Characterization of Insertion Sequence ISSau2 in the Human and Livestock-Associated Staphylococcus aureus

Mobile genetic elements play important roles in evolution and diversification of bacterial genomes. ISSau2 is 1660bp in length with terminal 5'-TG and CA-3' dinucleotides and has two overlapping reading frames orfA and orfB. It has been found in a wide range of S. aureus, such as HA-MRSA252, LGA251, MRSA S0385 and ED133. To determine distribution of ISSau2, 164 S. aureus isolates from milk samples of mastitic cows from our laboratory and all the S. aureus strains from the National Center for Biotechnology Information (NCBI) database were screened for the presence of ISSau2. Next, in order to explore a potential relationship among S. aureus ISSau2-containing strains and isolates, a relationship among 10 ISSau2-positive S. aureus isolates and 27 ISSau2-positive S. aureus strains was investigated by a phylogenetic analysis. These ISSau2 isolates and strains could be classified into four groups (A, B, C and D). The strains or isolates in Group D were all isolated from mammary glands, suggesting tissue specificity. All strains in Group B had an identical ISSau2 derivative, termed ISSau21628, with 32bp deletion at the 3' terminus. ISSau21628 in strain ST398 from Group B was closely related to ISSau2 in strain LGA251 from Group D.