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Pediatric Infectious Disease Journal:
Original Studies

Antimicrobial-Resistant Shigella sonnei: Limited Antimicrobial Treatment Options for Children and Challenges of Interpreting In Vitro Azithromycin Susceptibility

Jain, Sanjay K. MD*; Gupta, Amita MD†; Glanz, Brian BS‡; Dick, James PhD‡; Siberry, George K. MD, MPH*

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Background: Antimicrobial-resistant Shigella sonnei is a growing problem in the United States and poses treatment challenges particularly among children. Azithromycin is recommended as an alternative oral agent for shigellosis.

Methods: All isolates of Shigella submitted to Johns Hopkins clinical laboratory during the outbreak year (2002) were compared with a historical comparison group (1996–2000). Isolates were considered multiresistant if they were resistant to ampicillin and trimethoprim-sulfamethoxazole (TS). Selected outbreak and reference isolates were tested for azithromycin susceptibility by E-test, disk diffusion and broth dilution methods.

Results: Between 1996–2000, among the 111 isolates submitted, 63% were from pediatric patients; 63% of isolates were resistant to ampicillin and 12% to TS. In 2002, among the 205 isolates submitted, 82% were from pediatric patients; 91% isolates were resistant to ampicillin and 67% to TS. The proportion of multiresistant isolates increased from 6% in 1996 to 65% in 2002 (P < 0.05). Azithromycin susceptibility by E-test and disk diffusion demonstrated 2 zones of inhibition for S. sonnei. Interpretation using the inner zone resulted in higher MICs (minimal inhibitory concentration) compared with the outer zones by E-test (P < 0.0001) and disk diffusion (P < 0.0001).

Conclusions: With increasing interest in using azithromycin for shigellosis, clinical laboratories should be aware of the interpretation difficulty caused by the dual-zone phenomenon seen with E-test and disk diffusion methods for S. sonnei.

© 2005 Lippincott Williams & Wilkins, Inc.


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