Peculiar Case of Brain Abscess Caused by Propionibacterium acnes in an Immunocompetent Individual Without Prior Neurosurgical Intervention: A Case Report and Literature Review

Propionibacterium acnes (P. acnes) is a slow-growing, anaerobic, gram-positive bacillus that commonly colonizes the skin and is a rare cause of CNS infections. It was previously viewed as a culture contaminant but is now recognized to infrequently cause indolent cases of CNS infections. It is even more rarely associated with abscesses in patients without a prior history of neurosurgical intervention. Due to being a slow-growing bacteria, P. acnes is frequently discovered to be the causative organism after 16S rRNA sequencing. In this case, the culture was positive. There are only five other reported cases of patients with a P. acnes abscess without prior neurosurgical intervention. Here we present the sixth case of an immunocompetent young male who was found to have a P. acnes brain abscess.


Introduction
Inflammation of the brain can be due to infectious or noninfectious causes.Among infectious etiologies, the most common route of infection is through direct or indirect invasion of the blood-brain barrier.Infection and inflammation of the brain parenchyma can also lead to abscess formation.The rate of uncomplicated recovery for patients with brain abscesses has increased from 33% to 70% over the past five decades [1,2].Despite such a successful recovery rate, the mortality and morbidity of brain abscess patients are very high, and clinical optimization through quick and accurate diagnosis in a timely manner with proper therapeutic intervention is crucial.
The most common clinical presentation of patients with brain abscesses is a triad of fever, headache, and focal neurological symptoms.Most importantly, the focal neurological deficit can be the sole presentation of a neurological disorder coexisting with a brain abscess.Approximately 86% of patients with abscesses have predisposing conditions such as sinusitis, otitis media, mastoiditis, and meningitis.Neurological surgical interventions are the main cause of brain abscesses, but brain abscesses can also rarely develop in patients without any pre-existing risk factors [3,4].
In immunocompetent individuals, approximately 95% of brain abscess cases are usually polymicrobial and bacterial in origin.The most commonly identified bacterial microorganisms are streptococci (i.e., S. mitis, S. mutans, and S. salivarius), staphylococci (i.e., S. aureus), anaerobes (i.e., actinomyces, Bacteroides), and Enterobacteriaceae.Empirical antibiotic treatment with third-generation cephalosporins and metronidazole is the standard of care.Neurosurgical interventions in the form of abscess aspiration and placement of an external ventricular drain, collection of biopsy fluid, and biopsy of brain parenchyma for diagnostic analysis are helpful diagnostic and therapeutic interventions [5].

Case Presentation
A 21-year-old African American male with no significant past medical history (except migraine headaches) was transferred to the University of South Alabama Health Hospital emergency department with a chief complaint of left-sided weakness.The patient reported that he had been hit in the head two weeks ago with a wooden stick, was knocked off balance, and started having some headaches and left-sided weakness.On physical examination, the patient denies blurry vision, headache, nausea, vomiting, chest pain, diaphoresis, or confusion.Emergency laboratory evaluation revealed an elevated WBC count; otherwise, the remaining labs were unremarkable.Workups for sexually transmitted diseases (STDs), including HIV, rapid plasma reagin (RPR), and hepatitis, were negative.Neurological examination revealed 4/5 grip strength (grips) in A CT of the head without contrast revealed a right periventricular white matter mass measuring 1.2 cm in greatest dimensions.The mass was peripherally hyperdense with an extensive amount of surrounding vasogenic edema, most suggestive of an intracranial abscess.MRI of the brain with intravenous gadolinium revealed a peripherally ring-enhancing lesion in the right corona radiata adjacent to the cingulate gyrus measuring 1.6 x 2.1 cm with continuous ring enhancement.The enhancing portion of the mass measured approximately 0.4 cm.There is significant T2/fluid attenuated inversion recovery (FLAIR) hyperintensity representing edema, which extended into the right frontal, temporal, and parietal white matter.Edema also crossed the midline via the corpus callosum.Edema also crossed the midline via the corpus callosum with a stable 0.8 cm leftward midline shift (Figure 1).The patient was admitted to the neurosurgical intensive care unit (NSICU), and neurosurgery was consulted.
A right burr hole craniotomy for biopsy with stealth navigation was performed.Pathology department was consulted, and a frozen section was performed for intraoperative diagnosis to confirm malignancy versus abscess formation.Evaluation of the frozen section revealed minute fragments of predominantly necrotic tissue with a few atypical glial cells (Figure 2).No definitive malignancy was identified in the frozen section specimen, and more tissue was requested for permanent sections for further work-up.

FIGURE 2: Low-power view showing minute fragments of necrotic debris and a few atypical glial cells (A, 4x; B, 10x). High-power view showing predominantly necrotic debris (C, 20x; D, 40x).
Histologic examination of permanent sections revealed fragments of benign brain parenchyma with gliosis, macrophages, and a focal dense perivascular inflammatory infiltrate.Abundant amounts of necrotic tissue were also identified, and multiple immunohistochemistry stains were ordered to rule out an infectious etiology (Figure 3).

Discussion
Our patient was hospitalized with left upper extremity and left lower extremity hemiparesis, incoordination, and a headache.A CT scan revealed a 1.2 x 1 cm lesion with vasogenic edema in the right corona radiata, with spectroscopy findings concerning glioblastoma multiforme.The patient underwent a burr-hole craniotomy for a biopsy.Tissue culture was positive for P. acnes after five days.An attempt was made to have 16S rRNA sequencing on the specimen, but the specimen was determined to be inadequate.We do not believe this influenced our conclusion because the slow growth of the organism was consistent with P. acnes.Despite the fact that P. acnes is normal skin flora and a potential contaminant, the sequencing may have provided proof of the infection.The slow growth does not imply contamination or poor collection, as it might with other organisms.
In our literature review, we looked at other cases of P. acnes causing a CNS infection in patients without immunosuppression or prior neurosurgical intervention.In Table 1, we summarize the five cases of brain abscesses caused by P. acnes reported in the literature [5][6][7][8][9].Most of the case reports found by our initial literature review were consistent with patients who had prior neurosurgical interventions.One study that characterized the microbes mostly involved in post-neurosurgical intracranial infections found P. acnes to be the second most commonly isolated organism after Staphylococcus aureus, at 28.6% [10].The incidence of P. acnes CNS infections in patients without prior neurosurgical intervention has yet to be described due to its rarity.The mechanism by which the infection occurs after neurosurgery is thought to be direct infection, as P. acnes is part of normal skin flora.
extremities, 5/5 grip strength (grips) in the right upper extremities, 2/5 strength in the left lower extremity, and 5/5 strength in the right lower extremity.

FIGURE 1 :
FIGURE 1: A) MRI with intravenous gadolinium showed a lesion with significant vasogenic edema and midline shift; B) Post-craniotomy MRI showing residual lesion.

FIGURE 6 :
FIGURE 6: Culture media showing growth of Propionibacterium acnes.