FIG. 4.
Secretion of CTB mutants is not impaired in ETEC. (A) Representative immunoblot of TCA-precipitated total culture samples, adjusted for CFU, showing induced expression of WT CTB, CTB[Q3K], CTB[E11K], and CTB[E11A] in strain jf570. The blot was probed with anti-CT antibody. (B) Strains expressing WT CTB and the indicated mutant pentamers were fractionated to isolate cell-free supernatant (Sup't) and periplasm (Pp). Each fraction was tested for pentamer levels by GM1 ELISA, with WT levels set to 100%. Supernatant levels were normalized to CFU (n ≥ 3), and periplasm levels were normalized to alkaline phosphatase activity (n ≥ 2).