A Case of Infective Endocarditis and Spinal Epidural Abscess Caused by Streptococcus mitis Bacteremia.

A 57-year-old man presented with abdominal pain, hematemesis, and melena. He reported taking high-dose ibuprofen for back pain and drinking several 24-ounce beers daily. Examination was remarkable for icteric sclera, poor dentition, tachycardia, and crescendo-decrescendo murmur at right upper sternal border, radiating to the carotids. Labs revealed leukocytosis, anemia, thrombocytopenia, and elevated liver function tests and INR. Endoscopy demonstrated antral ulcers, duodenitis, and esophagitis. Blood cultures were obtained and broad-spectrum antibiotics started; cultures later grew Streptococcus mitis, and antibiotic coverage was narrowed. Transthoracic echocardiogram (TTE) demonstrated aortic stenosis and regurgitation, but no vegetation. Repeat blood cultures were negative; however, the patient developed neurological symptoms concerning for cauda equina syndrome, and MRI revealed epidural abscess. Emergent decompression could not be performed as the patient developed hematemesis and required intubation. Transesophageal echocardiogram (TEE), initially deferred due to friable esophageal mucosa, was performed and revealed small aortic valve vegetation. Poor oral hygiene was felt to be the probable source of the patient's S. mitis bacteremia, epidural abscess, and infective endocarditis. The patient's neurological symptoms resolved without intervention and remaining teeth were extracted. This case demonstrates that Streptococcus mitis can result in clinically significant bacteremia, particularly in immunocompromised patients, including chronic heavy alcohol users.

Nematode-bacteria mutualism: Selection within the mutualism supersedes selection outside of the mutualism.

The coevolution of interacting species can lead to codependent mutualists. Little is known about the effect of selection on partners within verses apart from the association. Here, we determined the effect of selection on bacteria (Xenorhabdus nematophila) both within and apart from its mutualistic partner (a nematode, Steinernema carpocapsae). In nature, the two species cooperatively infect and kill arthropods. We passaged the bacteria either together with (M+), or isolated from (M-), nematodes under two different selection regimes: random selection (S-) and selection for increased virulence against arthropod hosts (S+). We found that the isolated bacteria evolved greater virulence under selection for greater virulence (M-S+) than under random selection (M-S-). In addition, the response to selection in the isolated bacteria (M-S+) caused a breakdown of the mutualism following reintroduction to the nematode. Finally, selection for greater virulence did not alter the evolutionary trajectories of bacteria passaged within the mutualism (M+S+ = M+S-), indicating that selection for the maintenance of the mutualism was stronger than selection for increased virulence. The results show that selection on isolated mutualists can rapidly breakdown beneficial interactions between species, but that selection within a mutualism can supersede external selection, potentially generating codependence over time.