Perhaps one of the most elegant demonstrations of
conjugation to the indigenous marine flora was the work of
Dahlberg et al. (1998). These investigators used the
conjugative plasmid pBF1 which contains the green
fluorescent protein (GFP) gene in Pseudomonas putida
as a donor. This gene is not expressed in the donor because
of a chromosomal repressor. Gene transfer was noted
when cultivated recipients were used as well as the natural
marine population. Transfer to the indigenous population
could be observed directly by epifluorescence microscopy,
without the need of cultivation of transconjugants. Transfer
frequencies ranged from 2x10-6 to 1.4 x10-4, with transfer
highest in filter mating compared to bulk matings. The
plasmid used also encoded for mercury resistance, but
selection in mercury did not facilitate or enhance transfer
frequencies. Thus, selective pressure may or may not
enhance transfer to the indigenous population.