Microbiology Case Study–Joint Pain After Left Knee Arthroscopy

Case history:

A 51 year old male with a history of recent left knee arthroscopy presented with increasing joint pain, swelling, and reduced range of motion accompanied by subjective fevers. He was receiving physical therapy and admitted to soaking the knee in a hot tub on two occasions after the therapy sessions. An aspirate of synovial fluid obtained at the clinic showed the following colony morphology and Gram stain:

Spready, rough, bluish-green colonies on blood agar.
Spready, rough, bluish-green colonies on blood agar.
Gram stain of the predominant colonies showing gram negative bacilli

Laboratory identification:

Pseudomonas aeruginosa is a non-spore forming, non-encapsulate, motile gram-negative bacillus. The bacteria typically form characteristic colonies with a metallic sheen and have a green-blue appearance due to production of soluble blue phenazine pigment (pyocyanin) and yellow-green pigment fluorescein (or pyoverdin). The organisms classically emit a grape-like odor in culture. They are oxidase positive. Lab diagnosis is based on colony morphology and the organism’s biochemical characteristics. P. aeruginosa is also the only clinically significant fluorescent pseudomonad that grows at 42oC. In our laboratory, the isolate was identified on the MALDI-TOF.


P. aeruginosa has been documented to infect any external site or organ. Community acquired infections are associated with otitis media, skin ulcers, corneal infection, and rashes secondary to contaminated hot tub water which is consistent with our patient’s history. Hospital acquired infections are typically related to catheters, bedsores, burns, and eye infections. People with cystic fibrosis are particularly susceptible to infection with this organism although asymptomatic colonization in these patients is also possible. Patients with extensive burns are also at risk for infection with Pseudomonas. Rarely, a septicemic infection characterized by black necrotic skin lesions known as ecthyma gangrenosum may happen.

The organism has two important virulence factors for pathogenesis- exotoxin A and exoenzyme S. It also produces various cytotoxic substances, all of which contribute to the local tissue destruction.

Pseudomonas infections can be treated with aminoglycosides, beta lactam, and fluoroquinolone antibiotics.

-Lauren Pearson, D.O. is a 2nd 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 Assistant Professor at the University of Vermont.

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