Microbiology Case Study: An 8 Year Old Male with Left Knee Pain

Case History

An 8 year old male with no significant medical history presented with left knee pain and swelling for one week. Physical examination revealed a temperature of 101.2°F and a left swollen, tender knee with reduced range of motion. A joint aspirate was performed, and synovial fluid and blood were sent for microbiological analysis.

Laboratory Findings

Synovial fluid analysis showed increased neutrophils, a nucleated cell count of 90,840 cells/cmm, and no crystals.

Blood cultures were negative. Gram smear of the joint fluid showed many neutrophils and no bacteria. Fluid culture grew convex tan-yellow colonies on blood and chocolate plates at 48 hours (Image 1). Gram smear revealed gram-negative cocci (Image 2). The organism was identified by MALDI-TOF as Aggregatibacter aphrophilus. Antibiotic susceptibility testing showed susceptibility to augmentin, ampicillin, ceftriaxone, and levofloxacin.

Image 1. Growth on anaerobic chocolate plate.
Image 2. Gram stain from anaerobic culture showing gram negative cocci.


Aggregatibacter aphrophilus is a gram negative coccobacillus that requires 5% CO2 and grows best on blood agar. It is oxidase negative and catalase negative. It is categorized as a HACEK organism, being a cause of culture-negative endocarditis. It is considered normal oral flora, and dental procedures can be a source of infection. Aggregatibacter endocarditis can cause a positive P-ANCA and be misdiagnosed as a vasculitis. It has also been reported as causes of sacroiliitis, bartholinitis, endophthalmitis, and brain abscesses. Treatment is generally ceftriaxone for 8 weeks. Identification is by biochemical methods or MALDI-TOF. Broad range PCR (br-PCR) has also been described, which targets a highly-conserved region of 16S rDNA, and then compares the sequences to database sequences.

The patient was given cefazolin, and his temperature downtrended. He was discharged prior to results but placed on oral augmentin. After susceptibility testing, infectious disease was consulted and he was placed on ceftriaxone for 8 weeks. He continued to improve and subsequent cultures were negative.


  1. Ratnayake L, Olver WJ, Fardon T. Aggregatibacter aphrophilus in a patient with recurrent empyema: a case report. J Med Case Rep. 2011;5:448. Published 2011 Sep 12. doi:10.1186/1752-1947-5-448
  2. Hirano K, Tokui T, Inagaki M, Fujii T, Maze Y, Toyoshima H. Aggregatibacter aphrophilus infective endocarditis confirmed by broad-range PCR diagnosis: A case report. Int J Surg Case Rep. 2017;31:150–153. doi:10.1016/j.ijscr.2017.01.041

-Jonathan Wilcock, MD is a 1st year anatomic and clinical pathology resident at the University of Vermont Medical Center.

-Christi Wojewoda, MD, is the Director of Clinical Microbiology at the University of Vermont Medical Center and an Associate Professor at the University of Vermont.

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