Aerococcus urinae

Aerococcus urinae is a Gram-positive bacterium associated with urinary tract infections as well as other infections in people, commonly older adults and those with weak immune systems.

Aerococcus urinae
Aerococcus urinae on blood agar[1]
Microscopy of Aerococcus urinae with gram stain, showing gram positive cocci.[1]
Scientific classification
Domain:	Bacteria
Phylum:	Bacillota
Class:	Bacilli
Order:	Lactobacillales
Family:	Aerococcaceae
Genus:	Aerococcus
Species:	A. urinae
Binomial name
Aerococcus urinae

Aerococcus urinae: A Rare but Emerging Pathogen

Aerococcus urinae is a Gram-positive, catalase-negative bacterium that’s slowly gaining attention in clinical microbiology, particularly for its role in urinary tract infections (UTIs) and, in rarer cases, more serious infections like endocarditis. Although it's been around for a while, it was only in the past few decades that researchers really began recognizing it as a distinct and clinically relevant species. Because it looks and acts like more well-known bacteria (like streptococci or staphylococci), it’s often misidentified—especially without advanced lab tools.

Classification

Aerococcus urinae is a Gram- positive, catalase- negative coccus that frequently grows in clusters. Belonging to the genus Aerococcus.

First Identified in 1953 from air and dust collections, the bacteria was later obtained from human urine and was officially seen as a unique species in 1992. ^[2] Later, it was found in the urine of patients with urinary tract infections^[3] and in 1992, A. urinae was assigned as distinct species.

Since being Identified as such a distinct species, today it is known to cause Urinary Tract infections (UTIs) , and other serious conditions such as bacteremia; which is often seen in elderly patients

What It Is and Where It’s Found

A. urinae is usually found in older adults, especially men, and is often associated with urinary tract infections. It’s not part of the normal flora in most people, and you’re not likely to find it in younger, healthy individuals. Instead, it shows up in people with urologic issues like enlarged prostate, kidney stones, or anyone using catheters long-term. These conditions make it easier for bacteria like A. urinae to get into the urinary tract and cause problems.

What makes it tricky is that under a microscope, A. urinae shows up as small Gram-positive cocci in clusters or pairs—similar to other bugs we see all the time. On blood agar, it can be alpha-hemolytic or even non-hemolytic, making it even harder to distinguish. In many labs, it was historically brushed off as a contaminant or misidentified entirely.

How It’s Identified

Traditional lab methods like biochemical tests often miss the mark with A. urinae. That’s changing now with the use of MALDI-TOF mass spectrometry and genetic testing (like 16S rRNA sequencing), which have made it easier to accurately pin down. MALDI-TOF is especially useful because it can quickly ID bacteria based on their protein profiles.

That said, not every lab has access to this tech, so A. urinae may still fly under the radar in smaller or less-equipped hospitals.

Symptoms and Infections It Can Cause

The most common condition linked to A. urinae is a urinary tract infection. Symptoms are typical of UTIs—burning when urinating, feeling the need to go often, pelvic discomfort, and possibly cloudy or bloody urine. Most patients recover without complications, especially if they get the right antibiotics early.

However, in rare cases, A. urinae has been linked to more serious infections like:

• Bacteremia (bacteria in the blood)
• Infective endocarditis (infection of the heart’s inner lining)
• Spondylodiscitis (infection of spinal discs)

These serious cases usually occur in people who already have health issues—things like heart valve disease, recent surgeries, or compromised immune systems. Endocarditis caused by A. urinae can present subtly, with general symptoms like fatigue or low-grade fever, so it’s easy to miss if you’re not looking for it.

Clinical relevance

A. urinae may also cause invasive infections including sepsis originating from the urinary tract and infective endocarditis, especially in elderly men with underlying urinary tract diseases.^[4][5]

A. urinae is sensitive to many commonly used antibiotics such as penicillins, cephalosporins, and vancomycin. Nitrofurantoin has been reported to be effective in one series of 42 clinical isolates.^[6] The bacterium can form biofilms on foreign materials and can aggregate human platelets, two features of potential importance for the disease causing capacity of this organism.^[7]

A. urinae is the most common aerococcus isolated from invasive human infections whereas Aerococcus sanguinicola is isolated from human urine as often as A. urinae.^[8]

Treatment and Antibiotic Sensitivity

Fortunately, A. urinae is usually sensitive to common antibiotics, especially beta-lactams like penicillin and amoxicillin. It also tends to respond to vancomycin and ceftriaxone. However, resistance to drugs like ciprofloxacin and TMP-SMX has been reported, so cultures and susceptibility testing are important, especially in more complicated or serious infections.

In severe infections like endocarditis, treatment might involve IV antibiotics for several weeks. Sometimes, a combination therapy is used, depending on how resistant the strain is and how sick the patient is.

Why It Matters

Even though it’s still considered rare, Aerococcus urinae is likely more common than we think—it just hasn't always been correctly identified. As labs adopt better diagnostic tools, we’re seeing more cases reported. For older patients, especially those in hospitals or long-term care with catheters or other risk factors, this bacterium could be more significant than it seems.

Understanding and recognizing A. urinae matters not only for correct treatment but also to avoid complications from underdiagnosing or mismanaging infections caused by it. With an aging population and increased use of medical devices, it’s possible we’ll continue seeing it pop up more frequently in clinical settings.