Staphylococci — MCQs

10 questions

Which of the following is a catalase-positive, beta-hemolytic staphylococcus?

Which of the following disorders would be more likely associated with Staphylococcus saprophyticus rather than Staphylococcus aureus?

What is the causative organism for the condition depicted in the image?

A 25-year-old female presented to the hospital on 3rd day of menstruation with complaints of high fever, vomiting and rash on her trunk and extremities. On investigations she had leukocytosis and negative blood culture. She is diagnosed as:

Toxic shock syndrome is due to the following virulence factor:

A child after consuming food in a party complains of diarrhea within 1-5 hours. The diagnosis is:

Which organism is incriminated in causing the following lesions? (Recent NEET Pattern 2016-17)

Following are true of Gram negative bacterial cell wall compared to Gram positive bacteria except:

A 2-year-old child presents with fever for 5 days and fast breathing. On examination breath sounds are reduced in left infra-axillary areas and left inframammary areas. CXR was performed and pleural tap was done. Gram stain of pus drained in pleural tap shows: (Recent NEET Pattern 2016-17)

Q10

An outbreak of staphylococcal infection involving umbilical cords of seven newborn babies was reported in the nursery. Bacteriological survey reveals that two nurses have a large number of Staphylococcus aureus in the nasopharynx. What test should be performed to determine whether these nurses may have been responsible for the outbreak?

Staphylococci Indian Medical PG Practice Questions and MCQs

Question 1: Which of the following is a catalase-positive, beta-hemolytic staphylococcus?

A. S. epidermidis
B. S. saprophyticus
C. S. aureus (Correct Answer)
D. None of the above

Explanation: ***Correct Option: S. aureus*** - *Staphylococcus aureus* is **catalase-positive** and characteristically produces **beta-hemolysis** (complete hemolysis with clear zones) on blood agar - S. aureus produces several hemolysins including **alpha-toxin** (alpha-hemolysin), which causes complete lysis of red blood cells, resulting in the characteristic beta-hemolytic pattern - It is also **coagulase-positive**, a key differentiating factor from other *Staphylococcus* species - This is the most clinically significant staphylococcus species, causing skin infections, pneumonia, endocarditis, and toxin-mediated diseases *Incorrect Option: S. epidermidis* - *Staphylococcus epidermidis* is **catalase-positive** but is **gamma-hemolytic** (non-hemolytic) on blood agar - It is **coagulase-negative** and a common cause of foreign body and prosthetic device infections due to its ability to form **biofilms** - Part of normal skin flora *Incorrect Option: S. saprophyticus* - *Staphylococcus saprophyticus* is **catalase-positive** but is **gamma-hemolytic** (non-hemolytic) on blood agar - It is **coagulase-negative** and well-known as a cause of **urinary tract infections (UTIs)**, especially in young, sexually active women - Resistant to **novobiocin**, a key distinguishing characteristic from S. epidermidis *Incorrect Option: None of the above* - This is incorrect because S. aureus clearly meets both criteria: catalase-positive and beta-hemolytic

Question 2: Which of the following disorders would be more likely associated with Staphylococcus saprophyticus rather than Staphylococcus aureus?

A. Burns
B. Tension pneumothorax
C. Osteomyelitis
D. Acute cystitis (Correct Answer)

Explanation: ***Acute cystitis*** - **Staphylococcus saprophyticus** is a common cause of **urinary tract infections (UTIs)**, particularly acute cystitis, in young sexually active women. - This bacterium has a high affinity for **uroepithelial cells**, facilitating its colonization and subsequent infection of the bladder. *Tension pneumothorax* - A **tension pneumothorax** is a medical emergency characterized by air accumulation in the pleural space, leading to lung collapse and mediastinal shift. - It is typically caused by trauma or iatrogenic factors, not directly by bacterial infection from either *Staphylococcus saprophyticus* or *Staphylococcus aureus*. *Burns* - Burn wounds are highly susceptible to bacterial colonization and infection, with **Staphylococcus aureus** being a primary pathogen in this context. - *Staphylococcus saprophyticus* is rarely associated with burn wound infections. *Osteomyelitis* - **Osteomyelitis**, an infection of the bone, is most frequently caused by **Staphylococcus aureus** via hematogenous spread or direct inoculation. - *Staphylococcus saprophyticus* is not a common pathogen in osteomyelitis.

Question 3: What is the causative organism for the condition depicted in the image?

A. Staphylococci (Correct Answer)
B. Candidal infection
C. Streptococcus
D. Actinomycetes

Explanation: ***Staphylococci*** - The image shows **impetigo** with **crusted lesions**, consistent with **_Staphylococcus aureus_** infection. - **Staphylococcus aureus** is a major causative organism of impetigo, particularly **bullous impetigo**, and commonly produces the characteristic **honey-colored crusts** seen in non-bullous forms as well. - This superficial bacterial skin infection is highly contagious and responds well to topical or systemic antibiotics. *Candidal infection* - **Candidal infections** (e.g., candidiasis) typically present as **erythematous patches** with satellite lesions, or white plaques in mucosal areas, which is not consistent with the image. - This fungal infection is often seen in immunocompromised individuals or in warm, moist skin folds, not as crusted superficial lesions. *Streptococcus* - While **_Streptococcus pyogenes_** can also cause impetigo (especially non-bullous impetigo), the clinical presentation in the image is most consistent with **staphylococcal infection**. - Streptococcal infections may present similarly but can also cause other conditions like cellulitis or erysipelas with distinct features. *Actinomycetes* - **Actinomycosis** is a rare, chronic bacterial infection that forms **abscesses and sinus tracts**, often with "sulfur granules," which is distinct from the superficial skin lesions shown. - This infection usually involves deeper tissues and presents as a chronic, indolent infection, unlike the acute superficial presentation of impetigo.

Question 4: A 25-year-old female presented to the hospital on 3rd day of menstruation with complaints of high fever, vomiting and rash on her trunk and extremities. On investigations she had leukocytosis and negative blood culture. She is diagnosed as:

A. staphylococcal food poisoning
B. toxic shock syndrome (Correct Answer)
C. scalded skin syndrome
D. varicella zoster infection

Explanation: ***Toxic shock syndrome*** - The combination of **high fever, vomiting, rash, leukocytosis, and menstruation** (especially tampon use) is highly suggestive of **Toxic Shock Syndrome (TSS)**, which is often caused by *Staphylococcus aureus* exotoxins. - A **negative blood culture** is common in TSS as it is a toxemia, not primarily a bacteremia, though bacteria are present at the local infection site. *Staphylococcal food poisoning* - While caused by *Staphylococcus aureus* toxins, **food poisoning** is typically characterized by rapid onset **gastrointestinal symptoms** (nausea, vomiting, diarrhea) [1] and usually resolves within 24 hours. - It does not typically present with the widespread **exfoliative rash** and systemic features seen in TSS. *Scalded skin syndrome* - **Staphylococcal scalded skin syndrome (SSSS)** is characterized by **blistering and exfoliation of the skin**, primarily affecting young children. - While both involve staphylococcal toxins, SSSS does not typically present with the prominent **fever, vomiting, and menstrual association** seen in this patient. *Varicella zoster infection* - **Varicella zoster infection** (chickenpox or shingles) presents with characteristic **vesicular lesions** in various stages of healing. - The rash described (trunk and extremities without vesicles) and the association with menstruation do not fit the clinical picture of a varicella zoster infection.

Question 5: Toxic shock syndrome is due to the following virulence factor:

A. M protein
B. Carbohydrate cell wall
C. Streptolysin O
D. Pyrogenic exotoxin (Correct Answer)

Explanation: ***Pyrogenic exotoxin*** - **Pyrogenic exotoxins**, specifically **Toxic Shock Syndrome Toxin-1 (TSST-1)** and other streptococcal pyrogenic exotoxins (SPEs), are **superantigens** responsible for the symptoms of **toxic shock syndrome**. - These superantigens bind directly to **MHC class II** and **T-cell receptors**, leading to a massive, non-specific release of **cytokines** that cause severe systemic inflammation and organ failure. *M protein* - **M protein** is a major virulence factor of **Streptococcus pyogenes**, contributing to its antiphagocytic properties and adherence. - While important for streptococcal infections, it is not the primary mediator of the **toxic shock syndrome** associated with *Staphylococcus aureus* or *Streptococcus pyogenes*. *Carbohydrate cell wall* - The **carbohydrate cell wall** is a structural component of bacteria, particularly Gram-positive bacteria, and can have some immunogenic properties. - However, it does not directly act as a specific virulence factor like a superantigen to cause the severe systemic symptoms characteristic of **toxic shock syndrome**. *Streptolysin O* - **Streptolysin O** is an **exotoxin produced by Streptococcus pyogenes** that causes **hemolysis** and is cardiotoxic and cytolytic. - While it contributes to tissue damage and can be involved in severe streptococcal infections, it is not the main superantigen responsible for the widespread systemic effects of **toxic shock syndrome**.

Question 6: A child after consuming food in a party complains of diarrhea within 1-5 hours. The diagnosis is:

A. S. aureus (Correct Answer)
B. Clostridium perfringens
C. Streptococcus
D. Clostridium botulinum

Explanation: ***S. aureus*** - **_Staphylococcus aureus_** food poisoning has a rapid onset (**1-6 hours**) because symptoms are caused by preformed **exotoxins** ingested with contaminated food, not by bacterial growth in the host. - Common sources include foods handled extensively and left at **room temperature**, allowing the bacteria to multiply and produce heat-stable toxins. *Clostridium perfringens* - **_Clostridium perfringens_** food poisoning typically has a longer incubation period (**6-24 hours**), as symptoms are caused by toxins produced by bacteria replicating in the host intestine. - It often results from eating large quantities of contaminated meat or poultry that has been improperly cooked or stored. *Streptococcus* - **_Streptococcus_** species are not primary causes of acute **foodborne diarrheal illness** with such a rapid onset. - While some strains can cause infections, their role in common food poisoning with preformed toxins is negligible compared to _S. aureus_. *Clostridium botulinum* - **_Clostridium botulinum_** causes **botulism**, a severe neuroparalytic illness, not primarily diarrhea, and typically has a longer incubation period (**12-36 hours**). - Symptoms are due to neurotoxins affecting the nervous system, leading to **flaccid paralysis**, not acute gastroenteritis.

Question 7: Which organism is incriminated in causing the following lesions? (Recent NEET Pattern 2016-17)

A. Streptococcus pyogenes (Correct Answer)
B. Streptococcus pneumoniae
C. Enterococcus
D. Staphylococcus aureus

Explanation: ***Streptococcus pyogenes*** - The image depicts **impetigo**, characterized by crusted lesions, often found on the face. - **Streptococcus pyogenes** (Group A Streptococcus) is a common cause of impetigo, either alone or in combination with *Staphylococcus aureus*. *Streptococcus pneumoniae* - *Streptococcus pneumoniae* is primarily associated with **respiratory tract infections**, such as pneumonia and otitis media, not skin lesions like impetigo. - While it can cause invasive diseases, its primary presentation is typically not superficial skin infections. *Enterococcus* - *Enterococcus* species are common inhabitants of the normal **gastrointestinal flora** and are frequently implicated in **urinary tract infections**, endocarditis, and hospital-acquired infections. - They are generally not a primary cause of impetigo or similar superficial skin infections. *Staphylococcus aureus* - While *Staphylococcus aureus* is a very common cause of **impetigo**, the question asks for "the organism" as if there is only one most specific answer without providing other context, suggesting **Streptococcus pyogenes** as a highly relevant primary pathogen, especially if non-bullous impetigo is implied by the crusted appearance. - *S. aureus* often presents with **purulent lesions** (e.g., boils, carbuncles) and bullous impetigo with fluid-filled blisters which eventually rupture and crust.

Question 8: Following are true of Gram negative bacterial cell wall compared to Gram positive bacteria except:

A. Thinner
B. Presence of lipopolysaccharide
C. Presence of outer membrane
D. Presence of Teichoic acid (Correct Answer)

Explanation: ***Presence of Teichoic acid*** - **Teichoic acid** is a unique component of the cell wall in **Gram-positive bacteria**, playing a role in cell wall structure and antigenicity. - Its presence is **not a characteristic of Gram-negative bacteria**, making this statement the exception. *Thinner* - The cell wall of **Gram-negative bacteria** is indeed **thinner** than that of Gram-positive bacteria. - This **thin peptidoglycan layer** (2-3 nm) is much less substantial compared to the thick peptidoglycan layer (20-80 nm) of Gram-positive bacteria. *Presence of lipopolysaccharide* - **Lipopolysaccharide (LPS)**, or endotoxin, is a characteristic component of the **outer membrane** of Gram-negative bacteria. - LPS contributes to the **pathogenicity** of Gram-negative bacteria and is absent in Gram-positive bacteria. *Presence of outer membrane* - **Gram-negative bacteria** have a unique **outer membrane** that lies external to the thin peptidoglycan layer. - This outer membrane contains LPS and porins, and is a distinguishing feature **absent in Gram-positive bacteria**, which have only a single cytoplasmic membrane.

Question 9: A 2-year-old child presents with fever for 5 days and fast breathing. On examination breath sounds are reduced in left infra-axillary areas and left inframammary areas. CXR was performed and pleural tap was done. Gram stain of pus drained in pleural tap shows: (Recent NEET Pattern 2016-17)

A. Meningococcus
B. Pneumococcus (Correct Answer)
C. Staphylococcus
D. Enterococcus

Explanation: ***Pneumococcus*** - The image provided shows **Gram-positive diplococci**, a classic morphology for *Streptococcus pneumoniae* (Pneumococcus). These bacteria are often seen in pairs or short chains. - *Streptococcus pneumoniae* is the **most common bacterial cause of community-acquired pneumonia** and associated empyema, especially in children. The clinical presentation of fever and fast breathing, with reduced breath sounds and a large pleural effusion seen on CXR (suggesting empyema), is highly consistent with pneumococcal infection. *Meningococcus* - *Neisseria meningitidis* (Meningococcus) is a **Gram-negative diplococcus**, whereas the morphology depicted in the Gram stain is **Gram-positive**, ruling out meningococcus. - Additionally, meningococcus primarily causes **meningitis** and bloodstream infections, not typically empyema in this clinical setting. *Staphylococcus* - *Staphylococcus* species are Gram-positive cocci that typically arrange in **clusters**, resembling grape-like formations, which is not what is shown in the image. - Although *Staphylococcus aureus* can cause empyema, especially after influenza or in healthcare-associated infections, its characteristic arrangement on Gram stain would be different. *Enterococcus* - *Enterococcus* species are Gram-positive cocci that typically form **chains** or **pairs**, similar to streptococci but often distinguishable by specific biochemical tests. - Although enterococci can cause infections, they are **less common causes of community-acquired empyema** in children compared to pneumococci. The morphology in the image, showing distinct diplococci, is most consistent with pneumococci.

Question 10: An outbreak of staphylococcal infection involving umbilical cords of seven newborn babies was reported in the nursery. Bacteriological survey reveals that two nurses have a large number of Staphylococcus aureus in the nasopharynx. What test should be performed to determine whether these nurses may have been responsible for the outbreak?

A. Coagulase testing
B. Nasopharyngeal culture on mannitol salt agar
C. Bacteriophage typing (Correct Answer)
D. Protein A typing

Explanation: ***Bacteriophage typing*** - **Bacteriophage typing** involves using specific **bacteriophages** to identify different strains within a bacterial species based on their susceptibility to lysis by these phages. - This method helps determine if the specific strain of **Staphylococcus aureus** found in the nurses' nasopharynx matches the strain causing the outbreak in the newborns' umbilical cords, thereby establishing an epidemiological link. - This is the **classical method** for *S. aureus* strain typing in outbreak investigations. Modern molecular methods like PFGE, MLST, and whole genome sequencing have largely replaced bacteriophage typing, but it remains a fundamental concept tested in medical examinations. *Coagulase testing* - **Coagulase testing** differentiates **Staphylococcus aureus** (coagulase-positive) from other coagulase-negative staphylococci. - While it identifies the species, it does not provide the **strain-level differentiation** needed to link a specific individual to an outbreak. *Nasopharyngeal culture on mannitol salt agar* - **Mannitol salt agar** is a selective and differential medium used to isolate and identify **Staphylococcus aureus** from mixed cultures due to its ability to ferment mannitol and tolerate high salt concentrations. - This test would confirm the presence of **Staphylococcus aureus** in the nasopharynx but would not provide the detailed **strain-specific information** required to trace the source of the outbreak. *Protein A typing* - **Protein A** is a common cell wall component of **Staphylococcus aureus** that binds to the Fc region of immunoglobulins. - While its presence is characteristic of **Staphylococcus aureus**, **Protein A typing** does not offer the necessary **strain-specific resolution** to epidemiologically link an individual carrier to a specific outbreak strain.

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