FIG 2.

Siderophore utilization bioassays. The indicated strains were seeded into LB agar containing the iron chelator EDDA. The siderophore or E. coli strain spotted on the agar is noted below each picture. The siderophore then diffuses onto the agar, and to give a positive signal in the assay, it must bind iron efficiently in competition with the chelator and be transported and used by the bacteria embedded in the agar. The growth around each siderophore was photographed after 24 h of incubation at 37°C. The numbers in parentheses are the average diameter in millimeters of the zone of growth ± standard deviation measured in at least three independent assays. The agar was seeded with the following strains: V. cholerae vibB mutant ALV101 (A), E. coli strain ARM110 (entF) or EWE1046 (entF fepA) (B), and V. cholerae strain ALV101 (vibB) carrying the plasmid pFepA, pFes1, or pFepA and pFes1 (C). The vibB (V. cholerae) and entF (E. coli) mutants were used to reduce the background growth in the assay.