D 4 Non-Hemolytic Streptococcus pyogenes: Invasive Infection and Prospective Survey

CIMOLAI, N., B.C.’s Children’s Hospital, Vancouver, B.C.

Objective: The finding of a blood-borne infection that was caused by a non-hemolytic variant of S. pyogenes raised concern for undetectable streptococcal pharyngitides. A prospective survey was designed to investigate the presence of similar bacteria among throat specimens.

Method: Throat swabs were screened for S. pyogenes during a three month period with the use of anaerobic 5% sheep blood agar and aerobic DNase Methyl Green agar (DMGA). The latter detects DNase-producing colonies which include S. pyogenes. Confirmation and susceptibility testing of S. pyogenes was accomplished by routine methods.

Results: A 7 yr. old male developed fever and sore throat and was treated with erythromycin. Ten days later, pharyngitis recurred. Non-mucoid, non-hemolytic S. pyogenes was recovered from blood cultures, and the child recovered after clindamycin therapy. The bacterium was M4T4, OF+, erythromycin resistant, and yielded large zones of clearing on DNase Methyl Green agar. Although the initial throat swab did not reveal S. pyogenes, culture on DMGA yielded a noticeable heavy growth of the bacterium. During the study period, 99/404 (24.5%) of throat swabs yielded S. pyogenes. Two of these isolates were recovered on blood agar only (both light growths). One isolate was a non-hemolytic S. pyogenes (1/404;0.25%) which grew in heavy quantitation and was detectable only on DMGA; it was M non-typable, T4, OF+ and erythromycin susceptible. Six Group C (S. equisimilis) and Group G large colony beta-hemolytic streptococci were also reported; these were variably DNase positive. Retrospective review of non-hemolytic streptococci from invasive specimens did not find non-hemolytic S. pyogenes. 

Conclusion: Non-hemolytic S. pyogenes are likely to be under-recognized. Fortunately, non-hemolytic S. pyogenes are uncommonly found in routine specimens such as throat swabs.