Association of LtxA with OMVs. (A) Western blot to detect LtxA on purified OMVs (lane 3) confirms that the vesicles contain full length LtxA (114 kDa). For comparison, purified LtxA is included in lane 2. (B) Western blot for LtxA comparing purified OMVs with the remaining supernatant (after ultracentrifugation). OMVs and OMV-free supernatant were diluted to the same volume to allow a direct comparison of the LtxA content in both fractions. Intact LtxA was detected in both the OMVs and OMV-free supernatant, with OMVs containing approximately one-third of the total secreted LtxA. (C) Dot blot comparisons of the LtxA concentration in OMVs and bacterial cells. The OMVs and bacteria were lysed then spotted on a nitrocellulose membrane at A280 values of 0.2, as marked. Standards of purified LtxA were also spotted at A280 values of 0.2, 0.1, 0.05, 0.025, 0.0125, and 0.00625. ImageJ was used to develop a standard curve of spot intensity versus concentration, which was then used to calculate the LtxA composition of the OMVs and cells.