Causes of Group B Strep
Group B Streptococcus (GBS) infections happen when a usually harmless bacterium enters areas of the body where it shouldn’t be, such as the blood, lungs, or spinal fluid. Although GBS naturally lives in many healthy people, it can become dangerous—especially in newborns, pregnant women, and those with weakened immune systems.
Understanding what causes GBS infections helps healthcare providers identify those at risk and take steps to prevent serious illness. While most people carry GBS without any symptoms, certain conditions can allow it to cause severe infections.
1. Natural Colonisation
Group B Strep, also known as Streptococcus agalactiae, normally lives in the:
- Gastrointestinal tract
- Vagina
- Rectum
- Urinary tract
Around 20–40% of healthy adults carry the bacteria without knowing. Colonisation may be:
- Temporary (lasting days or weeks)
- Intermittent (coming and going)
- Persistent (lasting months or years)
People can spread the bacteria through skin contact, sexual activity, or movement of bacteria from the gut to the genital area. However, doctors do not consider GBS a sexually transmitted infection.
2. Transmission During Labour and Birth
The most serious GBS infections in babies occur during labour and delivery. A baby may become infected if GBS passes from the mother’s birth canal into the baby’s body. This can happen when:
- Labour begins
- The mother’s water breaks
- The baby passes through the vaginal canal
Babies may breathe in or swallow the bacteria, which can lead to lung or blood infections. The risk increases if:
- The mother carries high levels of GBS
- Labour lasts a long time
- The water breaks early
- The baby is born prematurely
Without antibiotics, about 1 in 200 babies born to GBS-positive mothers may develop early-onset disease. With antibiotics, the risk drops to 1 in 4,000.
3. Weakened Immune Systems in Adults
In adults, GBS can cause invasive disease when the immune system is weak. The risk increases for people with:
- Diabetes
- Cancer or those on chemotherapy
- HIV/AIDS or other immune disorders
- Kidney or liver disease
- Recent surgery or serious injury
In these cases, the body cannot fight off the bacteria, which then spread into the blood, bones, joints, or lungs, leading to serious illness.
4. Damage to Natural Barriers
Group B Strep can enter the body more easily when natural protective barriers break down. This includes:
- Open wounds, ulcers, or surgical cuts in the skin
- Use of urinary catheters or bladder infections
- Damage to the vaginal or rectal lining
Once inside these vulnerable areas, GBS can multiply rapidly and cause inflammation, sepsis, or organ damage.
5. Pregnancy-Related Risk Factors
Pregnancy naturally changes the body’s environment, making it easier for GBS to grow. For example:
- Hormonal changes affect the vaginal flora
- The immune system weakens slightly, which helps prevent rejection of the baby
- Colonisation rates rise in the third trimester
During pregnancy, GBS may cause:
- Urinary tract infections (UTIs)
- Preterm labour
- Premature rupture of membranes
- Infections in the womb (intra-amniotic infections)
- Postpartum infections, especially after a C-section
Because of these risks, most healthcare systems include routine GBS screening and treatment in prenatal care.
6. Hospital-Associated Infections
Although rare, people can also get GBS infections in healthcare settings. This may happen in:
- Newborns in neonatal intensive care units
- Patients recovering from surgery
- Elderly adults in long-term care
Poor hand hygiene or contaminated equipment may contribute to GBS spreading in hospitals. However, these cases are uncommon and often preventable with proper infection control.
7. Unknown Triggers and Genetic Factors
Some people develop severe GBS infections even though they have no clear risk factors. Researchers believe this may happen due to:
- Differences in immune system genes
- Strain variations in the GBS bacteria
- Low levels of protective antibodies, especially in newborns
Ongoing studies aim to understand these factors better. These efforts may also support the development of GBS vaccines in the future.
Conclusion | Causes of Group B Strep
Group B Strep infections result from a mix of colonisation, weakened immunity, and exposure during childbirth. Although GBS often lives harmlessly in many people, it can turn dangerous under the right conditions. By learning how GBS spreads and who faces the greatest risk, doctors can screen, treat, and protect patients—especially newborns, pregnant women, and adults with chronic illness.
