[A case of osteomyelitis caused by mixed infection by Aggregatibacter aphrophilus and Staphylococcus aureus]

A 46-year-old man presented with a 10-day history of progressive weakness and numbness in both his legs. The spinal MRI and abdominal CT revealed discitis of L4/L5 and multiple epidural abscesses. The blood culture tested positive for methicillin sensitive Staphylococcus aureus (MSSA). The Gram stain of sample from abscess drainage showed Gram-positive cocci in clusters and small Gram-negative rods. The Gram-positive cocci in clusters were identified as MSSA using a general culture method and the small Gram-negative rods were identified as Aggregatibacter aphrophilus using a 16s ribosomal RNA sequencing method. The patient was treated four times with surgical abscess drainages and a long course of intravenous antibiotics therapy following which he recovered. No case of mixed infection by A. aphrophilus and S. aureus has been reported in MEDLINE; therefore, our case should be the first case report. Since we confirmed not only Gram-positive cocci in clusters but also Gram-negative rods in the abscess sample, we did not stop the identification procedure at the point when MSSA was identified and finally identified A. aphrophilus with 16s ribosomal RNA sequencing. Especially, in a mixed infection with organisms whose growth speed is different, an organism with slow growth might be missed if the Gram stain of sample were skipped. This case implies the significance of Gram staining as the identification procedure for organisms.