A 7 year old boy presents to the pediatrician because his mother noticed a "smoky" color to his urine. Upon questioning the mother, it is revealed that the child suffered a sore throat several weeks ago that was left untreated. Physical examination reveals hypeension and mild generalized edema. Urinalysis is significant for red blood cell casts. Which of the following accurately describes the microorganism responsible for this child's illness?
High-Yield Explanation
In this case, you need to figure out what disease the child has, what organism causes the disease, and which of the listed features is true of the microorganism. The disease in question is poststreptococcal glomerulonephritis, as evidenced by the smoky urine, hypeension, edema, and red blood cell casts in the urine sediment. The history of the prior sore throat is a tip-off that this is a nonsuppurative sequelae of an infection due to Streptococcus pyogenes (group A beta;-hemolytic Streptococci). You must remember the features of S. pyogenes. S. pyogenes can be differentiated from Streptococcus pneumoniae and viridans Streptococci by its hemolytic pattern; it is beta-hemolytic, while the others are alpha-hemolytic . It can be distinguished from the other beta-hemolytic Streptococci by its sensitivity to the a ntibiotic bacitracin. Other impoant things to remember about S. pyogenes are its many virulence factors, including M protein, antiphagocytic capsule, hyaluronidase, streptolysins O and S, and erythrogenic toxins. 2nd option is incorrect because Streptococci are catalase negative, in contrast to Staphylococci, which are catalase positive. 3rd option is incorrect because S. pyogenes is coagulase negative. (In fact, a coagulase test should NOT be done on a catalase-negative organism.) The coagulase test is an impoant means of differentiating Staphylococcus aureus, which are coagulase positive, from all other species of Staphylococcus, which are coagulase negative. Ref: Ray C.G., Ryan K.J. (2010). Chapter 25. Streptococci and Enterococci. In C.G. Ray, K.J. Ryan (Eds), Sherris Medical Microbiology, 5e.