Protein display for an OM-enriched sucrose density gradient fraction derived from stationary phase Y. pestis cells grown at 26°C. A crude membrane fraction was layered on top of a 3-step sucrose density gradient (15%, 54% and 70% w/V sucrose) and centrifuged at 105,000 × g for 17 h at 5°C. A whitish OM-enriched band was isolated, washed with a 2.5 M NaBr solution and extracted with 8 M urea/2 M thiourea/1% ASB-14 (usb-MBR fraction). Circa 120 μ g of the protein extract was analyzed in a 2D gel as described in the legend of Figure 3. OM proteins are denoted with their short names or gene locus tags.