Most of the time when I get consulted on a person with a bloodstream infection, the culprit organism is something totally ordinary like Staphylococcus aureus, E.coli, Streptococcus, etc.
Sometimes, however, an unexpected organism is isolated (usually in an immunocompromised patient). A few days ago, one such rare organism reared its head in one of my patients.
The patient I saw had a history of recurrent pancreatic cancer and had developed a liver abscess. Blood cultures were also positive and revealed...Clostridium cadaveris. Interestingly, it wasn't found in the abscess fluid.
Judging by its name, it's clear that this organism has some connection with death. In fact, it is one of the most prominent bacteria involved in the decomposition of human bodies after death--not exactly something you want to have in your bloodstream while you're alive. However, this bacteria is not usually a pathogen and lives symbiotically as part of the normal resident flora of the intestine.
Reports of true infections with this bacteria involve immunocompromised patients (pancreatic cancer certainly qualified my patient for that designation) but cases have occurred in the immunocompetent as well. Given the context of my patient's overall condition, this has to be treated as a true infection.
In a way, the practice of infectious disease can be thought of as managing the delicate balance in the human body between peaceful co-existence with its microbial flora (the microbiome) and preventing a microbial insurgency from occurring. During the 2008 IDSA meeting, the visionary Dr. Arturo Casadevall describe infectious disease physicians of the future as microbiome management experts who "protect the symbiont" (see his piece from 1996).