Nutritionally deficient Streptococcus: investigation of the hidden culprit in culture-negative endocarditis

Subacute bacterial endocarditis in a 72-year-old woman was found to be caused by nutritionally deficient ("satelliting") Streptococcus sanguis. The organism, which required pyridoxal compounds or thiol substitutes, was identified after the application of relatively unusual but simple laboratory procedures. Normally, the organism's nutritional requirements would be fulfilled by components of the human blood that are used to inoculate clinical blood cultures, provided that the dilutional effect of the media is less than 10. The difficulty of diagnosis in such cases appears to lie in the laboratory inability to identify the organisms, rather than in failure to grow in properly obtained blood cultures.