A) Scheme showing the conjugative transmission of a plasmid (red circle) in a population of susceptible cells under Density-Dependent Transmission (DDT). In this regime, conjugation (black arrows) occurs when a plasmid containing cell contacts a plasmid-free one. The number of transconjugant cells formed is thus equivalent to the number of Donor:Recipient encounters. The conjugation frequency thus increases linearly with increasingly available recipients. B) Scheme showing the conjugative transmission of a plasmid (red circle) in the same population of susceptible cells as in A), but under Frequency-Dependent Transmission (FDT). In this regime, conjugation (black arrows) is limited by the time required to complete the transfer event, thus not all Donor:Recipient contacts result in successful Transconjugant formation. The conjugation frequency is thus constant, regardless of the number of available recipients. C) Ideal number of transconjugants obtained per donor (y axis, T/D) as a function of the total cell concentration (x axis, N). In a Holling´s Type II functional response there is a density-dependent region where transconjugant formation increases proportionally with N (leftwards from the blue line). When N becomes saturating the formation of transconjugants does not scale any further with cell density and becomes frequency-limited (rightwards from the blue line). D, E, F) Transconjugants per donor ([T]/[D], y axis) obtained at different cell densities ([N], x axis) in conjugations using E. coli BW27783, on solid LB-agar surfaces matings and 1 h. conjugation time. Each graph corresponds to plasmids R388 (D), pKM101 (E) and RP4 (F). The black line represents the ideal DDT dynamics, while the red line represents the FDT.

Peer reviewed