Detection and enumeration of viable but non-culturable transconjugants of Escherichia coli during the survival of recipient cells in river water

Viable but non-culturable transconjugant cells were detected by a modification of the direct viable count (DVC) method. This modification involved the addition of parental antimicrobial markers (kanamycin and streptomycin) to the elongation medium in order to promote selective elongation of the transconjugant cells. Presence of viable, other than culturable, transconjugants was demonstrated in matings with parental cells from TSB culture as well as with recipient cells from survival in river water (under illuminated and non-illuminated systems). In matings with a recipient strain from illuminated systems, culturable transconjugants were not detected after the third day of recipient cell survival. In spite of this, viable transconjugants were detected in numbers that exceeded 10⁵cells ml⁻¹. These results clearly show that a fraction of non-culturable recipient cells is able to receive and express plasmids by conjugation processes and form viable but non-culturable transconjugant cells.