The phenomenon of staphylococcal interbacterial aggregate and net structures (SIAN) in community-associated methicillin-resistant Staphylococcus aureus CA-MRSA/J.

Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) usually causes skin and soft tissue infections (SSTIs), but occasionally causes invasive infections with a broad range of manifestations. Virulence factors and pathogenesis of CA-MRSA have been investigated in details with genotype ST8/SCCmecIVa (USA300), which first emerged in the United States. However, CA-MRSA evolves rapidly, with different clones dominating in different world regions; their pathogenesis remains unclear. CA-MRSA with genotype ST8/SCCmecIVl (CA-MRSA/J) emerged in 2003 in Japan, spreading widely with a fatal case. We have studied the genetic characteristics of CA-MRSA/J, and during the course of this study, we found that CA-MRSA/J has bacteriophage-like spikes with or without a hexagonal cap (spikes X and Xc). Here, we report that CA-MRSA/J strain NN55 has non-phage-like, one-μm-long/jerky spikes with or without a hexagonal cap (LSX/LSXc), and also that LSX/LSXc forms (staphylococcal) interbacterial aggregate/net structures (SIAN). Regarding the phenomenon of SIAN, NN55 first formed single short spike X, followed by multiple molecules of long and jerky LSX/LSXc, leading to the interbacterial construction of SIAN, in colonies with high cell densities. The LSX/LSXc and SIAN structures have not been reported in S. aureus. NN55 was invasive in a HEp-2 cell assay, exhibiting SIAN. The novel SIAN structures may be foci-skeletons or toxic aggregates in NN55's invasive infections. The phenomenon of SIAN suggests novel staphylococcal cell-surface dynamism, providing a new structure-and-function relationship model and advancing the understanding of CA-MRSA pathogenesis.