Case B: LL is a 12-year-old female presenting to her pediatrician, complaining o

Question

Case B: LL is a 12-year-old female presenting to her pediatrician, complaining of sore throat and cough. She has had some hoarseness in her voice over the past few days and subjective sweats but no documented fever. She has a history of seasonal allergies in the fall, and takes loratidine (antiallergic) only during that season. Upon review of systems, she complains of isolated throat pain, without any running nose, sinus pressure, or headache. Her mother has been taking her temperature at home, and they have fluctuated from 97.8°F-99.2°F. Throat examination showed yellow pus dots in the back. Rapid Strep Antigen (RSA) Throat Culture Negative Pending 1. Which is the most likely pathogen causing this infecting? (1pt) 2. What is the most likely reason for which the RSA was negative? (1pt) What do you expect to observe in the throat culture that indicates the likely presence of this pathogen? (1pt) 3. 4. Which culture conditions are most appropriate to culture this microorganism? (1pt)

Explanation / Answer

1. The most likely pathogen causing this infection could be Group A beta hemolytic Streptococcus pyogenes

2. Rapid Strep Antigen Test is used to detect GAS(Group A specific antigens) of Group A beta hemolytic Strep pyogenes. The virulence factors to be detected are Capsule (which inhibits the phagocytosis in the host immune system), Enzymes (Streptokinase, Hyaluronidase, Streptolysin O) and toxins (Erythrogenic toxin, Exotoxin A and Exotoxin B)

Explanation for negative result: It is common in 3-8% of the cases which does not rule out the presence of Strep. Since, the test is based on the principle of antigen antibody reactions, The negative result may be attributed to the less number of organisms present in the swab that does not have enough specific antigens of carbohydrates of cell wall to get the positive result. Sometimes sampling error may also be present which results in negative result and hence, should always be confirmed with the culture test

3. Strep pyogenes exhibit betahemolytic colonies on Blood agar when incubated for 24 hrs in aerobic conditions at 37 Celsius.

The growth on solid media shows discoid colonies with a diameter of 1-2mm. They produce capsular material often giving rise to mucoid colonies. Clear hemolysis is seeen around the colonies when grown at 10% CO2 on blood agar after 24hrs. All these characteristics indicate the likely presence of pathogen

4. The medium should be enriched with blood or serum. Temperature should be 37 celsius and aerobic conditions with 10% CO2 is most appropriate for culturing Strep. Glucose or serum broth can also be used using similar conditions in which the pathogen shows granular turbidity with a powdery deposit and no pellicle is formed.

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