There is no single number that answers this question, because “strep” covers an entire genus of bacteria with multiple species, and each species contains dozens to hundreds of distinct strains. The genus Streptococcus includes at least 20 recognized groups, and within just three of the most common disease-causing species, scientists have identified well over 385 unique strain types combined. The answer depends on which level of classification you’re looking at.
How Strep Bacteria Are Classified
Streptococcus bacteria are sorted using two main systems. The first, developed by microbiologist Rebecca Lancefield in the 1930s, divides them into groups labeled A through V based on a sugar molecule on their surface. There are 20 Lancefield groups total, though only a handful regularly cause illness in humans. The second system sorts strep by what they do to red blood cells in a lab dish: beta-hemolytic strains completely destroy the cells, alpha-hemolytic strains partially damage them, and gamma-hemolytic strains leave them alone.
These two systems overlap but don’t perfectly align. Some important strep species, like the viridans group, don’t fit neatly into any Lancefield category. And within each group or species, researchers further subdivide bacteria into serotypes or strain types based on genetic markers or capsule structures. That’s where the numbers get large.
Group A Strep: Over 275 Strain Types
Group A Streptococcus, the species behind strep throat, scarlet fever, and flesh-eating infections, is classified into more than 275 emm types. The emm typing system looks at a specific surface protein gene, and each variation gets its own number. This is the system the CDC uses to track which strains are circulating and causing outbreaks. Not all 275-plus types are equally common or dangerous. A relatively small number cause the majority of severe invasive infections, while others circulate without causing much trouble.
Group B Strep: 10 Serotypes
Group B Streptococcus is best known for causing serious infections in newborns during the first weeks of life, though it increasingly affects adults as well. This species is divided into 10 serotypes, designated Ia, Ib, II, III, IV, V, VI, VII, VIII, and IX. The classification is based on differences in the sugar capsule surrounding each bacterium. A small number of these serotypes, particularly type III, account for a disproportionate share of newborn infections.
Strep Pneumoniae: 100 Serotypes
Streptococcus pneumoniae, the bacterium responsible for most bacterial pneumonia as well as ear infections and meningitis, has 100 recognized serotypes. The most recently identified one, called 10D, was discovered when researchers found it carried a large genetic fragment from a different oral strep species. Each serotype has a slightly different capsule structure, which is why pneumococcal vaccines target specific serotypes rather than the whole species.
Before the year 2000, just seven serotypes caused most antibiotic-resistant pneumococcal infections in the United States. Conjugate vaccines introduced since then have dramatically reduced infections from those strains. When PCV7 was introduced, disease from six of those seven serotypes dropped, though serotype 19A initially surged to fill the gap. A newer vaccine covering 19A brought that strain under control too, and no single replacement serotype has emerged since.
The Viridans Group: A Catch-All Category
The viridans group streptococci are essentially what’s left over when you remove the beta-hemolytic strep, pneumococci, and enterococci from the genus. They’re classified into six major subgroups, including the S. mutans group (the primary driver of tooth decay), the S. salivarius group, and the S. anginosus group. The anginosus group alone contains three species that are known for causing abscesses in the brain, liver, and lungs.
Most viridans strep live harmlessly in your mouth and gut, but they can cause serious infections if they enter the bloodstream, particularly in people with damaged heart valves or weakened immune systems. Because they don’t fit into a single Lancefield group, they’re harder to classify neatly, and new species within this category continue to be identified.
Group D Strep and the Enterococcus Split
Group D streptococci have an unusual history. The group originally included enterococci, bacteria that live in the intestinal tract and are now among the most common hospital-acquired infections. However, scientists eventually reclassified enterococci into their own separate genus because they differ significantly from true streptococci in how they grow and behave in the lab. The remaining Group D streptococci, sometimes called the S. bovis group, now contain at least seven recognized divisions. These bacteria are notable because bloodstream infections with S. bovis group organisms are strongly associated with colon cancer, making their identification clinically important even though they’re relatively uncommon.
Why Strain Counts Keep Changing
Whole-genome sequencing has transformed how scientists identify and categorize strep bacteria. Older methods relied on surface molecules or lab behavior, which could lump genetically distinct bacteria together or split similar ones apart. Genomic analysis reveals finer distinctions, which is why the number of recognized strains and serotypes has grown steadily over the past two decades. The 100th pneumococcal serotype, for example, was only recently confirmed through genetic analysis. For Group A strep, the emm typing database maintained by the CDC continues to expand as new variants are submitted from around the world.
So while the genus contains around 20 broad groups, the total number of individually identifiable strains across all species is in the hundreds. The three species most relevant to human health alone account for more than 385 distinct types, and that number will continue to grow as sequencing technology improves.