Streptococcus pneumoniae US Medical PG Practice Questions and MCQs
Practice US Medical PG questions for Streptococcus pneumoniae. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Streptococcus pneumoniae US Medical PG Question 1: A 51-year-old man comes to the physician because of a 4-day history of fever and cough productive of foul-smelling, dark red, gelatinous sputum. He has smoked 1 pack of cigarettes daily for 30 years and drinks two 12-oz bottles of beer daily. An x-ray of the chest shows a cavity with air-fluid levels in the right lower lobe. Sputum culture grows gram-negative rods. Which of the following virulence factors is most likely involved in the pathogenesis of this patient's condition?
- A. IgA protease
- B. Exotoxin A
- C. Capsular polysaccharide (Correct Answer)
- D. P-fimbriae
- E. Heat-stable toxin
Streptococcus pneumoniae Explanation: ***Capsular polysaccharide***
- The patient's symptoms (fever, foul-smelling sputum, cavitation with air-fluid levels) and risk factors (smoking) suggest a **lung abscess** likely caused by **_Klebsiella pneumoniae_**.
- **Capsular polysaccharide** is a major virulence factor for _Klebsiella pneumoniae_, providing resistance to phagocytosis and contributing to its invasive potential.
*IgA protease*
- **IgA protease** is a virulence factor produced by bacteria such as _Neisseria gonorrhoeae_, _Neisseria meningitidis_, and _Haemophilus influenzae_ to cleave IgA antibodies.
- While important for mucosal infections, it is not characteristic of the severe lung pathology described, nor a primary virulence factor for a gram-negative rod causing lung abscesses like _Klebsiella_.
*Exotoxin A*
- **Exotoxin A** is a potent exotoxin produced by _Pseudomonas aeruginosa_, which inhibits protein synthesis by ADP-ribosylation of elongation factor 2.
- While _Pseudomonas_ can cause lung infections in compromised patients, the classic description of dark red, gelatinous sputum and the strong association with gram-negative rods causing lung abscesses points more directly to _Klebsiella_.
*P-fimbriae*
- **P-fimbriae** (pyelonephritis-associated fimbriae) are adhesion factors found on uropathogenic _E. coli_, enabling them to bind to uroepithelial cells and cause urinary tract infections.
- These fimbriae are not relevant to the pathogenesis of a lung abscess caused by gram-negative rods in this clinical context.
*Heat-stable toxin*
- **Heat-stable toxin** is typically associated with enterotoxigenic _E. coli_ (ETEC), causing watery diarrhea by activating guanylate cyclase.
- This toxin is involved in gastrointestinal infections and has no role in the pathogenesis of a lung abscess.
Streptococcus pneumoniae US Medical PG Question 2: A 2-year-old boy is brought in by his parents to his pediatrician. The boy was born by spontaneous vaginal delivery at 39 weeks and 5 days after a normal pregnancy. The boy has received all age-appropriate vaccinations as of his last visit at 18 months of age. Of note, the boy has confirmed sickle cell disease and the only medication he takes is penicillin prophylaxis. The parents state that they plan on enrolling their son in a daycare, which requires documentation of up-to-date vaccinations. The pediatrician states that their son needs an additional vaccination at this visit, which is a polysaccharide vaccine that is not conjugated to protein. Which of the following matches this description?
- A. Pneumovax (Correct Answer)
- B. Menactra
- C. Prevnar
- D. Hib vaccine
- E. Live attenuated influenza vaccine
Streptococcus pneumoniae Explanation: ***Pneumovax***
- **Pneumovax** (PCV23, PPSV23) is a **polysaccharide vaccine** that is not conjugated to a protein carrier. Children with **sickle cell disease** should receive this vaccine due to their immunocompromised state and increased risk of encapsulated bacterial infections.
- The Centers for Disease Control and Prevention (CDC) recommends PPSV23 for children aged 2 years and older with chronic medical conditions such as **sickle cell disease**, usually administered 8 weeks after their last PCV13 dose.
*Menactra*
- **Menactra** is a **quadrivalent meningococcal conjugate vaccine** (MCV4), meaning it contains a polysaccharide antigen conjugated to a protein carrier.
- This vaccine primarily targets *Neisseria meningitidis* and is different from the pneumococcal vaccine required here.
*Prevnar*
- **Prevnar** (PCV13) is a **pneumococcal conjugate vaccine**, meaning its polysaccharide antigens are conjugated to a protein carrier.
- While important for children with sickle cell disease, the question specifically asks for a vaccination that is a **polysaccharide vaccine that is not conjugated to protein**.
*Hib vaccine*
- The **Hib vaccine** (against *Haemophilus influenzae* type b) is a **conjugate vaccine**, meaning its polysaccharide capsule is linked to a protein carrier to enhance immunogenicity, particularly in infants.
- This vaccine is typically given earlier in childhood and is not the "additional" unconjugated polysaccharide vaccine described.
*Live attenuated influenza vaccine*
- The **live attenuated influenza vaccine (LAIV)** is a live virus vaccine, not a polysaccharide vaccine.
- It is also contraindicated in individuals with certain immunocompromising conditions, such as some patients with sickle cell disease.
Streptococcus pneumoniae US Medical PG Question 3: A 40-year-old man presents to the office complaining of chills, fever, and productive cough for the past 24 hours. He has a history of smoking since he was 18 years old. His vitals are: heart rate of 85/min, respiratory rate of 20/min, temperature 39.0°C (102.2°F), blood pressure 110/70 mm Hg. On physical examination, there is dullness on percussion on the upper right lobe, as well as bronchial breath sounds and egophony. The plain radiograph reveals an increase in density with an alveolar pattern in the upper right lobe. Which one is the most common etiologic agent of the suspected disease?
- A. Legionella pneumophila
- B. Chlamydia pneumoniae
- C. Mycoplasma pneumoniae
- D. Streptococcus pneumoniae (Correct Answer)
- E. Haemophilus influenzae
Streptococcus pneumoniae Explanation: ***Streptococcus pneumoniae***
- This patient presents with classic symptoms of **community-acquired pneumonia (CAP)**, including fever, chills, productive cough, and specific findings on physical exam (dullness, bronchial breath sounds, egophony) and chest X-ray (**lobar consolidation**).
- **_Streptococcus pneumoniae_** is the most common bacterial cause of CAP worldwide, accounting for a significant percentage of cases, especially in adults.
*Legionella pneumophila*
- While _Legionella_ can cause severe pneumonia, it often presents with **GI symptoms** (diarrhea, nausea) and **neurological symptoms** (confusion) in addition to respiratory symptoms, which are not described here.
- Risk factors typically include exposure to **contaminated water sources**, and the pneumonia can be rapidly progressive.
*Chlamydia pneumoniae*
- _Chlamydia pneumoniae_ typically causes a more **atypical pneumonia**, often with a more insidious onset, prolonged cough, and less severe systemic symptoms.
- It usually presents as a **walking pneumonia** with milder findings on chest X-ray, unlike the clear lobar consolidation described.
*Mycoplasma pneumoniae*
- Like _Chlamydia pneumoniae_, _Mycoplasma pneumoniae_ is a common cause of **atypical pneumonia**, often with a gradual onset, hacking cough, and less pronounced fever.
- It rarely causes the classic lobar consolidation seen in this patient and is often referred to as "walking pneumonia."
*Haemophilus influenzae*
- _Haemophilus influenzae_ is a significant cause of CAP, especially in patients with **underlying lung disease** (like COPD) or other comorbidities.
- While certainly a possibility given the patient's smoking history, **_Streptococcus pneumoniae_** remains the overall most common cause of bacterial CAP in otherwise healthy adults.
Streptococcus pneumoniae US Medical PG Question 4: You are treating a neonate with meningitis using ampicillin and a second antibiotic, X, that is known to cause ototoxicity. What is the mechanism of antibiotic X?
- A. It binds the 50S ribosomal subunit and inhibits formation of the initiation complex
- B. It binds the 30S ribosomal subunit and inhibits formation of the initiation complex (Correct Answer)
- C. It binds the 30S ribosomal subunit and reversibly inhibits translocation
- D. It binds the 50S ribosomal subunit and inhibits peptidyltransferase
- E. It binds the 50S ribosomal subunit and reversibly inhibits translocation
Streptococcus pneumoniae Explanation: ***It binds the 30s ribosomal subunit and inhibits formation of the initiation complex***
- The second antibiotic, X, is likely an **aminoglycoside**, such as **gentamicin** or **amikacin**, which are commonly used in combination with ampicillin for neonatal meningitis and are known to cause ototoxicity.
- Aminoglycosides exert their bactericidal effect by **irreversibly binding to the 30S ribosomal subunit**, thereby **inhibiting the formation of the initiation complex** and leading to misreading of mRNA.
*It binds the 50S ribosomal subunit and inhibits formation of the initiation complex*
- This mechanism is characteristic of **linezolid**, which targets the 50S ribosomal subunit to prevent the formation of the initiation complex.
- While linezolid can cause side effects, **ototoxicity** is less commonly associated with it compared to aminoglycosides, and it is not a primary drug for neonatal meningitis alongside ampicillin.
*It binds the 50S ribosomal subunit and inhibits peptidyltransferase*
- This is the mechanism of action for **chloramphenicol**, which inhibits **peptidyltransferase** activity on the 50S ribosomal subunit, preventing peptide bond formation.
- Although chloramphenicol can cause **ototoxicity** and **aplastic anemia**, its use in neonates is limited due to the risk of **Gray Baby Syndrome**.
*It binds the 30s ribosomal subunit and reversibly inhibits translocation*
- This describes the mechanism of action of **tetracyclines**, which reversibly bind to the 30S ribosomal subunit and prevent the attachment of aminoacyl-tRNA, thereby inhibiting protein synthesis.
- Tetracyclines are **contraindicated in neonates** due to their potential to cause **tooth discoloration** and **bone growth inhibition**, and ototoxicity is not their primary adverse effect.
*It binds the 50s ribosomal subunit and reversibly inhibits translocation*
- This mechanism of reversibly inhibiting translocation by binding to the 50S ribosomal subunit is characteristic of **macrolides** (e.g., erythromycin, azithromycin) and **clindamycin**.
- While some macrolides can cause **transient ototoxicity**, they are not typically the second antibiotic of choice for neonatal meningitis in combination with ampicillin, and clindamycin's side effect profile is different.
Streptococcus pneumoniae US Medical PG Question 5: A 44-year-old woman with type 2 diabetes mellitus comes to the physician with a 3-day history of fever, right calf pain, and swelling. Her temperature is 38.7°C (101.7°F). Physical examination shows a 5 x 6-cm erythematous, warm, raised skin lesion with well-defined margins over the right upper posterior calf. The organism isolated from the lesion forms large mucoid colonies on blood agar. Further evaluation shows that the organism has a thick hyaluronic acid capsule. The causal organism of this patient's condition is most likely to have which of the following additional characteristics?
- A. Solubility in bile
- B. Resistance to optochin
- C. Positive coagulase test
- D. Susceptibility to bacitracin (Correct Answer)
- E. Negative pyrrolidonyl arylamidase test
Streptococcus pneumoniae Explanation: ***Susceptibility to bacitracin***
- This patient presents with **fever**, **right calf pain and swelling**, and a **well-defined erythematous, raised lesion (erysipelas)** with **large mucoid colonies** and a **thick hyaluronic acid capsule**, all characteristic features of **Group A Streptococcus (GAS)**, specifically *Streptococcus pyogenes*.
- *S. pyogenes* is characteristically **susceptible to bacitracin** (zone A disk), which is the classic laboratory test used to differentiate Group A Strep from other beta-hemolytic streptococci.
- The **bacitracin susceptibility test** is highly specific for presumptive identification of GAS in clinical microbiology laboratories.
*Solubility in bile*
- **Bile solubility** is characteristic of *Streptococcus pneumoniae*, which lyses in the presence of bile salts.
- *S. pyogenes* is **not bile-soluble**.
*Resistance to optochin*
- While *S. pyogenes* is **resistant to optochin**, this characteristic is primarily used to differentiate *S. pneumoniae* (susceptible) from other alpha-hemolytic streptococci (resistant).
- This is not the primary distinguishing test for GAS identification in the context of beta-hemolytic streptococci.
*Positive coagulase test*
- A **positive coagulase test** is characteristic of *Staphylococcus aureus*, not streptococci.
- *S. pyogenes* is **coagulase-negative**.
*Negative pyrrolidonyl arylamidase test*
- This is **incorrect**. *Streptococcus pyogenes* is actually **PYR-POSITIVE**, not PYR-negative.
- The **positive PYR test** is one of the key biochemical tests used to identify GAS, along with bacitracin susceptibility.
- A negative PYR test would suggest a different organism, not *S. pyogenes*.
Streptococcus pneumoniae US Medical PG Question 6: A 71-year-old woman presents with high-grade fever and chills, difficulty breathing, and a productive cough with rust-colored sputum. She complains of a sharp left-sided chest pain. Physical examination reveals increased fremitus, dullness to percussion, and bronchial breath sounds on the lower left side. A chest X-ray shows left lower lobe consolidation. The offending organism that was cultured from the sputum was catalase-negative and had a positive Quellung reaction. The organism will show which gram stain results?
- A. Gram-negative diplococci
- B. Cannot be seen with gram staining since the organism lacks a cell wall
- C. Gram-positive cocci in clusters
- D. Gram-negative rod
- E. Gram-positive diplococci (Correct Answer)
Streptococcus pneumoniae Explanation: ***Gram-positive diplococci***
- The clinical presentation (high fever, chills, productive cough with **rust-colored sputum**, sharp chest pain, signs of **consolidation**) is classic for **pneumococcal pneumonia**.
- The organism responsible for pneumococcal pneumonia, *Streptococcus pneumoniae*, is a **Gram-positive, catalase-negative diplococcus** that exhibits a **positive Quellung reaction** due to its polysaccharide capsule.
*Gram-negative diplococci*
- This describes organisms such as **Neisseria meningitidis** or **Neisseria gonorrhoeae**, which cause meningitis or gonorrhea, respectively, not typical pneumonia.
- While *Moraxella catarrhalis* is a Gram-negative diplococcus that can cause respiratory infections, it typically causes otitis media or sinusitis and less commonly severe pneumonia with rust-colored sputum.
*Cannot be seen with gram staining since the organism lacks a cell wall*
- This description typically refers to **Mycoplasma pneumoniae**, which causes **atypical pneumonia** and lacks a cell wall, rendering it unstainable by Gram stain.
- Mycoplasma pneumonia usually presents with a more indolent course, a non-productive cough, and rarely causes rust-colored sputum or lobar consolidation seen on X-ray.
*Gram-positive cocci in clusters*
- This morphology is characteristic of **staphylococci**, such as *Staphylococcus aureus*, which can cause pneumonia, often in immunocompromised individuals or as a complication of influenza.
- However, *Staphylococcus aureus* is **catalase-positive**, and its pneumonia presentation can be more fulminant, often leading to abscess formation, differing from the typical presentation of pneumococcal pneumonia.
*Gram-negative rod*
- This morphology is characteristic of various bacteria including **Klebsiella pneumoniae**, **Pseudomonas aeruginosa**, or **Haemophilus influenzae**.
- **Klebsiella pneumoniae** can cause severe pneumonia with **currant jelly sputum** but is a Gram-negative rod and would not exhibit a Quellung reaction in the same manner as *S. pneumoniae*.
Streptococcus pneumoniae US Medical PG Question 7: A 46-year-old woman presents to her primary care physician with complaints of increasing left upper quadrant discomfort. She has a known history of type 1 Gaucher disease. On physical examination, her spleen is palpable 8 cm below the costal margin. Routine laboratory work reveals severe pancytopenia. After consultation with the patient on the risks of her condition, the patient decides to undergo a splenectomy. Which of the following is more likely to occur as a consequence of splenectomy in this patient?
- A. Thrombocytopenia
- B. Leukopenia
- C. Pneumococcal septicemia (Correct Answer)
- D. Staphylococcal septicemia
- E. Anemia
Streptococcus pneumoniae Explanation: ***Pneumococcal septicemia***
- Patients who undergo splenectomy are at significantly increased risk of **overwhelming post-splenectomy infection (OPSI)**, particularly from **encapsulated bacteria** like *Streptococcus pneumoniae*.
- The spleen plays a crucial role in filtering encapsulated bacteria and producing opsonizing antibodies, and its removal compromises this immune function.
*Thrombocytopenia*
- **Thrombocytopenia** is typically a symptom *before* splenectomy in Gaucher disease due to hypersplenism.
- After splenectomy, the platelet count often **increases**, not decreases, due to the removal of the organ that sequesters platelets and destroys them.
*Leukopenia*
- **Leukopenia** (specifically neutropenia) is a pre-existing condition in severe Gaucher disease due to hypersplenism and bone marrow involvement.
- Post-splenectomy, the white blood cell count, particularly neutrophils, generally **increases** as the sequestration and destruction in the spleen are eliminated.
*Staphylococcal septicemia*
- While *Staphylococcus* can cause serious infections, it is **not the primary pathogen** associated with OPSI in asplenic patients.
- Encapsulated bacteria like *Streptococcus pneumoniae* are the most common and dangerous cause of post-splenectomy sepsis.
*Anemia*
- **Anemia** is a common finding in Gaucher disease due to hypersplenism and bone marrow infiltration.
- Splenectomy typically **improves** anemia by removing the site of red blood cell destruction and reducing abnormal cytokine production that inhibits erythropoiesis.
Streptococcus pneumoniae US Medical PG Question 8: A 6-year-old boy and his parents present to the emergency department with high-grade fever, headache, and projectile vomiting. The boy was born at 39 weeks gestation via spontaneous vaginal delivery. He is up to date on all vaccines and is meeting all developmental milestones. Past medical history is noncontributory. He has had no sick contacts at school or at home. The family has not traveled out of the area recently. He likes school and playing videogames with his younger brother. Today, his blood pressure is 115/76 mm Hg, heart rate is 110/min, respiratory rate is 22/min, and temperature is 38.4°C (101.2°F). On physical exam, the child is disoriented. Kernig’s sign is positive. A head CT was performed followed by a lumbar puncture. Several aliquots of CSF were distributed throughout the lab. Cytology showed high counts of polymorphs, biochemistry showed low glucose and elevated protein levels, and a gram smear shows gram-positive lanceolate-shaped cocci alone and in pairs. A smear is prepared on blood agar in an aerobic environment and grows mucoid colonies with clearly defined edges and alpha hemolysis. On later evaluation they develop a ‘draughtsman’ appearance. Which one of the following is the most likely pathogen?
- A. Streptococcus pneumoniae (Correct Answer)
- B. Staphylococcus aureus
- C. Neisseria meningitidis
- D. Staphylococcus epidermidis
- E. Streptococcus agalactiae
Streptococcus pneumoniae Explanation: ***Streptococcus pneumoniae***
- The CSF findings of **high polymorphs**, **low glucose**, and **elevated protein** are classic for bacterial meningitis. The Gram stain showing **Gram-positive, lanceolate-shaped cocci in pairs** is highly characteristic of *Streptococcus pneumoniae*.
- The growth of **mucoid colonies** with **alpha hemolysis** on blood agar in an aerobic environment, which later develop a **'draughtsman' appearance**, are further confirmatory characteristics of *S. pneumoniae*.
*Staphylococcus aureus*
- *Staphylococcus aureus* is a Gram-positive coccus but typically presents in **clusters** on Gram stain, not lanceolate pairs.
- While it can cause meningitis, it usually exhibits **beta-hemolysis** and is catalase-positive, unlike *S. pneumoniae*.
*Neisseria meningitidis*
- *Neisseria meningitidis* is a **Gram-negative diplococcus**, which would appear distinctly different on Gram stain compared to the described Gram-positive lanceolate cocci.
- Although it is a common cause of bacterial meningitis, its colonial morphology and Gram stain characteristics do not match the case.
*Staphylococcus epidermidis*
- *Staphylococcus epidermidis* is a **Gram-positive coccus in clusters**, similar to *S. aureus*, and is commonly a **skin commensal** or found in infections related to indwelling devices.
- It typically exhibits **gamma-hemolysis** (non-hemolytic) and is usually **coagulase-negative**, differentiating it from the alpha-hemolytic, 'draughtsman' appearing colonies described.
*Streptococcus agalactiae*
- *Streptococcus agalactiae* (**Group B Streptococcus**) is a Gram-positive coccus that typically grows in **chains** and causes **beta-hemolysis**, particularly in neonates.
- While it can cause meningitis, its characteristic hemolytic pattern and arrangement on Gram stain differ from the alpha-hemolytic, lanceolate-shaped cocci in pairs described.
Streptococcus pneumoniae US Medical PG Question 9: A 35-year-old man comes to the emergency department with fever, chills, dyspnea, and a productive cough. His symptoms began suddenly 2 days ago. He was diagnosed with HIV 4 years ago and has been on triple antiretroviral therapy since then. He smokes one pack of cigarettes daily. He is 181 cm (5 ft 11 in) tall and weighs 70 kg (154 lb); BMI is 21.4 kg/m2. He lives in Illinois and works as a carpenter. His temperature is 38.8°C (101.8°F), pulse is 110/min, respirations are 24/min, and blood pressure is 105/74 mm Hg. Pulse oximetry on room air shows an oxygen saturation of 92%. Examinations reveals crackles over the right lower lung base. The remainder of the examination shows no abnormalities. Laboratory studies show:
Hemoglobin 11.5 g/dL
Leukocyte count 12,800/mm3
Segmented neutrophils 80%
Eosinophils 1%
Lymphocytes 17%
Monocytes 2%
CD4+ T-lymphocytes 520/mm3(N ≥ 500)
Platelet count 258,000/mm3
Serum
Na+ 137 mEq/L
Cl- 102 mEq/L
K+ 5.0 mEq/L
HCO3- 22 mEq/L
Glucose 92 mg/dL
An x-ray of the chest shows a right lower-lobe infiltrate of the lung. Which of the following is the most likely causal organism?
- A. Streptococcus pneumoniae (Correct Answer)
- B. Legionella pneumophila
- C. Pneumocystis jirovecii
- D. Staphylococcus aureus
- E. Cryptococcus neoformans
Streptococcus pneumoniae Explanation: ***Streptococcus pneumoniae***
- This patient presents with **fever, chills, productive cough, dyspnea, leukocytosis with neutrophilia, and a lobar infiltrate on chest X-ray**, which are classic signs of **community-acquired bacterial pneumonia**.
- Although the patient is **HIV-positive**, his CD4+ count is >500/mm3 and he is on antiretroviral therapy, indicating relatively preserved immune function, making *S. pneumoniae* the most common cause of pneumonia even in HIV-infected individuals with controlled disease.
*Legionella pneumophila*
- While *Legionella* can cause pneumonia with fever and dyspnea, it is often associated with **gastrointestinal symptoms** (e.g., diarrhea) and **hyponatremia**, which are not present here.
- Exposure to contaminated water sources is a common risk factor, and the lobar infiltrate is less typical than diffuse or patchy infiltrates.
*Pneumocystis jirovecii*
- *Pneumocystis pneumonia (PJP)* is typically seen in **HIV patients with severely suppressed immune systems (CD4+ count <200/mm3)**.
- The patient's CD4+ count (520/mm3) is above this threshold, and PJP usually presents with diffuse interstitial infiltrates rather than a lobar infiltrate.
*Staphylococcus aureus*
- *S. aureus* pneumonia often occurs in the context of recent **influenza infection, intravenous drug use, or hospitalization**, or can present rapidly with **necrotizing pneumonia** or **empyema**.
- While possible, the absence of these specific risk factors or severe features makes it less likely than *S. pneumoniae* in this specific presentation.
*Cryptococcus neoformans*
- *Cryptococcus neoformans* is an opportunistic fungus that typically causes **pulmonary or central nervous system infections**, especially in severely immunocompromised patients (CD4+ count usually <100/mm3).
- Pulmonary cryptococcosis often manifests as **nodules or cavitary lesions**, or can be asymptomatic, which differs from the acute lobar pneumonia presented.
Streptococcus pneumoniae US Medical PG Question 10: While testing various strains of Streptococcus pneumoniae, a researcher discovers that a certain strain of this bacteria is unable to cause disease in mice when deposited in their lungs. What physiological test would most likely deviate from normal in this strain of bacteria as opposed to a typical strain?
- A. Quellung reaction (Correct Answer)
- B. Hemolytic reaction when grown on sheep blood agar
- C. Bile solubility
- D. Optochin sensitivity
- E. Motility
Streptococcus pneumoniae Explanation: ***Quellung reaction***
- The **Quellung reaction** tests for the presence of the **polysaccharide capsule**, which is the primary virulence factor of *S. pneumoniae*.
- An **avirulent strain** that cannot cause disease would most likely lack the capsule and show a **negative Quellung reaction** (no capsular swelling), deviating from the **positive reaction** seen in typical encapsulated pathogenic strains.
- The capsule enables *S. pneumoniae* to evade phagocytosis and complement-mediated killing, which is essential for establishing infection in the lungs.
*Hemolytic reaction when grown on sheep blood agar*
- Both virulent and avirulent strains of *S. pneumoniae* typically exhibit **alpha-hemolysis** (partial hemolysis, producing a greenish discoloration) on sheep blood agar due to the production of pneumolysin.
- This characteristic does not differentiate between pathogenic and non-pathogenic strains in terms of disease-causing ability.
*Bile solubility*
- *S. pneumoniae* is characteristically **bile-soluble** due to the presence of autolysin enzymes that are activated by bile salts, leading to cellular lysis.
- This property is a **species characteristic** present in both virulent and avirulent strains, thus it would not explain the inability to cause disease.
*Optochin sensitivity*
- *S. pneumoniae* is universally **sensitive to optochin**, a chemical agent that inhibits its growth and is used for laboratory identification.
- This characteristic is used for **species identification** but does not correlate with strain virulence or disease-causing ability.
*Motility*
- *Streptococcus pneumoniae* is a **non-motile** bacterium; it lacks flagella.
- This characteristic is consistent across all strains and is not a virulence factor for this species.
More Streptococcus pneumoniae US Medical PG questions available in the OnCourse app. Practice MCQs, flashcards, and get detailed explanations.
