Interaction of Staphylococcus aureus cells and silk threads in vitro and in mouse skin.

Staphylococcus aureus cell suspension was epicutaneously inoculated on the back skin of cyclophosphamide-treated mice with silk stitches and these sites were occluded. Biopsy specimens were taken from three mice at 1, 3, 6, 12, 24, 48, and 72 h after inoculation and were examined by electron microscopy. Fibril-like structures (glycocalyx) were seen around the S. aureus cells at 1 h. At 3 h, they had extended towards the silk threads. There were microcolonies on the surfaces of the silk threads and at 12 h the S. aureus cells were enclosed in membrane-like structures. The electron density of the membrane-like structures increased over time. After ruthenium red staining, the membrane-like structures and the fibril-like structures were stained positive, suggesting that these structures contain polysaccharide components. With a combination chemotherapy using clarithromycin and ofloxacin, S. aureus cells in the membrane-like structures were degenerated, whereas the use of clarithromycin or ofloxacin alone had little effect. Chlorhexidin gluconate and povidone iodine were effective if they were able to reach the biofilm. The fibril-like structures appeared in vitro only in the presence of silk threads, and were enhanced by the presence of mouse plasma. These structures did not form with formaldehyde-killed S. aureus cells. Thus, S. aureus cells may interact with foreign bodies to form biofilms, thereby evading the effect of antibacterial agents.