In vivo internalization of Staphylococcus aureus by embryonic chick osteoblasts

Staphylococcus aureus is the primary pathogen associated with osteomyelitis , an acute and recurrent bone disease. Internalization of S. aureus by cult ured embryonic chick calvarial osteoblasts has been observed. The purpose o f this study was to demonstrate that internalization of bacteria by embryon ic chick calvarial and tibial osteoblasts occurs in vivo. In initial experi ments, 10(8) colony forming units (cfu) of S. aureus, strain UAMS-1 or Cowa n, were injected subcutaneously under the scalp skin of 17 day chick embryo s. After 45 min, calvariae were harvested acid processed for transmission e lectron microscopy (TEM). In subsequent experiments, 10(9) cfu of UAMS-1 we re injected into the allantoic sac of 17 day chick embryos via a small open ing in the egg shell. After 48 h, calvariae and tibiae were harvested for T EM. S, aureus cells were found in approximately 14% of the calvarial osteob lasts after subcutaneous injection and in 11% of calvarial and tibial osteo blasts following intraallantoic injection. Endosomes were observed in some cells, but most bacteria internalized appeared to be free in the cytoplasm, Osteoblasts with as few as five bacteria had a greater loss of cytoplasmic integrity and a more heterochromatic nucleus than osteoblasts with fewer b acteria or than uninfected osteoblasts, S, aureus cells in calvariae and ti biae were also observed in the cytoplasm of approximately 4% of the osteocy tes in mineralized bone matrix. Thus, internalization of S, aureus by osteo blasts in vivo augments the previous observation in vitro, This study has a lso shown that osteoblasts with few bacteria continue differentiating into osteocytes. Results of these experiments support the hypothesis that intern alization of S, aureus by osteoblasts may play a role in the etiology of os teomyelitis. (C) 2000 by Elsevier Science Inc. All rights reserved.