The use of two-dimensional gradient plates to investigate the range of conditions under which conjugal plasmid transfer occurs.

Gel-stabilized two-dimensional gradient plates were used to study the effects of pH, salt concentration and temperature on the conjugal transfer of plasmid RP4 between strains of Escherichia coli and Pseudomonas putida. The combinations of pH and salt concentration that permitted conjugation were mapped as a two-dimensional growth area occupied by transconjugants following conjugation. This conjugation domain was less extensive than the areas that supported growth of the parental strains, and showed evidence for the interactive effects of pH and salt concentration in determination of conditions that permitted conjugation. The size and shape of the conjugation domain was influenced by time, temperature, the identities of the donor and recipient bacteria, and the combination of donor and recipient bacteria.