The reason why some children develop allergies and others don't may be partly due to the amounts of different bacteria in their intestines, even before their first allergic symptoms appear.

An international group of researchers followed infants from Estonia and Sweden, measuring the composition of microbes in their stool starting a few days after birth and then periodically during the first year of their lives.

The investigators found that children from both countries who developed allergies by the time they turned 2 years of age had different amounts of certain bacteria in their guts than those without allergies, with more of some bacteria and less of others.

Specifically, the researchers note that more nonallergic children had enterococci and bifidobacteria, while more allergic infants exhibited Staphylococcus aureus and higher amounts of clostridia.

This research supports a growing body of data linking allergies to the presence or absence of different bacteria in the intestines of infants.

More children develop allergies in Sweden than Estonia. Demonstrating that gut bacterial composition can precede allergy development in children from these two disparate countries helps eliminate the influence of standard of living, reinforcing the connection between these microbes and allergies.

Although diet can influence the amount of bacteria in a child's gut, the differences in microbe composition between allergic and healthy children were more likely due to hygiene.

All of the bacteria tested in this study are more or less ubiquitous in the environment, so how much of one or the other an infant has in her gut likely stems from how much is present in the delivery ward, with cleaner hospitals supporting less bacteria.

Modern hygiene standards in rooming may have reduced the microbial load, thus delaying colonization slightly.

Although it is not clear why infants benefit from the presence of some bacteria, researchers suggest that these bacteria stimulate the babies' developing immune systems, helping them ward off allergies.

Indeed, comparing the intestines of both groups of children provides an interesting historical perspective, for the children from Estonia have similar flora to children from Western countries 40 years ago.

The microbial environment, or ecology, has changed in Western industrialized societies. The gut flora in Estonia is rather similar to that prevailing in infants in Sweden and West Germany in the early 1960s, so the flora has changed in Sweden."

As this study demonstrates, cleaner is not necessarily better, since these changes in hygiene may be responsible for the different allergy rates between the two countries.

Among the babies followed in this study, almost all of the Swedes, but only 9 out of 15 Estonian children, developed allergies by the age of 2.

Journal of Allergy and Clinical Immunology 2001;108:516-520