Don’t Call Me Viridans: I Have a Name

Today, I want to dive into a topic that fascinates me and builds on my previous post about endocarditis imaging. This time, the focus is on streptococcal bloodstream infections and their risk of infective endocarditis (IE)—a connection that’s far more nuanced than we often acknowledge. One of the most useful tools in understanding this complexity comes from a well known Danish group, which we actually printed and posted up in our fellow’s office because of just how insightful it is.

Let me begin with a bold suggestion:
We should stop using the term “viridans streptococci.”

Here’s why. When it comes to bloodstream infections (BSIs), the risk of endocarditis varies dramatically between individual streptococcal species. Thanks to advancements in microbiology, we now have the ability to identify these organisms at the species level—and we should use it. Grouping everything under the “viridans” umbrella not only oversimplifies the picture but can also obscure meaningful differences that affect clinical decision-making.

A Quick Microbiology Refresher

The term viridans group streptococci traditionally refers to catalase-negative, usually alpha-hemolytic organisms. But this includes a wide variety of species across several groups:

• The Streptococcus mitis group—which includes S. mitis, S. oralis, S. sanguinis, S. parasanguinis, and S. gordonii—are alpha-hemolytic.

• The Streptococcus anginosus group (S. anginosus, S. constellatus [formerly S. milleri], and S. intermedius) can be alpha-, beta-, or gamma-hemolytic.

• The Streptococcus bovis group, now classified under Streptococcus gallolyticus, are typically alpha- or gamma-hemolytic.

• Streptococcus mutans and Streptococcus salivarius are generally alpha-hemolytic, although their hemolytic patterns can sometimes be subtle or variable depending on the strain and growth conditions.

This variety is important—because as you’ll see, not all of these organisms carry the same endocarditis risk.

The Danish Study: What You Need to Know

The study I’m referring to was conducted by Chamat-Hedemand and colleagues in Denmark (PMID: 32580572). It examined data from 6,224 patients over a ten-year period, offering a robust view of streptococcal bloodstream infections and their association with IE. Importantly, the study didn’t limit itself to just the viridans group, but rather looked at all streptococcal species. That said, most IE cases fell within what we typically consider the “viridans” group.

Here’s how the study defined its key criteria:

• A bloodstream infection (BSI) was defined as having at least one positive blood culture bottle for a streptococcal species.

• Infective endocarditis (IE) was identified based on diagnostic coding—specifically ICD-10 codes assigned for IE diagnoses.

Once all cases were reviewed, the authors categorized the streptococcal species in two major ways:

1. By how commonly they were seen in BSIs:

   • Common: more than 5% of total BSIs

   • Moderately common: 1% to 5%

   • Uncommon: 0.1% to 1%

   • Rare: less than 0.1%

2. By the prevalence of endocarditis associated with each species:

   • Low risk: less than 3% of cases developed IE

   • Moderate risk: 3% to 10%

   • High risk: 10% to 30%

   • Very high risk: more than 30%

What Did They Find?

Among the most common streptococcal species (those responsible for over 5% of BSIs), the study found that Streptococcus pneumoniae and Streptococcus pyogenes were associated with a low risk of IE—less than 3%. In contrast, S. dysgalactiae, S. anginosus, and S. agalactiae carried a moderate risk (3% to 10%). Most strikingly, S. mitis and oralis had a high risk of endocarditis, between 10% and 30%.

In the moderately common group (1%–5% of BSIs), the prevalence of endocarditis was especially alarming in several species. S. gallolyticus, S. sanguinis, and S. gordonii all had a very high risk of IE—over 30%. In fact, S. gordonii stood out, with approximately 40–50% of bloodstream infections leading to endocarditis. S. parasanguinis carried a high risk (10%–30%), while S. salivarius fell into the moderate risk range (3%–10%).

For uncommon causes of BSI, S. mutans demonstrated a very high risk of IE, and Granulicatella adiacens had a high risk as well.

Lastly, in the rarest group—species that are mostly “zoonotic” like S. suis, S. equi, and Granulicatella elegans—no cases of IE were observed.

Why This Matters in Practice

The next time you see “viridans streptococci” reported in a blood culture, remember: that label is too vague to guide care. Knowing the species—not just the group—can significantly affect your diagnostic and therapeutic approach.

Even within the same subgroup, risk varies widely. For example:

• In the mitis group:

  • Streptococcus peroris had 0% IE risk

  • Streptococcus gordonii had a risk of around 44%

• In the bovis group:

  • Streptococcus alactolyticus had 0% IE cases

  • Streptococcus gallolyticus had around 30%

This is a clear case where better speciation can—and should—change your clinical management. In a future post, I’ll go into more detail about what to do with these findings: which patients need further workup, and when to reach for furtehrimaging such a TTE or aTEE. But, as with many decisions in ID, there’s nuance…

🎵 Musical Coda: What Does Viridans Mean?

In Latin, “viridans” means “greenish.” It refers to the greenish hue seen around colonies of viridans group streptococci when grown on blood agar—thanks to partial hemolysis (alpha-hemolysis) that oxidizes hemoglobin.