Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 2431: Which of the following conditions is caused by Staphylococcus aureus?

A. Corynebacterium minutissimum infection
B. Haemophilus ducreyi infection
C. Propionibacterium acnes infection
D. Bullous impetigo (Correct Answer)

Explanation: ***Bullous impetigo*** - Bullous impetigo is a superficial skin infection characterized by **blisters (bullae)**, and is specifically caused by **Staphylococcus aureus** producing exfoliative toxins. - The toxins produced by *S. aureus* cause intraepidermal cleavage, leading to the formation of the characteristic **flaccid bullae**. *Corynebacterium minutissimum infection* - *Corynebacterium minutissimum* causes **erythrasma**, a chronic superficial skin infection characterized by well-demarcated reddish-brown patches, often in intertriginous areas. - It does not cause bullous impetigo and is typically diagnosed by its coral-red fluorescence under a **Wood's lamp**. *Haemophilus ducreyi infection* - *Haemophilus ducreyi* is the causative agent of **chancroid**, a sexually transmitted infection characterized by painful genital ulcers with a necrotic base and often accompanied by swollen, tender regional lymph nodes. - It is not associated with skin blistering or bullous impetigo. *Propionibacterium acnes infection* - *Propionibacterium acnes* (now *Cutibacterium acnes*) is a bacterium commonly implicated in **acne vulgaris**, contributing to inflammation and comedone formation within hair follicles. - It causes inflammatory lesions like papules, pustules, nodules, and cysts, rather than bullous lesions.

Question 2432: Which of the following statements about Mycoplasma is correct?

A. Mycoplasma pneumoniae can cause lung infections. (Correct Answer)
B. Penicillin is effective against Mycoplasma.
C. Mycoplasma has a thick cell wall.
D. Mycoplasma can be easily cultured on standard blood agar.

Explanation: ***Mycoplasma pneumoniae can cause lung infections.*** - **_Mycoplasma pneumoniae_** is a well-known cause of **atypical pneumonia**, often referred to as "walking pneumonia." - It infects the respiratory tract, leading to symptoms like **cough**, **fever**, and **sore throat**. - It is one of the most common causes of community-acquired pneumonia, especially in young adults and children. *Penicillin is effective against Mycoplasma.* - **Penicillin** and other beta-lactam antibiotics target bacterial **cell wall synthesis**. - **_Mycoplasma_** species inherently **lack a cell wall**, rendering these antibiotics ineffective. - Treatment requires antibiotics that target other mechanisms like protein synthesis (macrolides, tetracyclines). *Mycoplasma has a thick cell wall.* - **_Mycoplasma_** species are unique among bacteria because they **completely lack a cell wall**, making them pleomorphic (variable in shape). - This absence of a cell wall is a key characteristic that distinguishes them from most other bacteria. - The lack of cell wall makes them resistant to beta-lactam antibiotics and allows them to pass through bacterial filters. *Mycoplasma can be easily cultured on standard blood agar.* - **_Mycoplasma_** species are extremely **fastidious organisms** that require **specialized culture media** such as PPLO (pleuropneumonia-like organism) agar enriched with serum and yeast extract. - They cannot grow on standard blood agar or routine bacterial culture media. - Colonies are very small ("fried egg" appearance) and may take **1-3 weeks** to grow, making culture challenging.

Question 2433: Which of the following organisms is least likely to cause Fournier's gangrene?

A. Staphylococcus aureus (Correct Answer)
B. Streptococcus pyogenes
C. Bacteroides fragilis
D. Clostridium perfringens

Explanation: ***Staphylococcus aureus*** - While *Staphylococcus aureus* can cause various soft tissue infections, it is **less commonly implicated** in the polymicrobial, synergistic infection characteristic of Fournier's gangrene compared to the other listed organisms. - Fournier's gangrene is typically caused by a **synergy of aerobic and anaerobic bacteria**, where *S. aureus* is primarily a notable aerobic pathogen but not a dominant one in the specific context of this necrotizing fasciitis. *Streptococcus pyogenes* - *Streptococcus pyogenes* (Group A Streptococcus) is a well-known cause of **necrotizing fasciitis**, including in the perineal region, often referred to as "flesh-eating bacteria." - It rapidly invades tissues and releases toxins, contributing significantly to the severe tissue destruction seen in Fournier's gangrene. *Bacteroides fragilis* - *Bacteroides fragilis* is a **strict anaerobe** and a common inhabitant of the gut, making it a frequent pathogen in infections originating from the perineum or gastrointestinal tract. - Its presence is highly characteristic of the **polymicrobial nature** of Fournier's gangrene, contributing to the anaerobic environment that promotes tissue necrosis. *Clostridium perfringens* - *Clostridium perfringens* is a **strict anaerobe** notorious for causing **gas gangrene (clostridial myonecrosis)**, a severe form of necrotizing soft tissue infection. - Its ability to produce gas and toxins contributes to rapid tissue destruction and is a significant component of the severe mixed infections seen in Fournier's gangrene.

Question 2434: Which of the following statements is false regarding yersiniosis?

A. Gram-negative bacillus
B. Caused by Yersinia pseudotuberculosis
C. Caused by Yersinia pestis (Correct Answer)
D. Caused by Yersinia enterocolitica

Explanation: ***Caused by Yersinia pestis*** - **Yersinia pestis** is the causative agent of **plague**, not typical yersiniosis affecting the gastrointestinal tract. - Yersiniosis generally refers to infections caused by other *Yersinia* species like *Y. enterocolitica* and *Y. pseudotuberculosis*. *Gram-negative bacillus* - *Yersinia* species, including *Y. enterocolitica* and *Y. pseudotuberculosis*, are indeed **Gram-negative bacilli**. - This characteristic is fundamental to their classification and bacterial structure. *Caused by Yersinia pseudotuberculosis* - **Yersinia pseudotuberculosis** is a recognized cause of **yersiniosis**, particularly affecting children. - It can cause symptoms such as **abdominal pain** and **mesenteric lymphadenitis**, mimicking appendicitis. *Caused by Yersinia enterocolitica* - **Yersinia enterocolitica** is the **most common cause of yersiniosis** in humans. - It is responsible for a range of symptoms including **gastroenteritis**, **acute enteritis**, and **mesenteric lymphadenitis**.

Question 2435: Which atypical mycobacteria are known for pigment production?

A. M. fortuitum, M. chelonae
B. M. xenopi, Mycobacterium avium complex (MAC)
C. M. gordonae, M. szulgai (Correct Answer)
D. M. ulcerans (non-pigment producing)

Explanation: **Correct: M. gordonae, M. szulgai** - **M. gordonae** is a classic **scotochromogen**, producing yellow-orange pigment in both light and dark conditions - **M. szulgai** is unique as it shows **dual chromogenicity**: photochromogen at 25°C and scotochromogen at 37°C - These are the classic examples of **pigment-producing atypical mycobacteria** used for classification purposes - Pigment production (Runyon classification) is a key characteristic differentiating atypical mycobacteria from *M. tuberculosis* *Incorrect: M. fortuitum, M. chelonae* - These are **rapid growers** (Runyon Group IV) and are typically **non-chromogens** - Not primarily known or highlighted for pigment production as a defining feature - Clinically important for causing **skin and soft tissue infections**, especially in post-traumatic or post-surgical settings *Incorrect: M. xenopi, Mycobacterium avium complex (MAC)* - **M. xenopi** is a **non-chromogen** (no pigment production) and is thermophilic - **MAC** (*M. avium* and *M. intracellulare*) are also **non-chromogens** - MAC is an important cause of disseminated disease in immunocompromised patients (especially AIDS) and pulmonary disease in patients with pre-existing lung disease *Incorrect: M. ulcerans (non-pigment producing)* - Correctly identified as a **non-chromogen** (no pigment production) - Causes **Buruli ulcer**, a severe necrotizing skin disease - Distinguished by production of **mycolactone toxin**, not pigment characteristics

Question 2436: Which bacterium is most commonly associated with severe corneal ulcers in contact lens wearers?

A. Pseudomonas aeruginosa (Correct Answer)
B. Moraxella
C. Corynebacterium
D. E. coli

Explanation: ***Pseudomonas aeruginosa*** - *Pseudomonas aeruginosa* is a common cause of severe eye infections, particularly **corneal ulcers**, especially in contact lens wearers. - It rapidly causes significant tissue damage due to its production of various **virulence factors**. *Moraxella* - *Moraxella catarrhalis* can cause **conjunctivitis** and occasionally other minor ocular infections, but is less commonly associated with severe vision-threatening infections than *Pseudomonas*. - It is also known for causing **otitis media** and respiratory tract infections. *Corynebacterium* - While *Corynebacterium diphtheriae* can cause a form of **conjunctivitis with pseudomembrane formation**, other *Corynebacterium* species are often part of the normal ocular flora. - Most *Corynebacterium* species associated with the eye are considered **commensals** or opportunistic pathogens in immunocompromised individuals. *E. coli* - *Escherichia coli* is generally a less common cause of primary eye infections but can be implicated in cases of **neonatal conjunctivitis** (ophthalmia neonatorum) or infections in immunocompromised individuals. - It is more commonly associated with **gastrointestinal** and **urinary tract infections**.

Question 2437: What is the correct order of application of reagents in the complete Gram staining procedure?

A. Gentian violet → Iodine → Alcohol/Acetone → Safranin (Correct Answer)
B. Iodine → Gentian violet → Alcohol/Acetone → Safranin
C. Safranin → Gentian violet → Iodine → Alcohol/Acetone
D. Gentian violet → Alcohol/Acetone → Iodine → Safranin

Explanation: **Gentian violet → Iodine → Alcohol/Acetone → Safranin** - **Gentian violet** (or crystal violet) is the **primary stain** that colors all cells purple. - **Iodine** acts as a **mordant**, forming a crystal violet-iodine complex within the cell walls. - **Alcohol/Acetone** is the **decolorizer**, washing out the primary stain from Gram-negative cells but not from Gram-positive cells. - Finally, **Safranin** is the **counterstain** that stains decolorized Gram-negative cells pink or red. *Iodine → Gentian violet → Alcohol/Acetone → Safranin* - **Iodine** is a mordant and needs a primary stain (like gentian violet) to bind to and form a complex; applying it first would not effectively stain the cells. - The correct sequence requires the primary stain to be applied before the mordant can fix it. *Safranin → Gentian violet → Iodine → Alcohol/Acetone* - **Safranin** is a counterstain and should be applied last to stain the decolorized Gram-negative cells, not as the first reagent. - Applying reagents out of order would lead to incorrect staining results, as **safranin** is meant to provide contrast after decolorization. *Gentian violet → Alcohol/Acetone → Iodine → Safranin* - **Alcohol/Acetone** (decolorizer) is applied too early in this sequence; it should be used after the mordant (iodine) has formed a complex with the primary stain. - Applying the decolorizer before the mordant would prevent the formation of the crystal violet-iodine complex, leading to incorrect differentiation between Gram-positive and Gram-negative bacteria.

Question 2438: Which disease is classified as 'multibacillary'?

A. Leprosy (Correct Answer)
B. Tuberculosis
C. Trachoma
D. Tetanus

Explanation: **Leprosy** - Leprosy, caused by *Mycobacterium leprae*, is classified by the **WHO** into **paucibacillary (PB)** and **multibacillary (MB)** forms based on bacterial load and number of skin lesions. - **Multibacillary leprosy** is defined as having more than 5 skin lesions with positive skin smears, requiring a longer treatment regimen (12 months of multidrug therapy with rifampicin, dapsone, and clofazimine). - This classification is specific to **leprosy** and is crucial for determining appropriate treatment duration and drug combinations. *Tuberculosis* - Tuberculosis is classified by **anatomical location** (pulmonary vs. extrapulmonary), **drug susceptibility** (drug-sensitive vs. drug-resistant), or **smear status** (smear-positive vs. smear-negative). - The term **'multibacillary'** is not used in TB classification, making this an incorrect option. *Trachoma* - Trachoma is a chronic **keratoconjunctivitis** caused by *Chlamydia trachomatis*, classified by **clinical stages** (TF, TI, TS, TT, CO) according to the WHO simplified grading system. - The paucibacillary/multibacillary classification does **not apply** to trachoma. *Tetanus* - Tetanus is a **toxin-mediated disease** caused by *Clostridium tetani*, manifesting as muscle spasms and lockjaw. - Classification is based on **clinical severity** (localized, generalized, cephalic, neonatal), not bacterial load, making the term 'multibacillary' inapplicable.

Question 2439: Which of the following statements about Chlamydia is false?

A. Reticulate body is metabolically active
B. Replicate by binary fission
C. Obligate intracellular organism
D. Gram positive (Correct Answer)

Explanation: ***Gram positive*** - Chlamydia are **Gram-negative** bacteria, despite their unique cell wall structure which lacks a peptidoglycan layer but contains an outer membrane with lipopolysaccharide (LPS) [1]. - The statement that Chlamydia are Gram positive is therefore **FALSE**, making this the correct answer. *Obligate intracellular organism* - This statement is TRUE. Chlamydia are indeed **obligate intracellular organisms**, meaning they can only replicate inside host cells [2]. - They rely on the host cell for ATP and other metabolic precursors, earning them the nickname "energy parasites" [2]. *Reticulate body is metabolically active* - This statement is TRUE. The **reticulate body (RB)** is the metabolically active and replicative form of Chlamydia that resides within the host cell [1]. - It undergoes binary fission to produce more RBs before differentiating back into elementary bodies [1]. *Replicate by binary fission* - This statement is TRUE. The **reticulate bodies** of Chlamydia replicate primarily through **binary fission** within the inclusion bodies inside the host cell cytoplasm [1]. - This process allows for the rapid amplification of the bacteria.

Question 2440: Necrotizing fasciitis is caused by -

A. Beta hemolytic streptococci (Correct Answer)
B. Pneumococcus
C. Staphylococcus aureus
D. Clostridium perfringens

Explanation: ***Beta hemolytic streptococci*** - **Group A Streptococcus (GAS)**, specifically *Streptococcus pyogenes*, is the most common cause of **Type II necrotizing fasciitis**. - Its virulence factors, like **exotoxins**, contribute to rapid tissue destruction and systemic toxicity. *Staphylococcus aureus* - While *S. aureus* can cause severe skin and soft tissue infections, it is more commonly associated with **cellulitis**, **abscesses**, and **septic arthritis**. - It can be a co-pathogen in **polymicrobial (Type I) necrotizing fasciitis** but is less frequent as a sole cause compared to GAS for Type II. *Pneumococcus* - *Streptococcus pneumoniae* (Pneumococcus) is primarily known for causing respiratory infections like **pneumonia**, **otitis media**, and **meningitis**. - It is not a typical causative agent of necrotizing fasciitis. *Clostridium perfringens* - This bacterium is the primary cause of **gas gangrene** (clostridial myonecrosis), a severe form of necrotizing soft tissue infection involving muscle tissue. - While also a flesh-eating bacterium, its clinical presentation and typical affected tissues differ from those of necrotizing fasciitis caused by streptococci.

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

Practice Questions

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