FIG 1.

Characterization of Stx released by phage lysis. (A) Summary of regulation of Stx expression in E. coli. The A and B subunits of Stx are encoded on lysogenic bacteriophages. Stx is not expressed by lysogenic bacteria. DNA damage or stalled replication (e.g., via ciprofloxacin treatment) can activate the bacterial SOS response, resulting in cleavage of the phage repressor. Loss of repression leads to induction of the lytic cycle, during which new phage particles and Stx are released following phage-mediated lysis. (B) Western blot of purified Stx1 standards and three independent supernatants (s/n) from C600::H19B. (C) Western blot of purified Stx2a standards and three independent supernatants from C600::933W. (D) Amounts of toxin (means ± standard deviations; n = 3) in culture supernatants were determined from standard curve dilution series of purified Stx subunits. An A:B subunit ratio of 1:3 was obtained for Stx1, and a ratio of 1:8 was obtained for Stx2a.