Background: For Yersinia pestis, Burkholderia pseudomallei, and Burkholderia mallei, conventional broth microdilution (BMD) is considered the gold standard for antimicrobial susceptibility testing (AST) and, depending on the species, requires an incubation period of 16-20 h, or 24-48 h according to the Clinical and Laboratory Standards Institute (CLSI) guidelines. After a diagnosis of plague, melioidosis or glanders during an outbreak or after an exposure event, the timely distribution of appropriate antibiotics for treatment or post-exposure prophylaxis of affected populations could reduce mortality rates.
Results: Herein, we developed and evaluated a rapid, automated susceptibility test for these Gram-negative bacterial pathogens based on time-lapse imaging of cells incubating in BMD microtitre drug panels using an optical screening instrument (oCelloScope). In real-time, the instrument screened each inoculated well containing broth with various concentrations of antibiotics published by CLSI for primary testing: ciprofloxacin (CIP), doxycycline (DOX) and gentamicin (GEN) for Y. pestis; imipenem (IPM), ceftazidime (CAZ) and DOX for B. mallei; and IPM, DOX, CAZ, amoxicillin-clavulanic acid (AMC) and trimethoprim-sulfamethoxazole (SXT) for B. pseudomallei. Based on automated growth kinetic data, the time required to accurately determine susceptibility decreased by ≥70% for Y. pestis and ≥ 50% for B. mallei and B. pseudomallei compared to the times required for conventional BMD testing. Susceptibility to GEN, IPM and DOX could be determined in as early as three to six hours. In the presence of CAZ, susceptibility based on instrument-derived growth values could not be determined for the majority of B. pseudomallei and B. mallei strains tested. Time-lapse video imaging of these cultures revealed that the formation of filaments in the presence of this cephalosporin at inhibitory concentrations was detected as growth. Other β-lactam-induced cell morphology changes, such as the formation of spheroplasts and rapid cell lysis, were also observed and appear to be strain- and antibiotic concentration-dependent.
Conclusions: A rapid, functional AST was developed and real-time video footage captured β-lactam-induced morphologies of wild-type B. mallei and B. pseudomallei strains in broth. Optical screening reduced the time to results required for AST of three Gram-negative biothreat pathogens using clinically relevant, first-line antibiotics compared to conventional BMD.
Keywords: Antimicrobial susceptibility testing; Cell morphology; Gram-negative biothreat agents.