About 8% to 10% of children may continue carrying strep bacteria after antibiotics, even when they feel healthy.
Researchers found that bacterial genetics alone did not explain why some children continued carrying strep.
The findings point to the possible role of the body's immune response and other bacteria living in the throat.
Strep throat can be miserable for children, causing fever, swollen tonsils, and a painful sore throat. While antibiotics can treat the infection, some children continue carrying Group A Streptococcus (GAS) bacteria after their symptoms disappear.
These children are known as carriers. They may feel completely healthy, but the bacteria can remain in their throats for months or even years and potentially spread to others.
“Around 8 to 10% of kids will stay colonized with strep even after the antibiotics are done,” Binghamton University Associate Professor of Biological Sciences Laura Cook said in a news release. “They no longer have symptoms. They feel healthy.”
Researchers from Binghamton University, the University of Illinois Chicago and the University of Wisconsin School of Medicine and Public Health wanted to understand why this happens. Their study, published in Cell Reports, explored whether certain characteristics of the bacteria could explain why they persist after treatment.
How researchers studied strep infections
The research team analyzed samples from 205 children ages 5 to 15 who visited pediatric clinics in Madison, Wisconsin, between 2012 and 2023. Children who tested positive for strep throat received antibiotics and returned about four weeks later for another throat swab.
Researchers compared children whose infections had cleared with those who still carried strep despite no longer having symptoms. They used whole-genome sequencing to examine the bacteria's genetic makeup and RNA sequencing to study which genes were active during infection and after treatment.
The team also tested the bacteria's ability to form biofilms, which are communities of bacteria that can stick together, and examined their antibiotic susceptibility.
What the findings mean for consumers
The researchers found no consistent genetic differences that could explain why some strep strains persisted after treatment. They also found no major differences in biofilm formation or antibiotic susceptibility between bacteria from carriers and non-carriers.
However, the bacteria showed different gene activity during active infections compared with the post-treatment carrier state. Importantly, those gene activity patterns during the initial infection did not predict which children would become carriers.
That suggests the explanation may involve more than the bacteria themselves. Researchers believe factors such as a child's immune response or the other bacteria living in their throat could play a role.
The team plans to investigate whether differences in children's throat microbiomes — the communities of microorganisms living there — could help explain or predict strep carriage.
For families, the findings offer new insight into why a child may test positive for strep even after feeling better. They do not establish a new treatment or mean antibiotics have failed to treat the original illness. Instead, they highlight questions researchers are still working to answer about why strep bacteria sometimes remain in the throat after treatment.