A Case of Streptococcus iniae Bacteremia Misdiagnosed by Biochemical Testing, but Diagnosed Definitively by Mass Spectrometry and 16S rRNA Gene Sequencing

A 76-year-old man was admitted to our hospital with a history of fever and difficulty in moving. Blood culture at admission grew β-hemolytic streptococci, identified by biochemical testing (BD CRYSTAL GP) as Streptococcus porcinus. S. porcinus is a swine-derived streptococcal species, and has been reported as a causative bacterium of female genitourinary infection. The clinical background of our patient did not match with the detected bacterium, so further testing by MALDI-TOF MS and 16S rRNA gene sequencing was performed, which identified the bacterium as Streptococcus iniae. There are many case reports of cellulitis caused by this bacterium in persons cooking infected fish, and some reports of arthritis, endocarditis, and empyema. Reports of S. iniae from Japan are rare, and since this bacterium may not be included in some biochemical identification databases, it might not have been identified correctly. In cases with inconsistency between the culture-identified organisms and the clinical course, performing additional identification tests may be useful to obtain an accurate diagnosis.