It is now well established that PAIs comprise a large family of mobile genetic elements and may undergo horizontal transfer to a new host species (Sakellaris et al., 2004; Qiu et al., 2006; Coburn et al., 2007) . To determine whether the 89K PAI is transmissible from a donor S. suis strain containing 89K to a recipient strain without this element, we performed conjugative mating experiments. To obtain a marked version of 89K, a 3′ single cross-over mutant of strain 05ZYH33 constructed in our previous work was used as the donor, which was tagged with a spectinomycin marker located downstream of the SalK/R two-component system within 89K (Li et al., 2008) . We designated this derivative 05ZYH33-89KSpc and the modified 89K island as 89K*. SS2-N, a virulent strain of S. suis 2 that does not naturally contain 89K, was engineered to be chloramphenicol-resistant by replacing the recA gene with a cat gene cassette (creating SS2-NDrecA) and used as the recipient. Medium containing both chloramphenicol and spectinomycin was used to select exconjugants that received 89K*. In the filter mating assay, double antibioticresistant transconjugants were obtained, and transfer of 89K* was detected at a mean frequency of 5.4 ¥ 10 -6 per donor (Table 1) . Using specific primers, the integrated 89K* was detected in 10 randomly selected transconjugants, providing further evidence that 89K* was transferred into the recipients. Transfer of 89K* was also observed at a similar frequency by using other S. suis 2 strains as recipients, including the virulent S735 strain and the avirulent (DNase I) SS2-NDrecA 4.9 ¥ 10 -6 05ZYH33-89KSpc (supernatant) SS2-NDrecA < 10 -8 05ZYH33-89KSpc S735::cat 6.0 ¥ 10 -6 05ZYH33-89KSpc 7996::cat 2.5 ¥ 10 -6 05ZYH33DmobA89K SS2-NDrecA < 10 -8 CDmobA89K SS2-NDrecA 9.6 ¥ 10 -5 05ZYH33DmobC89K SS2-NDrecA