Bacterial infections in babies most often come from the mother’s own body, passed during pregnancy or delivery. The birth canal naturally harbors bacteria, and roughly 80% of early infections in newborns trace back to organisms a baby encounters before or during birth. Other infections develop later, picked up from the hospital environment, caregivers, or contaminated food sources during pregnancy.
The Most Common Bacteria Behind Newborn Infections
Three bacteria account for the vast majority of serious newborn infections in industrialized countries: Group B Streptococcus (GBS), E. coli, and Listeria.
GBS is the single biggest culprit, responsible for more than 40% of all early-onset infections. Between 10% and 30% of pregnant women carry GBS in their vaginal or rectal area without any symptoms. Of babies born to colonized mothers, about 1% to 2% develop invasive disease. GBS has been the leading bacterial infection linked to illness and death among newborns in the United States since the 1970s.
E. coli is the second most common cause, accounting for roughly 30% of early infections. It’s especially prevalent among premature and very low birth weight babies, where it actually surpasses GBS as the top threat. Mothers can carry pathogenic strains of E. coli in the vaginal and intestinal tract, and these transfer to the baby during delivery.
Listeria works differently. Rather than passing through the birth canal, it typically reaches the fetus through the placenta after the mother eats contaminated food, often soft cheeses, deli meats, or unpasteurized products. The mother may only feel mild flu-like symptoms or nothing at all, but the consequences for the baby can be severe. Nearly 25% of pregnancy-associated Listeria cases result in fetal loss or death of the newborn.
How Bacteria Reach the Baby
During Labor and Delivery
The most common route is vertical transmission, meaning bacteria travel from the mother’s body directly to the baby. During a vaginal birth, the newborn passes through the birth canal and comes into contact with the mother’s vaginal and fecal flora. If pathogenic bacteria are present, the baby can swallow or inhale them, or the organisms can enter through the eyes, ears, or any break in the skin. The longer labor takes, the greater the exposure. Research shows that when labor extends beyond 12 hours, the odds of vertical transmission roughly double compared to shorter labors.
Water breaking early also creates risk. Once the membranes rupture, the protective barrier between the baby and the outside environment is gone. The longer the gap between membrane rupture and delivery, the more time bacteria have to ascend into the uterine environment. When this gap exceeds 12 hours, the diversity of potentially harmful bacteria in the baby’s gut increases significantly, including organisms picked up from the surrounding hospital environment.
Before Birth
Some infections cross the placenta while the baby is still in the womb. Listeria is the classic example. The mother’s bloodstream carries the bacteria to the placenta, where it can infect the fetus directly. This can happen weeks before delivery, leading to preterm labor, stillbirth, or a baby born already fighting a serious infection.
After Birth
Late-onset infections, those appearing after the first week of life, often come from the hospital environment rather than the mother. Babies in neonatal intensive care units face particular risk because they frequently need IV lines, breathing tubes, and other invasive equipment that can introduce bacteria past the body’s natural defenses. The organisms involved tend to be different too, including hospital-associated bacteria like Klebsiella and Pseudomonas rather than the GBS and E. coli that dominate early infections.
Why Some Babies Are More Vulnerable
Premature babies face a dramatically higher risk of bacterial infection, and the reasons go beyond simply being smaller. Their immune systems are genuinely underdeveloped in multiple, specific ways. The protective waxy coating on the skin (vernix) doesn’t begin forming until the third trimester, so preterm babies lack this barrier entirely. Their skin itself is thinner and more permeable to bacteria. Inside their airways, they have fewer of the hair-like cells that sweep pathogens out of the lungs, making respiratory infections easier to establish.
At the molecular level, premature babies produce significantly lower amounts of the antimicrobial proteins that serve as a first line of defense against bacteria. These proteins normally kill or neutralize bacteria on contact, and their absence leaves preterm infants relying on an immune system that hasn’t yet learned to mount a strong response.
Beyond prematurity, several maternal factors raise the risk. A mother who develops a fever during labor, delivers prematurely, or has a urinary tract infection during pregnancy passes along higher risk to her baby. Maternal UTIs during pregnancy have been associated with up to a 5.9-fold higher risk of urinary tract infection in the infant, likely because the mother harbors bacteria that transfer to the baby and then cause an ascending infection.
Types of Infections Babies Develop
Bacterial infections in newborns can take several forms depending on where the bacteria settle. Sepsis, a bloodstream infection, is the most dangerous. Meningitis occurs when bacteria reach the fluid surrounding the brain and spinal cord, with GBS and E. coli together causing the majority of cases. Pneumonia develops when bacteria are inhaled into the lungs during delivery. Urinary tract infections are also common in newborns, with E. coli responsible for 40% to 72% of cases, followed by Klebsiella species.
Doctors classify newborn infections by timing. Early-onset infections appear within the first week of life (most within 24 to 48 hours) and account for about 80% of all neonatal bacterial infections. These almost always trace back to bacteria encountered during birth. Late-onset infections develop after the first week and up to about three months of age, with sources that may include the hospital environment, caregivers, or community exposure.
Warning Signs in a Newborn
Bacterial infections in newborns don’t always look the way you’d expect. A baby may not develop an obvious fever. Instead, the signs are often subtle and behavioral. Watch for changes in body temperature in either direction (too warm or too cool), breathing that seems faster or more labored than usual, reduced interest in feeding, or noticeably less movement and activity. A swollen belly, vomiting, diarrhea, seizures, or yellowing of the skin and eyes can also signal infection. A heart rate that’s unusually fast or slow is another red flag.
These symptoms overlap with many other newborn conditions, which is precisely why they’re easy to miss. The key pattern is a baby who seems “off” in a way that’s hard to pinpoint, especially one who was feeding well and suddenly isn’t.
How Screening Reduces the Risk
The most impactful preventive measure is screening for GBS during pregnancy. The current recommendation from the American College of Obstetricians and Gynecologists is a simple swab test during the 36th or 37th week of each pregnancy. If the test comes back positive, antibiotics given through an IV during labor dramatically reduce the chance of transmission. This screening protocol has cut the rate of early-onset GBS disease by more than half since it was widely adopted.
For Listeria, prevention falls on food safety during pregnancy: avoiding soft cheeses made with unpasteurized milk, deli meats unless heated until steaming, and ready-to-eat foods that have been refrigerated for extended periods. Because the mother’s symptoms can be so mild, many cases aren’t caught until the baby is already affected, making prevention through diet the most effective strategy.