Characterization of an F1 Deletion Mutant of Yersinia pestis CO92, Pathogenic Role of F1 Antigen in Bubonic and Pneumonic Plague, and Evaluation of Sensitivity and Specificity of F1 Antigen Capture-Based Dipsticks

We evaluated two commercial F1 antigen capture-based immunochromatographic dipsticks, Yersinia Pestis (F1) Smart II and Plague BioThreat Alert test strips, in detecting plague bacilli by using whole-blood samples from mice experimentally infected with Yersinia pestis CO92. To assess the specificities of these dipsticks, an in-frame F1-deficient mutant of CO92 (Delta caf) was generated by homologous recombination and used as a negative control. Based on genetic, antigenic/immunologic, and electron microscopic analyses, the Delta caf mutant was devoid of a capsule. The growth rate of the Delta caf mutant generally was similar to that of the wild-type (WT) bacterium at both 26 and 37 degrees C, although the mutant's growth dropped slightly during the late phase at 37 degrees C. The Delta caf mutant was as virulent as WT CO92 in the pneumonic plague mouse model; however, it was attenuated in developing bubonic plague. Both dipsticks had similar sensitivities, requiring a minimum of 0.5 mu g/ml of purified F1 antigen or 1 x 10(5) to 5 x 10(5) CFU/ml of WT CO92 for positive results, while the blood samples were negative for up to 1 x 10(8) CFU/ml of the Delta caf mutant. Our studies demonstrated the diagnostic potential of two plague dipsticks in detecting capsular-positive strains of Y. pestis in bubonic and pneumonic plague.