **Sterols** are primarily components of eukaryotic cell membranes, with **Mycoplasma** being a notable exception among bacteria. *Cell wall of Mycoplasma* - **Mycoplasma** species are characterized by the complete **absence of a cell wall**, which makes them resistant to many common antibiotics that target cell wall synthesis. - Therefore, **sterols** cannot be found in their cell wall, as it does not exist.

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

Practice Questions

Pseudomonas and Related Bacteria

Practice Questions

Haemophilus and HACEK Group

Practice Questions

Bordetella and Brucella

Practice Questions

Mycobacteria

Practice Questions

Spirochetes

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