Group A Streptococcus (Streptococcus pyogenes) is the most common bacterial cause of tonsillitis, responsible for the vast majority of bacterial cases. That said, bacteria only account for 5% to 15% of tonsillitis in adults and 15% to 30% in children aged 5 to 15. The rest is viral. Knowing which bacteria are involved matters because it determines whether antibiotics will help and which complications to watch for.
Group A Strep: The Primary Cause
When doctors talk about “strep throat,” they mean an infection caused by group A beta-hemolytic Streptococcus pyogenes. This is the bacterium behind most cases of acute bacterial tonsillitis, and it’s the one clinicians are specifically testing for when they swab your throat.
Group A Strep is particularly good at latching onto tonsillar tissue. The bacteria use hair-like structures on their surface called pili to make initial contact with the cells lining your tonsils. Once attached, proteins on the bacterial surface bind to molecules in your tissue, creating a strong hold. The bacteria can even push their way inside your cells, which helps them evade your immune system and makes the infection harder to clear without treatment.
This bacterium is the primary concern because untreated infections can lead to serious complications: rheumatic fever (which can damage heart valves), kidney inflammation, and peritonsillar abscess, where a pocket of pus forms near the tonsil. These complications are rare with proper treatment, but they’re the reason strep throat gets taken more seriously than a typical sore throat.
Other Bacteria in Acute Tonsillitis
While Group A Strep dominates, several other bacteria can cause acute tonsillitis, though far less commonly. Mycoplasma pneumoniae, the bacterium better known for causing “walking pneumonia,” occasionally infects the throat. Chlamydia pneumoniae and Corynebacterium diphtheriae (the cause of diphtheria) are rare culprits. In sexually active individuals, Neisseria gonorrhoeae can cause pharyngitis and tonsillitis through oral contact. Arcanobacterium haemolyticum is recognized as a notable cause of throat infections in Scandinavia and the United Kingdom, though it gets less attention in the United States.
Fusobacterium: A Growing Concern in Teens
Fusobacterium necrophorum has emerged as an important and underrecognized cause of bacterial tonsillitis, particularly in teenagers and young adults. Studies have found its prevalence among patients aged 15 to 45 ranges from 10% to 48%, rivaling Group A Strep in some age groups. In patients between 14 and 20 years old, about 13.5% of pharyngitis cases involve this bacterium, compared to just 1.9% in younger children.
What makes Fusobacterium necrophorum especially concerning is its link to Lemierre’s syndrome, a rare but life-threatening condition where infection spreads to the jugular vein, causing blood clots and potentially seeding infections throughout the body. Roughly 1 in 400 cases of Fusobacterium throat infection progresses to Lemierre’s syndrome. Because of this risk, routine screening for this bacterium is becoming common practice in parts of Europe, though it hasn’t been widely adopted elsewhere.
Bacteria in Recurrent and Chronic Tonsillitis
The bacterial picture shifts when tonsillitis keeps coming back or becomes chronic. In recurrent tonsillitis, the most commonly isolated bacteria are Streptococcus pneumoniae, Staphylococcus aureus, and Haemophilus influenzae. The most common anaerobic bacterium (one that thrives without oxygen) found in recurrent cases is Bacteroides fragilis.
Adults and children with recurrent tonsillitis harbor different bacterial communities. Adults tend to have a wider variety of bacterial species, with higher rates of certain anaerobic bacteria like Prevotella and Porphyromonas species. Children with recurrent tonsillitis are more likely to show Group A Strep.
Chronic tonsillitis typically involves a mix of multiple bacterial species rather than a single dominant one. Alpha- and beta-hemolytic streptococci, Staphylococcus aureus, Haemophilus influenzae, and Bacteroides species are all commonly identified. In cases where tonsils become significantly enlarged (hypertrophic tonsils), Haemophilus influenzae is the most frequently isolated bacterium.
How Bacterial Tonsillitis Is Diagnosed
Because most tonsillitis is viral, not every sore throat needs testing. Clinicians use a scoring system called the Centor score to estimate the likelihood that a sore throat is caused by bacteria. It assigns one point for each of four features: white or yellow coating on the tonsils, swollen and tender lymph nodes at the front of the neck, fever above 38°C (100.4°F), and the absence of cough. A modified version, the McIsaac score, adds a point for patients aged 3 to 14 and subtracts one for those 45 and older.
Scores of 0 to 2 generally suggest managing symptoms at home without antibiotics. Scores of 3 or higher typically prompt a rapid antigen detection test (a quick throat swab that gives results in minutes) or a throat culture. Throat culture remains the gold standard, though it takes a day or two for results. For children over 3, guidelines recommend following up a negative rapid test with a throat culture because the rapid test can miss some cases. For adults, a backup culture after a negative rapid test usually isn’t necessary.
Neither scoring system is highly accurate on its own, which is why testing matters. Clinical signs alone can’t reliably distinguish bacterial from viral tonsillitis.
Antibiotic Treatment for Bacterial Tonsillitis
Penicillin and amoxicillin are the go-to antibiotics for Group A Strep tonsillitis. No clinical isolate of Group A Strep has ever been found to be resistant to penicillin, making it a reliably effective choice. A typical course runs 10 days, though some guidelines note that 5 days may be enough to resolve symptoms (with 10 days offering a better chance of fully clearing the bacteria).
If you’re allergic to penicillin, alternatives include certain cephalosporin antibiotics (unless your allergy causes an immediate reaction like hives or swelling), clindamycin, or macrolide antibiotics like azithromycin and clarithromycin. One caveat with macrolides: resistance is a real issue. In Japan, for instance, over 34% of Group A Strep samples showed macrolide resistance. Resistance rates vary by region and change over time, which is why penicillin remains the preferred first-line choice wherever possible.
You can generally expect to feel noticeably better within 2 to 3 days of starting antibiotics. Finishing the full course helps prevent complications and reduces the chance of the infection rebounding. Most people are no longer contagious after 24 hours on antibiotics.
What Happens Without Treatment
Viral tonsillitis resolves on its own, and even bacterial tonsillitis often clears without antibiotics. The concern with skipping treatment for confirmed Group A Strep is the risk of complications. Peritonsillar abscess is the most common local complication, forming when infection spreads beyond the tonsil into surrounding tissue. The bacteria found in peritonsillar abscesses are typically a mix of anaerobes (Prevotella, Fusobacterium, and Peptostreptococcus species) alongside Group A Strep, Staphylococcus aureus, and Haemophilus influenzae. In patients aged 15 to 24, Fusobacterium necrophorum is found more often in abscesses than Group A Strep.
Rheumatic fever and post-streptococcal kidney inflammation are rarer but more serious systemic complications. Rheumatic fever is exceedingly uncommon in developed countries today, partly because of widespread antibiotic use, but it remains a concern in lower-resource settings. Antibiotic treatment of strep throat is one of the few situations where treating a self-limiting infection genuinely prevents a more dangerous outcome down the line.