EVALUATION OF THE WISCAN HERMES MICROSCOPIC SCANNING SYSTEM FOR RAPID DETERMINATION OF ANTIBIOTIC SUSCEPTIBILITY OF YERSINIA PESTIS AND FRANCISELLA TULARENSIS

Yersinia pestis and Francisella tularensis, the etiologic agents of plague and tularemia respectively, are defined as category-A potential bio-weapons. Inhalation of aerosol droplets containing the pathogens leads to high mortality if not properly treated with antibiotics. Doxycycline and ciprofloxacin are the major recommended options for post-exposure prophylaxis against both pathogens. Antibiotic treatment must be started as soon as exposure is suspected, and bacterial susceptibility must be rapidly determined to verify proper treatment. The standard antimicrobial susceptibility testing (AST) method recommended by the Clinical and Laboratory Standard Institute (CLSI) is microdilution, in which growth inhibition in the presence of the antibiotics is determined by un-aided eye after 24 h for Y. pestis and 48 h for F. tularensis.

Many clinical laboratories analyze microdilution tests by automatic AST machines which monitor the change in optical density (OD) of the tested culture, using advanced growth curves analysis algorithms, resulting in a shorter time needed for an answer. In this study, we evaluated the microscopic scanning system (Wiscan Hermes) ability to shorten the duration time of the standard microdilution susceptibility assay for doxycycline and ciprofloxacin, compared to growth curves obtained by optical density reading in a plate reader (Tecan Infinite F200).

The results show that for F. tularensis, both OD measurements and the Wiscan Hermes system could not be used to shorten the standard 48 h incubation period. However, for Y. pestis, OD measurements may shorten the test duration from 24 h to 16-18 h. Further improvement may be achieved by the Wiscan Hermes system which was able to quantify the concentration of Y. pestis cultures yielding equivalent MIC values and allowing shortening of test duration to 10-12 h.