Question

A 65-year-old male presents to his family physician with a rapid onset fever, chest pain and cough productive of rusty-yellow sputum. Chest X-ray shows focal lobar infiltrates. A Gram stain of a sputum sample contained many polymorphonuclear leukocytes and extracellular gram-positive diplococci. Capsule-specific antibodies bound to the diplococci resulted in a positive Quellung reaction. Which of the following is the most likely pathogen? A. Streptococcus pneumoniae B. Enterococcus faecium C. Streptococcus pyogenes D. Streptococcus agalactiae E. Enterococcus faecalis

Short answer

The most likely pathogen is Streptococcus pneumoniae, option A.

Step-by-step solution

Analyze Clinical Symptoms

Firstly, identify the symptoms exhibited by the patient: fever, chest pain and coughing up rusty-yellow sputum. These symptoms suggest a respiratory infection.

Interpret Diagnostic Findings

Look at the findings from the lab. The chest X-ray shows focal lobar infiltrates, which typically indicates pneumonia. The sputum sample contains many polymorphonuclear leukocytes, frequent indicators of a bacterial infection, and extracellular gram-positive diplococci. The positive Quellung reaction, which is used to identify encapsulated bacteria, suggests that our pathogen has a capsule.

Match Symptoms and Findings with Pathogens

Now, each of these clues can be paired with a particular bacterium. The presence of gram-positive diplococci, as well as a positive Quellung reaction, suggest Streptococcus pneumoniae as this bacterium commonly appears under a microscope as pairs of gram-positive cocci and has a capsule. Additionally, Streptococcus pneumoniae is the most common cause of community-acquired pneumonia, fitting with our patient's symptoms.

Choose the Right Option

Given all information, the most likely pathogen causing this infection, is Streptococcus pneumoniae. This corresponds to option A.

Key concepts

Gram stain

Gram staining is a simple yet powerful laboratory technique used to classify bacteria into two broad categories: Gram-positive and Gram-negative. Named after its inventor, Hans Christian Gram, this stain uses a violet dye to initially color bacteria.

When treated with a decolorizer, Gram-positive bacteria retain the violet color while Gram-negative bacteria do not, instead taking up a red counterstain. This difference is due to the structure of their cell walls.

• In our context, the Gram stain of the sputum sample revealed gram-positive diplococci.
• These are bacteria that appear as pairs and stain violet, indicating a thicker peptidoglycan layer in their cell walls.
• For the diagnosis at hand, this finding is consistent with Streptococcus pneumoniae, a known gram-positive bacterium.

The identification of gram-positive cocci is pivotal in determining the most likely causative bacterial pathogen.

pneumonia diagnosis

Diagnosing pneumonia involves both clinical evaluation and diagnostic imaging. Pneumonia, a lung infection that inflames air sacs, causes symptoms like fever, cough, and chest pain.

A chest X-ray can reveal infiltrates in the lungs, which are indicative of pneumonia.

• In the case presented, the X-ray showed focal lobar infiltrates, points often associated with lobar pneumonia.
• This pattern suggests an infection isolated within a lobe of the lung, commonly caused by bacterial pathogens like Streptococcus pneumoniae.

Additional tests such as sputum analysis add further detail to a pneumonia diagnosis by identifying the causative organism, as seen here with the detection of gram-positive diplococci.

Quellung reaction

The Quellung reaction is a laboratory test used to detect the presence of capsules in certain bacteria. "Quellung" means "swelling" in German, signifying the capsule's appearance when it becomes apparent under a microscope.

This test involves adding specific antibodies to a microbial preparation:

• If the bacterium has a capsule, such as Streptococcus pneumoniae, the antibodies bind to it.
• The capsule, which may be difficult to see otherwise, swells and becomes visible, indicating a positive reaction.

Since Streptococcus pneumoniae is known for its polysaccharide capsule, the positive Quellung reaction in our scenario strongly points to this bacterium.

lobar pneumonia

Lobar pneumonia is a type of pneumonia that affects a large and continuous area of a lobe in the lungs. Unlike other forms, which might spread throughout the lung, lobar pneumonia is often confined to a single lobe.

This condition is associated with the following characteristics:

• Smooth, dense infiltrates seen on a chest X-ray.
• Associated with symptoms such as coughing, fever, and rusty sputum.
• Streptococcus pneumoniae is the most common causative organism.

In our case study, the presence of focal lobar infiltrates on the X-ray, accompanied by the symptoms, aligns well with a diagnosis of lobar pneumonia caused by Streptococcus pneumoniae.

bacterial pathogens

Bacteria are among the various pathogens that can cause diseases in humans. Knowing the type of bacteria involved is crucial for correct diagnosis and treatment.

The patient exhibits infection signs suggestive of a bacterial origin:

• The symptoms include fever and cough with rusty-yellow sputum.
• The identification of gram-positive diplococci is specific to certain pathogenic bacteria.
• Streptococcus pneumoniae, a well-known bacterial pathogen, often resides harmlessly in the respiratory tract but can cause severe infections, like pneumonia, when the immune system is compromised.

Recognizing the bacterial pathogen involved helps direct therapy, ensuring that the treatment targets the specific organism responsible.