Fig 1. F1 capsule production.
(A) V674, V674pF1, and VTnF1 bacteria grown at 37°C were re-suspended in India ink and observed by microscopy. (B) To evaluate the stability of F1 production, isolated colonies of Y. pestis CO92, or Y. pseudotuberculosis V674pF1, and VTnF1 were obtained after three subcultures in vitro. Isolated VTnF1 colonies were also obtained by culturing an homogenate of Peyer’s patches taken from mice previously inoculated with VTnF1 (108 CFU i.g.; noted "in vivo"). Serial dilutions of bacteria were tested using an F1-specific ELISA and the F1 index shown was calculated as 1/CFU yielding DO450nm = 1 in the ELISA. The un-encapsulated V674 was used as a negative control. (C) F1 production by VTnF1 was compared to that of Y. pestis CO92 after growth at 28°C or 37°C, using an F1-specific ELISA, and CO92Δcaf was used as negative control.