In general, HGT occurs in bacteria through three main mechanisms: transformation, phage-mediated transduc- tion and conjugation (Thomas and Nielsen, 2005) . The likelihood of transformation was excluded based on our observation that use of DNase I in the filter mating assay did not abolish the transfer ability of 89K (Table 1) . Furthermore, no 89K transfer was detected when the donor culture supernatant was incubated with the recipient, which also ruled out the possibility of phage-mediated transduction (Table 1) . Therefore, conjugation is the most likely mechanism for inter-strain transfer of 89K.