Infants delivered by cesarean section developed cavity-inducing oral bacteria almost a year earlier than those born vaginally, researchers reported.
Despite the caveats that women in the study were all poor and many had a history of STDs, poor oral health, and alcoholism, the cesarean connection still held, the researchers reported in the September issue of the Journal of Dental Research.
Cesarean infants acquired cavity-causing Streptococcus mutans 11.7 months earlier than did those born vaginally, said investigators led by Yihong Li, Dr. PH, D.D.S., an associate professor of craniofacial biology at New York University here.
They said cesarean-born infants may have weakened defenses against bacteria because of reduced bacterial exposure during delivery. By contrast, vaginally born infants receive more intense bacterial exposure from the vagina and perineum, which may help strengthen their immunity. This may help explain why cesarean babies show greater bacteria levels at an earlier age, Dr. Li said.
Among cesarean infants, "an atypical microbial environment may prevail, providing more potential biological binding niches for 'latecomers' such as S. mutans," the researchers wrote. "The mode of delivery may be an appropriate question to be included in past medical history for further determination of why some children are at greater risk for caries than others."
In a prospective cohort study, Dr. Li and colleagues followed 156 low-income primigravid women and their infants in Birmingham, Ala., from 1995 to 1999. A total of 127 infants were delivered by spontaneous vaginal delivery. The remaining 29 births were elective cesareans.
A total of 31% of the cesarean infants and 36.7% of the vaginal infants had detectable levels of S. mutans, but the cesarean infants acquired S. mutans on average around age 17.1 months compared with 28.8 months (p=0.038) for the vaginally delivered children, the researchers reported. Saliva and plaque samples were collected every three months from both mother and child.
The DNA of the bacteria from the cesarean infants was also identical to their mothers' bacteria, whereas three different genotypes were found in the vaginal-delivery pairs. This suggests, the researchers wrote, that by avoiding passage through the birth canal, cesarean infants are less likely to be exposed to various bacterial species.
The infant's gender, birth weight, or breastfeeding did not affect S. mutans colonization. However, other maternal factors, including socioeconomic status, did play a role.
Ninety-one percent of the participants were African-American. Maternal age ranged from 16.3 to 34.5. Participants' annual incomes were on average $10,000 a year or less. Average maternal age was 21. Forty-five percent of the mothers had a history of STDs, and 91.7% were treated for their infection before or during pregnancy. A total of 8.3% of the mothers drank alcohol, and 11.8% were regular smokers. Three-quarters of the mothers had cavities.
Factors -- beyond the mode of delivery -- affecting bacterial colonization included being an older mother (p=0.049), prolonged gestational age (p=0.04), prior STD infection in the mother (p=0.01), higher S. mutans levels (p=0.02) in the mother's saliva, and extreme poverty (annual income of less than $1,000) (p<0.01). All these "had an effect on either the percentage of colonization or the age of acquisition of S. mutans in infants," they wrote.
It is true that lower socioeconomic status has been linked to poor oral health, Dr. Li said, adding that whether similar findings would be seen among more affluent populations is unknown. Yet even after accounting for these factors, Dr. Li added, the association between the timing of S. mutans's arrival and cesareans persisted.
What researchers could not determine is whether the mothers' antibiotic treatments for their STDs had any role in the acquisition and the timing of S. mutans. "It's very possible that antibiotics might have had a role," Dr. Li said.
More study is needed, Dr. Li said, to see if these findings could occur among other populations and what the biological mechanism might be behind the association between the timing of S. mutans growth and cesarean delivery.
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