This study describes for the first time heterogeneity of antibiotic resistance profiles among group A Streptococcus isolates originating from a single throat swab in patients with acute pharyngitis. For each throat swab, 10 group A Streptococcus colonies were randomly selected from the primary plate and subcultured to a secondary plate. These isolates were characterized by various phenotypic and genotypic methods. Our results demonstrated that differing antibiotic resistance profiles were present in 19% of pediatric patients with acute pharyngitis before antimicrobial treatment. This heterogeneity likely resulted from horizontal gene transfer among streptococcal isolates sharing the same genetic background. As only a minority of colonies displayed antibiotic resistance among these heterogeneous samples, a classical diagnostic antibiogram would have classified them in most instances as “susceptible,” although therapeutic failure could be caused by the proliferation of resistant strains after initiation of antibiotic treatment.
From the *Bacterial Genetics and Physiology Laboratory, Institut de Biologie et de Médecine Moléculaires, Faculté des Sciences; †ENT Department, University Children’s Hospital Reine Fabiola; ‡Microbiology Department, Brugmann Hospital; §Paediatric Department, University Children’s Hospital Reine Fabiola, Université Libre de Bruxelles, Bruxelles, Belgium; ¶Department of Paediatrics, Centre for International Child Health, University of Melbourne, Royal Children’s Hospital; and ‖Murdoch Children Research Institute, Melbourne, Australia.
Accepted for publication October 25, 2012.
P.R.S. is a postdoctoral researcher from the Belgian research agency (Fonds National de Recherche Scientifique). The authors have no other funding or conflicts of interest to disclose.
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Address for correspondence: Pierre R. Smeesters, MD, PhD, Bacterial Genetics and Physiology Laboratory, Université Libre de Bruxelles, 12 Rue des Professeurs Jeener et Brachet, B: 6041 Gosselies, Belgium. E-mail: email@example.com.