FIG. 6.

V. cholerae classical strain 569B does not produce extracellular proteases and does not provide a growth advantage to El tor N16961. (A) BSA (2 μg; lane a) was incubated with 4 μl (lane b) or 8 μl (lane c) of 20-h culture filtrate of classical strain 569B for 60 min and analyzed by SDS-PAGE. (B) The El Tor N16961 and classical 569B strains were grown separately for 24 h and then mixed at a ratio of 1:1. The individual cultures and the mixed culture were incubated for a further 9 h, and the numbers of CFU of each strain in the individual and mixed cultures were determined at regular intervals. Circles and squares represent the number of CFU per ml of each strain in individual cultures and mixed cultures, respectively. (C) Growth of 20-h El Tor N16961 cells in 20-h culture filtrates from strain 569B. The number of CFU of N16961 suspended in N16961 (a), 569B (b), or O395 (c) culture filtrate or proteinase K (20 μg per ml)-treated culture filtrate of strain N16961 (d) or 569B (e) was determined after 6 h. Growth yields were calculated by dividing the number of cells after 6 h by the number of cells at inoculation. Data represent the average of three independent experiments; error bars indicate standard error of the means.