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Pediatr Infect Dis J. 2013 Mar;32(3):296-8. doi: 10.1097/INF.0b013e31827c9796.

Group A streptococcus colonies from a single throat swab can have heterogeneous antimicrobial susceptibility patterns.

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Bacterial Genetics and Physiology Laboratory, Institut de Biologie et de Médecine Moléculaires, Faculté des Sciences, Belgium.


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.

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