Bacteriology — MCQs

A 7-year-old boy presented to the OPD with complaints of pain and discharge from the left ear. On examination, the patient was febrile, with neck rigidity and a positive Kernig's sign. Gram-positive cocci were visualized in the Gram-stained specimen of the discharge. What is the most likely pathogen?

Q2499

A farmer presents with pustules that show Gram-positive cocci on smear. The culture shows beta hemolysis, and the organisms are catalase-negative. To confirm that the identified organism is group A streptococcus, which of the following tests should be performed?

Q2500

All the following statements regarding cervicofacial actinomycosis are true, except:

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 2491: Which organism is associated with the Ehrlich phenomenon, particularly in relation to diphtheria toxin?

A. Proteus
B. Mycobacterium TB
C. Corynebacterium (Correct Answer)
D. Staphylococcus

Explanation: ***Corynebacterium*** - The Ehrlich phenomenon is historically linked to the standardization of **diphtheria antitoxin**, which targets the toxin produced by *Corynebacterium diphtheriae*. - This bacterium is the causative agent of **diphtheria**, a severe throat infection characterized by a pseudomembrane and systemic complications due to its potent exotoxin. *Proteus* - **Proteus** species are commonly associated with **urinary tract infections** and are known for their motility and swarming growth. - They are not known to produce diphtheria toxin or be involved in the Ehrlich phenomenon. *Mycobacterium TB* - *Mycobacterium tuberculosis* causes **tuberculosis**, a chronic infectious disease primarily affecting the lungs. - This bacterium is acid-fast and has a waxy cell wall, but it is not relevant to diphtheria toxin or the Ehrlich phenomenon. *Staphylococcus* - **Staphylococcus** species, such as *Staphylococcus aureus*, cause a wide range of infections, including **skin infections**, food poisoning, and toxic shock syndrome. - While they produce various toxins, none are diphtheria toxin, and they are unrelated to the Ehrlich phenomenon.

Question 2492: Atrophic dry nasal mucosa, extensive encrustations, and a woody hard external nose are suggestive of which of the following conditions?

A. Bartonella henselae
B. Klebsiella pneumoniae (Correct Answer)
C. Bordetella pertussis
D. Bacillus anthracis

Explanation: ***Klebsiella pneumoniae*** - This clinical presentation is classic for **rhinoscleroma** (scleroma), a chronic granulomatous infection caused by *Klebsiella pneumoniae* subspecies *rhinoscleromatis*. - Rhinoscleroma presents with a characteristic triad: **atrophic dry nasal mucosa**, **extensive encrustations**, and a **woody hard external nose** due to progressive fibrosis in the sclerotic stage. - The disease progresses through three stages: catarrhal (atrophic with crusts), granulomatous (nodular lesions), and sclerotic (fibrosis causing the woody hardness). - Histologically, it shows **Mikulicz cells** (foamy macrophages containing bacteria) and **Russell bodies**, which are pathognomonic features. *Bacillus anthracis* - Causes **cutaneous anthrax**, which presents with a characteristic **black eschar** (painless necrotic ulcer with surrounding edema), not chronic atrophic changes. - Anthrax causes acute tissue necrosis and edema, not the chronic fibrotic "woody hard" nose seen in rhinoscleroma. - Does not produce extensive encrustations or chronic atrophic nasal mucosa. *Bordetella pertussis* - Causes **whooping cough**, a respiratory tract infection characterized by paroxysmal coughing with an inspiratory "whoop." - Affects the **tracheobronchial tree** causing inflammation and excessive mucus production, not localized nasal pathology. - Does not cause nasal atrophy, encrustations, or external nasal deformities. *Bartonella henselae* - Primarily causes **Cat-Scratch Disease**, presenting with regional lymphadenopathy following cat scratches or bites. - May cause skin papules at inoculation sites and systemic manifestations in immunocompromised patients. - Does not cause chronic nasal pathology, atrophic mucosa, or woody hardness of the nose.

Question 2493: Which organism, when grown on agar, is likely to show green-colored colonies?

A. Staphylococcus aureus
B. Streptococcus pneumoniae
C. Pseudomonas aeruginosa (Correct Answer)
D. Escherichia coli

Explanation: ***Pseudomonas aeruginosa*** - This bacterium produces **pyoverdine** and **pyocyanin**, pigments that give colonies a characteristic **metallic sheen** and a **greenish-blue color** on agar. - The green pigmentation is a key identifying feature on selective and differential media like **cetrimide agar**. *Staphylococcus aureus* - Typically forms colonies that are **cream-colored to golden-yellow** due to the production of carotenoid pigments. - It is known for its **beta-hemolysis** on blood agar but does not produce green colonies. *Streptococcus pneumoniae* - Characteristically forms **small, gray, mucoid colonies** with an area of **alpha-hemolysis** (partial hemolysis) on blood agar, giving a greenish discoloration around the colonies due to methemoglobin formation. - The colonies themselves are not primarily green, but the surrounding agar may appear green. *Escherichia coli* - Generally forms **large, gray, flat colonies** that are typically **non-hemolytic (gamma-hemolysis)** on blood agar. - On **MacConkey agar**, *E. coli* produces **pink colonies** due to lactose fermentation and acid production.

Question 2494: Which of the following statements about Clostridium tertium is true?

A. It is Gram variable and has terminal spores. (Correct Answer)
B. It has terminal spores and produces exotoxins.
C. It causes septic arthritis.
D. It produces exotoxins.

Explanation: ***It is Gram variable and has terminal spores.*** * *Clostridium tertium* is unique among clostridial species for often staining **Gram-variable or Gram-negative**, which can make initial identification challenging. * It forms characteristic **terminal spores**, a feature common among many **Clostridium** species, which contributes to its survival in harsh conditions. *It has terminal spores and produces exotoxins.* * While **Clostridium tertium** does form **terminal spores**, and many *Clostridium* species are known for producing potent **exotoxins**, *C. tertium* itself is generally **not recognized for significant exotoxin production** that causes typical clostridial diseases. * Its pathogenicity is more often associated with **invasive infection** rather than toxin-mediated effects. *It causes septic arthritis.* * While **Clostridium tertium** can cause various infections, including **bacteremia** and wound infections, **septic arthritis** is not a common or characteristic manifestation. * **Septic arthritis** is more frequently caused by bacteria like *Staphylococcus aureus* or *Streptococcus* species. *It produces exotoxins.* * Unlike species such as *Clostridium botulinum* or *Clostridium tetani*, **Clostridium tertium** is **not widely known for producing clinically significant exotoxins** that drive its pathogenic mechanisms. * Its role in infection is more often due to its ability to cause **invasive infections** in immunocompromised hosts.

Question 2495: Kanagawa phenomenon on Wagatsuma agar is shown by which of the following organisms?

A. V. Parahemolyticus (Correct Answer)
B. Vibrio cholerae
C. Vibrio vulnificus
D. Vibrio alginolyticus

Explanation: ***V. Parahemolyticus*** - The **Kanagawa phenomenon** refers to the production of a particular thermostable direct hemolysin (TDH) by *Vibrio parahaemolyticus*, which causes beta-hemolysis on a specialized agar called **Wagatsuma agar** (high salt mannitol medium containing sheep red blood cells). - This hemolytic activity detected on Wagatsuma agar is strongly correlated with the **virulence** of *V. parahaemolyticus* strains, indicating their potential to cause gastroenteritis. *Vibrio cholerae* - *Vibrio cholerae* is primarily recognized for causing **cholera** through the production of cholera toxin. - While it can be grown on various selective media, it does not typically exhibit the **Kanagawa phenomenon** on Wagatsuma agar. *Vibrio vulnificus* - *Vibrio vulnificus* is associated with severe wound infections and septicemia, particularly in individuals with pre-existing liver disease, and is often linked to the consumption of raw seafood. - It does not demonstrate the **Kanagawa phenomenon** on Wagatsuma agar. *Vibrio alginotyticus* - *Vibrio alginolyticus* is an opportunistic pathogen primarily associated with wound infections, otitis externa, and occasionally bacteremia, often in marine environments. - It is not known to produce the **Kanagawa hemolysin** or exhibit the phenomenon on Wagatsuma agar.

Question 2496: Which organism is primarily responsible for Angular conjunctivitis?

A. Pneumococcus
B. Moraxella (Correct Answer)
C. Streptococcus
D. None of the above

Explanation: ***Correct: Moraxella*** - **Moraxella lacunata** is the primary causative agent of **angular conjunctivitis** (angular blepharoconjunctivitis), characterized by inflammation concentrated at the outer and inner canthi of the eye. - This organism produces a **proteolytic enzyme** that can liquify tissue, leading to maceration of the skin at the corners of the eyelids. - The condition typically presents with redness, irritation, and excoriation at the angles of the eyes. *Incorrect: Pneumococcus* - **Streptococcus pneumoniae (Pneumococcus)** can cause acute bacterial conjunctivitis, but it typically presents with diffuse redness and purulent discharge, not specifically angular inflammation. - It is more commonly associated with **otitis media**, **pneumonia**, **meningitis**, and systemic infections. *Incorrect: Streptococcus* - Various species of **Streptococcus**, including **Streptococcus pyogenes**, can cause acute conjunctivitis with generalized redness and swelling, but they are not characteristically linked to the angular form. - **Streptococcus** species are more often associated with pharyngitis, skin infections, cellulitis, and rheumatic fever. *Incorrect: None of the above* - This option is incorrect because **Moraxella lacunata** is a well-established and classic cause of angular conjunctivitis.

Question 2497: Fever, leukopenia, disseminated intravascular coagulation, and hypotension caused by members of the Enterobacteriaceae family are most strongly associated with which of the following structures?

A. Lipid A (Correct Answer)
B. Polysaccharides
C. H antigens (flagellar antigens)
D. K antigens (capsular antigens)

Explanation: **Correct: Lipid A** - **Lipid A** is the endotoxic component of **lipopolysaccharide (LPS)** found in the outer membrane of Gram-negative bacteria like Enterobacteriaceae. - It is responsible for triggering a severe inflammatory response, leading to systemic effects such as **fever, leukopenia, disseminated intravascular coagulation (DIC)**, and **hypotension (septic shock)**. - Lipid A stimulates macrophages to release pro-inflammatory cytokines (TNF-α, IL-1, IL-6), which mediate the clinical manifestations of endotoxic shock. *Incorrect: Polysaccharides* - This term is too general; while **LPS contains a polysaccharide component (O antigen)**, it is the **Lipid A** portion that exerts the potent endotoxic effects. - The **O antigen** is significant for serotyping and can modulate immune responses, but it is not the primary mediator of the severe systemic symptoms listed. *Incorrect: H antigens (flagellar antigens)* - **H antigens** are located on the bacterial flagella and are primarily involved in **motility** and aiding in bacterial identification and serotyping. - They do not directly cause the severe systemic symptoms like DIC or septic shock, although flagella can contribute to inflammation in other ways. *Incorrect: K antigens (capsular antigens)* - **K antigens** are associated with the bacterial **capsule**, which provides protection against phagocytosis and aids in adhesion. - While capsules contribute to virulence and immune evasion, they are not directly responsible for the endotoxic effects seen with disseminated infection.

Question 2498: A 7-year-old boy presented to the OPD with complaints of pain and discharge from the left ear. On examination, the patient was febrile, with neck rigidity and a positive Kernig's sign. Gram-positive cocci were visualized in the Gram-stained specimen of the discharge. What is the most likely pathogen?

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

Explanation: ***Streptococcus pneumoniae*** - *Streptococcus pneumoniae* is a common cause of **otitis media**, which, if untreated, can lead to complications such as **meningitis**, presenting with neck rigidity and a positive Kernig's sign. - Gram-stain revealing **Gram-positive cocci** is consistent with *Streptococcus pneumoniae*, particularly in the context of ear discharge and meningeal signs. *Staphylococcus aureus* - While *Staphylococcus aureus* can cause ear infections, it is less commonly associated with the rapid progression to **meningitis** in children following otitis media compared to *S. pneumoniae*. - *S. aureus* typically presents with **abscess formation** and cellulitis and, while Gram-positive cocci, the clinical picture strongly favors *S. pneumoniae*. *Staphylococcus epidermidis* - *Staphylococcus epidermidis* is a common **commensal skin organism** and rarely causes serious infections in immunocompetent individuals like this child. - It is often found as a contaminant in cultures and is not a primary pathogen for otitis media leading to **meningitis**. *Haemophilus influenzae* - *Haemophilus influenzae* is a common cause of **otitis media** and historically meningitis, but it is a **Gram-negative coccobacillus**, which contradicts the Gram-stain finding of Gram-positive cocci. - Meningitis due to *H. influenzae* has significantly decreased due to widespread vaccination with the **Hib vaccine**.

Question 2499: A farmer presents with pustules that show Gram-positive cocci on smear. The culture shows beta hemolysis, and the organisms are catalase-negative. To confirm that the identified organism is group A streptococcus, which of the following tests should be performed?

A. Bacitracin sensitivity (Correct Answer)
B. Novobiocin sensitivity
C. Bile solubility
D. Optochin sensitivity

Explanation: ***Bacitracin sensitivity*** - **Group A Streptococcus (GAS)** is uniquely sensitive to bacitracin, distinguishing it from most other beta-hemolytic streptococci. - A definitive zone of inhibition around a **bacitracin disk** on a blood agar plate indicates GAS. *Optochin sensitivity* - This test is used to identify **Streptococcus pneumoniae**, which is alpha-hemolytic, not beta-hemolytic like the organism described. - *S. pneumoniae* is **sensitive to optochin**, while other alpha-hemolytic streptococci are resistant. *Novobiocin sensitivity* - This test is primarily used to differentiate coagulase-negative staphylococci, specifically to identify **Staphylococcus saprophyticus** (resistant) from other coagulase-negative staphylococci (sensitive). - It is not applicable to the identification of streptococci. *Bile solubility* - This test is used to differentiate **Streptococcus pneumoniae** (bile soluble) from other alpha-hemolytic streptococci. - The described organism is **beta-hemolytic**, making this test inappropriate for its identification.

Question 2500: All the following statements regarding cervicofacial actinomycosis are true, except:

A. The discharging pus contains visible 'sulphur granules'
B. There is usually history of trauma such as tooth extraction or a blow to the jaw
C. Cultures on blood agar often produce a typical 'molar tooth' morphology
D. 10-20% of actinomycosis cases occur in cervicofacial region. (Correct Answer)

Explanation: ***10-20% of actinomycosis cases occur in cervicofacial region.*** - This statement is incorrect because the **cervicofacial region** is the most common site of actinomycosis, accounting for **50-70% of all cases**, not 10-20%. - Therefore, cases in this region represent a significant majority of actinomycosis infections. *The discharging pus contains visible 'sulphur granules'* - **Sulphur granules** are characteristic macroscopic aggregates of **Actinomyces colonies** embedded in a calcium phosphate matrix, which are typically found in the pus of actinomycosis infections. - Their presence is a strong diagnostic indicator, aiding in the identification of the condition. *There is usually history of trauma such as tooth extraction or a blow to the jaw* - **Trauma**, such as **dental procedures** (e.g., tooth extraction), fractures, or other injuries, can disrupt mucosal barriers, allowing **Actinomyces** species, which are normal oral flora, to invade deeper tissues and cause infection. - This often provides the anaerobic environment necessary for the bacteria to proliferate. *Cultures on blood agar often produce a typical 'molar tooth' morphology* - **Actinomyces colonies** grown on blood agar under **anaerobic conditions** often exhibit a distinctive **'molar tooth' morphology** (rough, nodular, and convex appearance) after several days of incubation. - This specific colonial morphology is a key feature used in the laboratory diagnosis of actinomycosis.

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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