The amount of donor DNA determines transformation frequencies. V. cholerae strains A1552 (WT; lanes 1-4) and A1552Δdns (5-8), respectively, were naturally transformed on crab shell fragments with increasing amounts of donor genomic DNA (gDNA) of strain A1552-LacZ-Kan. Amounts of donor gDNA provided: 0.2 μg (lanes 1 and 5), 2 μg (lanes 2 and 6), 20 μg (lanes 3 and 7) and 200 μg (lanes 4 and 8). Average of at least three independent experiments. Student's t test: * statistically significant difference between lowest and highest amount of donor gDNA (p < 0.05); ** statistically significant difference between wild-type and nuclease minus strain (p < 0.01).