Bacterial conjugation, mediated mainly by conjugative plasmids such as F, represents a potential health threat through the sharing of genes for pathogenicity or antibiotic resistance. Given that more than 400 different species of bacteria coinhabit a healthy human gut and more than 200 coinhabit human skin, Francisco Dionisio [(2002) Genetics 162:1525–1532] investigated the ability of plasmids to undergo between-species conjugal transfer. The following data are presented for various species of the enterobacterial genus Escherichia. The data are presented as 'log base 10' values; for example, -2.0 would be equivalent to 10⁻² as a rate of transfer. Assume that all differences between values presented are statistically significant. Donor _ Recipient E. chrysanthemi E. blattae E. fergusonii E. coli E. chrysanthemi -2.4 -4.7 -5.8 -3.7 E. blattae -2.0 -3.4 -5.2 -3.4 E. fergusonii -3.4 -5.0 -5.8 -4.2 E. coli -1.7 -3.7 -5.3 -3.5 In what species is within-species transfer most likely? In what species pair is between-species transfer most likely?