Group B strep (GBS) is caused by the bacterium Streptococcus agalactiae, which naturally lives in the gut and genital tract of healthy adults. Roughly 10 to 30 percent of pregnant women carry GBS in their gastrointestinal or vaginal tract at any given time, usually without any symptoms at all. The bacterium only becomes a medical concern when it reaches a vulnerable person, most often a newborn during birth or an older adult with a weakened immune system.
Where the Bacteria Normally Lives
GBS is not an infection you “catch” from a contaminated surface or a sick person in the way you might catch the flu. It is a normal inhabitant of the human body. The bacteria colonize the lower intestine and, in many women, the vagina and rectum. Colonization can come and go over time. A person might test positive for GBS one month and negative the next, then positive again later. This fluctuation is normal and does not indicate a worsening health problem.
Because GBS lives quietly in so many people, carrying it is not considered an illness. Most colonized adults will never develop any symptoms or complications from the bacteria. The distinction between harmless colonization and active infection depends almost entirely on who is exposed and how their immune system responds.
How GBS Spreads to Newborns
The most well-known risk from GBS involves newborns. When a pregnant woman carries the bacteria in her vagina or rectum, the baby can be exposed during labor and delivery as it passes through the birth canal. This vertical transmission is the primary route for what doctors call early-onset GBS disease, which appears within the first week of life.
Late-onset GBS disease, which develops between one week and three months of age, is less well understood. In some cases the source may still be the mother, but for many infants with late-onset disease, the exact source of infection remains unknown. Unlike early-onset disease, there is no reliable way to prevent late-onset GBS with antibiotics given during labor.
What Makes GBS Dangerous When It Invades
GBS is not an especially aggressive organism for healthy adults, but it has a sophisticated set of tools for evading the immune system when it does reach deeper tissues. The bacterium is coated in a sugar-based capsule that mimics molecules already found in the human body. This disguise makes it harder for immune cells to recognize and destroy the bacteria.
GBS also produces a toxin that can damage lung cells, blood vessel walls, and brain tissue. This toxin helps the bacteria cross from the surface of the lungs or gut into the bloodstream, which is a critical step in causing serious infections like sepsis or meningitis. Another enzyme the bacteria produce breaks down the connective tissue between cells, essentially clearing a path for the bacteria to spread deeper into the body. A separate surface protein disables part of the immune system’s early warning signals, further slowing the body’s response.
In a healthy adult with a fully functioning immune system, these tactics rarely succeed. The immune system contains and eliminates the bacteria before they can establish a foothold. In a newborn whose immune system is still immature, or in an older adult whose immune defenses are compromised, these same tactics can lead to life-threatening infection.
Risk Factors for Newborn Infection
Not every baby born to a GBS-positive mother will become infected. Several factors increase the likelihood that colonization progresses to active disease:
- Premature birth before 37 weeks, when the immune system is even less developed
- Prolonged rupture of membranes lasting longer than 18 hours before delivery
- Maternal fever during labor of 100.4°F (38°C) or higher
- A previous baby with GBS infection
- Maternal HIV exposure
The more of these factors present at delivery, the higher the risk that the baby will develop an active GBS infection rather than simply being exposed and clearing the bacteria on its own.
GBS in Adults Outside of Pregnancy
GBS is increasingly recognized as a cause of serious infections in non-pregnant adults, particularly those over 65. The bacteria can cause bloodstream infections, urinary tract infections, skin and soft tissue infections, and occasionally meningitis in adults whose immune systems are weakened. Conditions that raise the risk include diabetes, liver disease, heart disease, HIV, and cancer or a history of cancer treatment. In these individuals, the same bacteria that sit harmlessly in the gut can enter the bloodstream through small breaks in tissue and overwhelm an immune system that lacks the resources to contain them.
How GBS Is Detected
For pregnant women, screening is straightforward. The American College of Obstetricians and Gynecologists recommends a vaginal and rectal swab between 36 and 37 weeks of pregnancy. The swab is placed in a nutrient broth that encourages GBS to grow, and results typically come back within one to two days. Advanced culture methods and molecular testing (which detects the bacteria’s DNA directly) both achieve sensitivity above 95 percent, meaning they miss very few true positives.
Screening is recommended for all pregnant women regardless of whether they plan a vaginal or cesarean delivery. The exception is women who already have a clear indication for preventive antibiotics, such as GBS found in a urine culture during the current pregnancy or a prior infant who developed GBS disease.
How Colonization Is Managed
If you test positive for GBS during pregnancy, the standard approach is antibiotics given through an IV during labor. This does not eliminate GBS from your body permanently. It reduces the number of bacteria present in the birth canal at the time of delivery, dramatically lowering the chance that your baby is exposed to a large enough dose to cause infection. The antibiotics work best when given at least four hours before delivery.
There is no recommended treatment for GBS colonization outside of the labor period. Taking oral antibiotics weeks before your due date will not reliably clear the bacteria, because GBS often recolonizes from the gut. The timing of treatment during active labor is what makes it effective.
For non-pregnant adults who develop an active GBS infection, treatment depends on where the infection has taken hold. Bloodstream infections and meningitis require hospitalization and IV antibiotics, while urinary tract infections can sometimes be treated with oral antibiotics. The underlying condition that allowed the infection to develop, such as uncontrolled diabetes or an immunosuppressive illness, also needs to be addressed to reduce the chance of recurrence.