Introduction Brain abscesses remain common among HIV-infected immunocompromised patients. In addition to the pyogenic organisms found in the healthy host, toxoplasma gondii, mycobacteria, fungi, protozoa, and Listeria species have all been reported as causative pathogens. We report here an unusual case of a brain abscess caused by Streptococcus acidominimus in an HIV-infected patient that partially responded to initial antimicrobial treatment. Case A 29-year-old Hispanic female with AIDS (CD4 18cells/μl and viral load 34 350 copies/ml) not on highly active antiretroviral treatment (HAART) and an active history of IV heroin abuse was found disoriented and confused. A brain CT scan revealed an irregularly shaped ring-enhancing lesion in the left frontal lobe with extensive vasogenic edema producing left-to-right shift (Fig. 1a). The immunoglobulin G (IgG) antitoxoplasma antibody was negative. The patient was sent to the operating room for emergent craniotomy and brain biopsy during which, a pocket of pus was found and the fluid was drained and sent for culture. As the finding was believed to be consistent with a pyogenic brain abscess, the patient was placed on empiric antibiotics including vancomycin, cefepime, and metronidazole, pending further microbiologic data. Postoperatively the patient was aphasic and right-sided hemiparesis was notable in the neurologic examination. Final cultures of the aspirate yielded Streptococcus acidominimus and her antibiotic regimen was changed to ceftriaxone and metronidazole for a total of 8 weeks of treatment.Fig. 1: MRI series of the pyogenic brain abscess in toxoplasmosis lesion in a patient with AIDS.Seven days after neurosurgery the patient was awake, alert and cooperative with a marked improvement of her neurologic signs (normal speech, hemiparesis resolved). Brain MRI at weeks 1, 2 and 3 (Fig.1b–e) after the initiation of treatment did not show any decrease in the abscess size. However, the edema and midline were significantly diminished. At this point the immunoglobulin M (IgM) antitoxoplasma antibody was positive and therapy for Toxoplasma gondii encephalitis started. Two weeks after the initiation of pyrimethamine, sulfadiazine and leukovorin the brain MRI revealed significant decrease of abscess size (Fig. 1f). Discussion The incidence of focal neurological diseases in patients AIDS has decreased after the introduction of the highly active antiretroviral therapy [1]. The initial approach to management of expansive brain lesions in HIV-infected patients include considering the diagnosis of cerebral toxoplasmosis [2], the most common cause of intracranial mass lesion in HIV-infected patients. However, the differential diagnosis should also include viridans streptococci, methicillin-resistant Streptococcus aureus, Gram-negative bacilli, listeria monocytogenes, Mycobacterium tuberculosis, Nocardia and Rhodococcus species. Differently from cerebral toxoplasmosis, brain abscesses are usually characterized by a peripheral uniform ring enhancement after the injection of contrast, a central high signal on T2-weighted and markedly hyperintense signal on DWI, indicating restricted diffusion [3]. Conclusion Streptococcus acidominimus is a Gram-positive facultative anaerobic organism of the Strepotococcus viridans group, which includes oropharyngeal commensals that have been cultured on dental plaque [4]. As is the case for 20% of patients with brain abscesses, the primary source of infection in our patient's case remained cryptogenic despite a full investigation. On further questioning, however, the patient described the routine of holding the needle in her teeth just prior to injecting heroin. We hypothesize that transient bacteremia from the intravenous injection lead to the bacterial seeding of a preexisting brain lesion. However, initial antibiotics alone did not decrease the lesion. Serum toxoplasma IgG can be negative in 5% of patients with end stage AIDS. It was only after treating for toxoplasmosis that our patient's lesion finally resolved.