Aerococcus Viridians: Under-Recognized Cause of Pyomyositis

Introduction: Aerococcal infection is a rarely described pathogen in the literature. Aerococcus species may be isolated from urine, blood or other sources. Several sites of infection caused by aerococcal infection have been reported including arthritis, UTI, endocarditis and meningitis. However, to our knowledge there is not a single case of soft tissue infection caused by this germ. Case presentation: We report a case of middle aged patient with multiple comorbidities, presenting with right supraclavicular mass following chest trauma. Imaging study showed multilobulated collection involving thoracic and neck muscles and invading the clavicle. Intraoperative drainage of the pus grew exclusively Aerococcus viridens. Conclusion: Aerococcus viridans is unusual cause of soft tissue infection, but should be considered in the differential diagnosis when evaluating patients with suspected deep soft tissue infection. A. viridens infection is underestimated because of technical difficulties in the identification of this organism. The introduction of matrixassisted laser desorption ionization time-of-flight mass spectrometry carried great success in accurate identification, as early identification of the offending organism guides physicians to appropriate treatment.

Patient presented to physician seeking for medical help and for evaluation of right supraclavicular mass. History goes back to 3 weeks prior to presentation when the patient sustained chest trauma in car accident. At that time patient underwent imaging studies to rule out fractures. No fractures or chest contusion was detected on standard chest X-ray. In the following days after trauma, patient started to sense unpleasant feeling at right subclavicular area associated with sensation of growing mass.
No documented fever or chills. No weight loss, nausea, vomiting or decreased per os intake.
Upon presentation patient was hemodynamically stable, afebrile and not in acute distress. Physical exam was non-contributory except for soft tender mass around the right clavicle with presence of crepitus, erythema and hotness.

Imaging studies
Chest X-ray did not reveal any abnormalities. So cervical and thoracic Computed tomography was ( Figure 1) done and showed following findings: • Multilobulated collection containing numerous pockets of air at neck, thoracic inlet and in the anterior chest wall invading the deepest aspect of the pectoralis major and minor muscles. • Lesion extends within the right lung apex along the first and second ribs 9 cm in length. • Infection extend within right anterior scalene muscle • Dislocation of right sternoclavicular joint with anterior displacement of the clavicle. • Irregular appearance of right clavicle, which suggests erosion due to infection or related to trauma.
CT scan findings were consistent with necrotizing skeletal muscle infection that necessitated urgent surgical intervention. Patient was started empirically on Ceftaroline 600 mg intravenously twice a day and clindamycin 600 mg intravenously thrice daily.   Few days later patient started to have clinical and laboratory improvement and draining tubes were removed. Inflammatory markers also went down.
On day 12, patient was discharged on Moxifloxacin 400 mg PO daily for six weeks. No occurrence observed after one year of follow up.

Discussion
In this article, we present the first case of Aerococcus viridians induced pyomyositis. To our knowledge, there is not a single reported case of pyomyositis caused primarily by A. viridans.
In the described clinical case, the development of soft tissue infection was preceded by chest wall trauma 3 weeks prior to diagnosis, but not by surgical intervention. It is well known that in 20-50% of cases there is history of trauma to affected muscle [1][2][3][4][5][6]. Our patient sustained trauma to anterior chest where exist ribs and intercostals muscles.
Interestingly, the clinical symptoms were disproportion to radiological findings. There was discrepancy between the large-sized infected necrotized mass as shown by thoracic CT scan and the nonspecific clinical symptoms. The development of infection was not associated with clinical signs of intoxication and no documented fever upon presentation or during hospitalization stay. Numerous predisposing factors may play a role in the development of this silent infection such as immunodeficiency disorders [7][8][9][10] and diabetes mellitus [11].
The presenting symptoms of our patient were nonspecific, especially in the absence of intoxication signs. In our case, many diseases share same presenting symptoms and clinical signs. Differential diagnosis includes: Pyomyositis, necrotizing soft tissue infection, infected hematoma, deep neck space infection and osteomylitis of the clavicle. Based on radiological findings, laboratory results and clinical picture, the most likely diagnosis was pyomyositis complicated with clavicle osteomyelitis.
Pyomyositis is defined as purulent infection of skeletal muscle that arises from hematogenous spread/or trauma and can be complicated with abscess formation [12]. Our patient has risk factors for pyomyositis including DM type II and trauma which is found in up to 50% of cases. Staphylococcus aureus is the most common cause of pyomyositis; it causes up to 90 percent of tropical cases and up to 75 percent of temperate cases [7,8,10,13,14]. In minority of cases pyomyositis has been reported to be caused by non-group A streptococci, pneumococci, and gram-negative enteric bacilli. Mycobacterial pyomyositis has also been reported [15][16][17][18]. Pyomyositis can also be polymicrobial, particularly in diabetic patients [10]. In the presented case, intraoperative drained pus sample grew only A. viridens which is an uncommon pathogen to cause pyomyositis. Aerococcus species have been identified in numerous reports as rare cause of infectious diseases including infective endocarditis, meningitis, arthritis and UTI, but not muscle infection [19][20][21].
Aerococci is generally saprophytic and have long been regarded to be uncommon in human infections. It was first described as a potential human pathogen in 1967 [22][23][24]. Aerococcus is a catalase negative gram positive coccus growing in small clusters. The genus contains following species: A. christensenii, A. sanguinicola, A. suis, A. urinae, A. urinaeequi, A. urinaehominis and A. viridans. The clinical significance of these species has been clearly established for A. urinae and A. viridans. A. urinae is a rarely reported human pathogen that has been identified as responsible for urinary tract infections, septicemia, sepsis, endocarditis [25,26].
The microorganism isolated from our patient has grown in pure culture. The presence of pure culture indicated that the cultured germ is the cause of patient medical condition. Historically, A. viridans has been associated with different human infections such as endocarditis [27][28][29], urinary tract infections [30], arthritis [31], or meningitis [32]. There have been several cases of severe invasive aerococcal infections leading to mortality [33]. Moreover, these species are pathogens for crustaceans, causing gaffkemia in marine lobsters, and it has been associated with septicemia in sea turtles. One may think that the presence of Aerococcus viridens in the culture medium, cannot prove that it is the direct cause of pyomyositis or osteomyelitis, as the findings in our particular patient does not fully satisfy the criteria Koch's postulates. However, clinical and laboratory improvement after antibiotic therapy covering A. viridens indicated that the cultured microorganism is in direct relation to the disease.
Aerococcus virdens is a rarely reported pathogen [34][35][36], possibly due to difficulties in the identification of the organism [37], and incidence of aerococcal infections has been underestimated because they are easily misidentified as streptococci or staphylococci, and thus, the introduction of matrix-assisted laser desorption ionization timeof-flight mass spectrometry (MALDI-TOF MS) carried great access to a rapid and accurate method [38,39]. MALDI/TOF is a diagnostic tool with much potential because it allows for the rapid identification of proteins and changes to proteins without the cost or computing power of sequencing nor the skill or time needed to solve a crystal structure in X-ray crystallography. So in the settings of high clinical suspicion of Aerococcus species infection and negative regular culture, further workup with MALDI-TOF MS should be considered.

Conclusion
In this case we presented the first case of Aerococcus viridens induced pyomyositis. A.viridans is unusual cause of soft tissue infection, but should be considered in the differential diagnosis when evaluating patients with suspected deep soft tissue infection. A. viridens infection is underestimated because of technical difficulties in the identification of this organism. The introduction of matrix-assisted laser desorption ionization time-of-flight mass spectrometry carried great success in accurate identification, as early identification of the offending organism guides physicians to appropriate treatment.