Fever, urinary symptoms, and a heart murmur

An 81-year-old man from Mississippi with alcohol use disorder, hypertension, and benign prostatic hyperplasia presents with fever and malaise. He reports mild urinary urgency and generalized malaise for the past 10 days. He denies focal pain, cough, abdominal symptoms, or neurologic complaints.

Three months ago, he traveled to Arizona, staying near a farm.He lives with a dog and recently adopted two kittens from a local pet store and has had multiple skin excoriations. He also underwent a molar extraction 1 month ago. He denies any known allergies to antibiotics.

Physical examination is notable for temperature of 38.9°C, heart rate 118 bpm, respiratory rate of 20 bpm. On auscultation there is a II/VI systolic murmur in cardiac apex radiating to the axilla Labs are notable for leukocytosis (15.4 × 10⁹/L) and elevated CRP (138 mg/L). Blood cultures are reported to be growing an organism (Gram stain is shown below). On culture, the organism is catalase negative and produces alpha-hemolytic colonies on blood agar. A transthoracic echocardiogram demonstrates a small mitral valve vegetation.

Aerococcus urinae is a Gram-positive, catalase-negative coccus that was first described in the 1990s as a causative organism of urinary tract infections (1). Although the organism is considered to be of low pathogenicity and may not always need to be treated, there are descriptions of severe, sometimes fatal infections, such as endocarditis, often in the presence of urinary tract infections or symptoms typical of this infection.

Isolates of A. urinae may easily be mistaken for staphylococci on Gram stain, because they appear as Gram-positive cocci arranged in tetrads or clusters. Colonies on blood agar may be mistaken for streptococci, because these show alpha hemolysis and are catalase negative, resembling viridans streptococci and some enterococci. Primary identification is based upon Gram stain of alpha-hemolytic colonies that are catalase negative. Aerococcus species appear as Gram-positive cocci in clusters, which differentiates them from streptococci and enterococci.

Differentiation between A. urinae and Aerococcus viridans can be done by testing for pyrrolidinyl aminopeptidase (PYR), which is positive only for A. viridans, and leucine aminopeptidase (LAP), which is positive only for A. urinae. Automated systems such as Vitek are reliable for phenotypic identification, and 16S rRNA PCR may be used for genotypic confirmation. In some cases, blood cultures remain negative, in which case 16S rRNA PCR performed directly on blood culture may reveal A. urinae (PMID: 20660216).