T4SS-mediated transfer of 89K PAI in S. suis 2 1673 excision product. Detection of both integrated and circular 89K suggests that the populations of 05ZYH33 carrying this island are heterogeneous. Most cells carry an integrated island, while others contain an extrachromosomal circular one. Essentially, the same results were obtained for 98HAH12, another highly pathogenic S. suis 2 strain, which is predominantly responsible for the earlier 1998 STSS outbreak in China (data not shown).