TABLE 2.

Distances between outer membranes^a

Donor × recipient	Mean distance between outer membranes at zone of closest approach (nm) ± SE	No. of junctions measured
E. coli HB101 (Tra1 + Tra2) × E. coli SCS1 (Rifr)	14 ± 1	32
E. coli HB101 (null) × E. coli SCS1 (Rifr)	35 ± 4	14
E. coli SCS1 (Rifr) × E. coli SCS1 (Rifr)	39 ± 4	15

^aThe distances between outer membranes in junction zones (or the region of closest proximity for bacteria in control samples) in RP4-mediated matings (Tra1 plus Tra2) versus plasmidless controls (null) or recipient × recipient “pseudo-matings.” Actively conjugating bacteria were tightly appressed, as evidenced by the small mean distance between outer membranes.