Conjugational junctions: morphology of specific contacts in conjugating Escherichia coli bacteria

F-plasmid-mediated bacterial conjugation was studied with hfr (traDts) and tra I mutant Escherichia coli donor strains. This allowed us to observe a statistically significant number of conjugation-specific contacts by video and electron microscopy. Single mating events between E. coli were observed in real time by video-enhanced light microscopy. Conjugation in vivo takes place by initial contact formation via pili, followed by direct and transient wall-to-wall contact, during which DNA is transferred and disaggregated. Electron microscopic observations of the contact zone between donor and recipient bacteria were made by thin sectioning of mating pairs that were arranged in monolayers. We defined the conjugation-specific contact found in stabilized mating pairs as the conjugational junction. Within this junction no specific substructure such as plasma bridges by fusion could be detected during transfer of DNA.