Crecchio and Stotzky (1998) exposed chromosomal DNA and plasmids obtained from B. subtilis to humic acids and found that the DNA retained the ability to transform competent recipient cells of B. subtilis. Similarly, Nielsen et al. (2000a) reported that the presence of humic acids was non-inhibitory to natural transformation of Acinetobacter sp. cells in vitro. Moreover, incubation of chromosomal DNA in a solution of humic acids over 7 days did not substantially reduce the number of bacterial transformants obtained, as compared to solutions with pure DNA, indicating that humic acids do not negatively affect the transforming activity of extracellular DNA.
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