Question 1

An 11-year-old boy presents with a sore throat, fever, chills, and difficulty swallowing for the past 3 days. The patient’s mother says that last night he was short of breath and had a headache. Past medical history is unremarkable. The patient has not been vaccinated as his mother thinks it is "unnecessary". His temperature is 38.3°C (101.0°F), blood pressure is 120/70 mm Hg, pulse is 110/min, and respiratory rate is 18/min. On physical examination, the patient is ill-appearing and dehydrated. A grayish-white membrane and pharyngeal erythema are present in the oropharynx. Significant cervical lymphadenopathy is also present. A throat swab is taken and gram staining shows gram-positive club-shaped bacilli along with few neutrophils. Which of the following would most likely be the result of the bacterial culture of the throat swab in this patient?

Accepted Answer

Small black colonies on tellurite agar. ***Small black colonies on tellurite agar***
  - The clinical presentation, including **sore throat**, **fever**, **grayish-white membrane** in the oropharynx, and **cervical lymphadenopathy** in an **unvaccinated child**, strongly suggests **diphtheria** caused by *Corynebacterium diphtheriae*.
  - *Corynebacterium diphtheriae* produces **small gray-black colonies** on **potassium tellurite agar** (e.g., Blood Tellurite Agar or Tinsdale agar) due to the reduction of tellurite to elemental tellurium within the bacterial cells.
  - This is the **definitive culture characteristic** used for laboratory diagnosis of diphtheria.

*Hemolytic black colonies on blood agar*
  - **Hemolytic black colonies** are not characteristic of *Corynebacterium diphtheriae*.
  - *C. diphtheriae* may show minimal or no hemolysis on blood agar, and does not produce black colonies on this medium.
  - Black colonies with hemolysis might suggest other organisms but are not typical for diphtheria diagnosis.

*Metallic green colonies on eosin-methylene blue agar*
  - **Metallic green colonies** on **eosin-methylene blue (EMB) agar** are characteristic of **lactose-fermenting bacteria**, particularly *Escherichia coli*.
  - This finding is associated with **Gram-negative enteric bacteria**, not the Gram-positive club-shaped bacilli seen in this patient.

*Greyish-white colonies on Thayer-Martin agar*
  - **Greyish-white colonies** on **Thayer-Martin agar** are typically seen with **fastidious Gram-negative diplococci**, such as *Neisseria gonorrhoeae* or *Neisseria meningitidis*.
  - This medium is selective for *Neisseria* species and would not be used for isolating *Corynebacterium diphtheriae*, which is a Gram-positive rod.

*Creamy white colonies on Loeffler's serum*
  - **Loeffler's serum medium** is indeed used to enhance the growth of *Corynebacterium diphtheriae*, and the organism produces **creamy white to grayish colonies** on this medium.
  - However, Loeffler's medium is primarily used to demonstrate the characteristic **metachromatic granules** (Babes-Ernst bodies) on microscopy, not for definitive culture identification.
  - **Tellurite agar**, not Loeffler's medium, is the **gold standard** for culture diagnosis because the black colony appearance is pathognomonic for *C. diphtheriae*. [Practice more Microbiology questions on OnCourse]

Question 2

A 24-hour-old newborn presents to the emergency department after a home birth because of fever, irritability alternating with lethargy, and poor feeding. The patient’s mother says symptoms acutely onset 12 hours ago and have not improved. No significant past medical history. His mother did not receive any prenatal care, and she had rupture of membranes 20 hours prior to delivery. His vital signs include: heart rate 150/min, respiratory rate 65/min, temperature 39.0°C (102.2°F), and blood pressure 60/40 mm Hg. On physical examination, the patient has delayed capillary refill. Laboratory studies show a pleocytosis and a low glucose level in the patient’s cerebrospinal fluid. Which of the following is the most likely causative organism for this patient’s condition?

Accepted Answer

Group B Streptococcus. ***Group B Streptococcus***
  - This newborn presents with **fever, irritability/lethargy, poor feeding**, and signs of **sepsis (tachycardia, tachypnea, hypotension, delayed capillary refill)**, along with **abnormal CSF (pleocytosis, low glucose)**, indicating **neonatal meningitis**.
  - **Group B Streptococcus (GBS)** is the **most common cause of early-onset neonatal sepsis and meningitis**, especially with risk factors such as **lack of prenatal care** and **prolonged rupture of membranes (>18 hours)**, as seen in this case.

*Group A Streptococcus*
  - While Group A Streptococcus can cause severe infections, it is an **uncommon cause of neonatal sepsis and meningitis** compared to GBS.
  - More typically associated with **pharyngitis, impetigo, and necrotizing fasciitis** in older children and adults.

*Enterovirus*
  - Enteroviruses are a common cause of **viral meningitis in neonates and infants**, but typically present with a **lymphocytic pleocytosis** and **normal CSF glucose**, in contrast to the features (pleocytosis, low glucose) seen here.
  - While fever and irritability can be present, the CSF findings point more towards a bacterial infection.

*Streptococcus pneumoniae*
  - *Streptococcus pneumoniae* can cause bacterial meningitis but is **less common in the immediate neonatal period** (first 7 days of life) compared to GBS.
  - Risk factors often include **preterm birth** or **underlying immune deficiencies**, which are not specified here.

*Cryptococcus neoformans*
  - *Cryptococcus neoformans* is an **opportunistic fungal pathogen** that typically causes meningitis in **immunocompromised individuals**, such as those with HIV/AIDS.
  - It is **extremely rare** in immunocompetent newborns and would not be the most likely cause in this clinical scenario. [Practice more Microbiology questions on OnCourse]

Question 3

An 11-year-old boy is brought to the emergency department by his parents for confusion and fever. The patient began complaining of a headache yesterday afternoon that progressively got worse. After waking him up this morning, his mom noticed that “he seemed funny and wasn’t able to carry a conversation fully.” When asked about his past medical history, the dad claims that he’s been healthy except for 2-3 episodes of finger pain and swelling. Physical examination demonstrates a boy in moderate distress, altered mental status, and nuchal rigidity. A CSF culture reveals a gram-positive, diplococci bacteria. What characteristic would you expect in the organism most likely responsible for this patient’s symptoms?

Accepted Answer

Optochin sensitivity. ***Optochin sensitivity***
  - The patient's symptoms (fever, confusion, headache, nuchal rigidity in an 11-year-old) and CSF findings (gram-positive diplococci) are highly suggestive of **Streptococcus pneumoniae meningitis**.
  - **Streptococcus pneumoniae** is sensitive to optochin, which is a key characteristic used for laboratory identification.

*Maltose fermentation*
  - **Neisseria meningitidis**, another common cause of bacterial meningitis, ferments **maltose** and glucose.
  - While *N. meningitidis* is also a gram-negative diplococcus, the CSF microscopy showing **gram-positive diplococci** rules out this organism.

*Pyocyanin production*
  - **Pyocyanin** is a blue-green pigment produced by **Pseudomonas aeruginosa**, a gram-negative rod.
  - *Pseudomonas aeruginosa* is typically associated with infections in immunocompromised patients, burn victims, or hospital-acquired infections, and it does not present as a gram-positive diplococcus in CSF.

*K-capsule*
  - The **K-capsule** (or capsular antigen) is characteristic of **Escherichia coli**, particularly strains causing neonatal meningitis.
  - *E. coli* is a gram-negative rod, which is inconsistent with the gram-positive diplococci observed in the CSF.

*Culture on chocolate agar with factors V and X*
  - This growth requirement is characteristic of **Haemophilus influenzae**, a gram-negative coccobacillus.
  - *Haemophilus influenzae* meningitis typically presents with similar symptoms but is caused by a gram-negative organism, not a gram-positive one as seen in this case. [Practice more Microbiology questions on OnCourse]

Question 4

A 3-year-old male is brought to the ER with a sore throat and fever. Examination of the pharynx reveals a dark, inflammatory exudate. Cysteine-tellurite agar culture produces black, iridescent colonies. Microscopic features of the causal organism most likely include which of the following?

Accepted Answer

Metachromic granules. ***Metachromic granules***
  - The constellation of **sore throat**, **fever**, **dark inflammatory exudate** in the pharynx, and growth on **cysteine-tellurite agar** with **black, iridescent colonies** is highly characteristic of *Corynebacterium diphtheriae*.
  - *Corynebacterium diphtheriae* is known for exhibiting **metachromatic granules** (Babes-Ernst bodies) when stained, which are reserves of inorganic polyphosphate.

*Serpentine growth patterns*
  - **Serpentine growth patterns** are characteristic of *Mycobacterium tuberculosis* in liquid culture, not *Corynebacterium diphtheriae*.
  - This growth pattern is due to the arrangement of bacterial cells in long, cord-like structures.

*Kidney bean-shaped diplococci*
  - **Kidney bean-shaped diplococci** are characteristic of *Neisseria* species, such as *Neisseria gonorrhoeae* or *Neisseria meningitidis*.
  - These Gram-negative cocci are typically found in pairs with adjacent flattened sides, giving them a kidney bean appearance.
  - These organisms cause different clinical syndromes and have distinct culture characteristics.

*Long, branching filaments*
  - **Long, branching filaments** are a microscopic feature of certain bacteria like *Actinomyces* and *Nocardia*.
  - These organisms are responsible for actinomycosis and nocardiosis, which are typically chronic infections distinct from diphtheria.

*Lancet-shape*
  - The term **lancet-shape** is used to describe the morphology of *Streptococcus pneumoniae*, which are Gram-positive cocci typically found in pairs (diplococci).
  - *Streptococcus pneumoniae* causes pneumonia, meningitis, and otitis media, which differ from the presentation of diphtheria. [Practice more Microbiology questions on OnCourse]

Question 5

A microbiology graduate student was given a swab containing an unknown bacteria that caused an ear infection in a seven-year-old girl. The student identified the bacteria as a gram-positive, catalase-negative cocci showing alpha-hemolysis (greenish discoloration around colonies) when grown on blood agar. Which of the following characteristics is associated with this bacteria?

Accepted Answer

Positive quellung reaction. ***Positive quellung reaction***
- The description of **gram-positive**, **catalase-negative cocci** with **alpha-hemolysis** (greenish discoloration) strongly points to ***Streptococcus pneumoniae***.
- ***S. pneumoniae*** possesses a polysaccharide capsule, which causes a **positive quellung reaction** (capsular swelling) in the presence of specific antiserum, making the capsule appear swollen and more visible under a microscope.
- This is the hallmark diagnostic test for *S. pneumoniae* and directly associated with the organism's virulence.

*Bacitracin-sensitive*
- **Bacitracin sensitivity** is a characteristic used to identify **Group A Streptococcus** (*Streptococcus pyogenes*), which is **beta-hemolytic**, not alpha-hemolytic.
- The bacteria in question exhibits **alpha-hemolysis**, ruling out Group A Streptococcus.

*Growth in bile and 6.5% NaCl*
- The ability to **grow in bile and 6.5% NaCl** is a distinguishing feature of ***Enterococcus* species**.
- While *Enterococcus* is gram-positive and catalase-negative, it typically exhibits variable hemolysis and is not associated with otitis media in this clinical context.

*Bacitracin-resistant*
- **Bacitracin resistance** is seen in many bacterial species, including **Group B Streptococcus** (*Streptococcus agalactiae*), which is **beta-hemolytic**.
- While *S. pneumoniae* is bacitracin-resistant, this is not its distinguishing characteristic; the **quellung reaction** is the specific identifying feature.

*Negative quellung reaction*
- A **negative quellung reaction** would indicate the absence of a polysaccharide capsule, which would rule out ***S. pneumoniae***.
- Since all other characteristics strongly suggest *S. pneumoniae*, a negative quellung reaction would be contradictory. [Practice more Microbiology questions on OnCourse]

Microbiology Questions

Topics

Gram-positive

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Bacteria

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