Could bacteria present naturally in the body and sometimes added to food or dietary supplements keep colds at bay?

In a study done in China, small children who drank a mixture of such bacteria — known as probiotics — in milk twice a day during the winter and spring had fewer colds, needed fewer antibiotics, and missed fewer days of school than other children who drank plain milk instead.

Researchers have shown in some studies that probiotics can benefit those who are already ill with various conditions, and the bacteria are thought to boost the immune system's response to invaders. Whether they were effective at preventing sickness, however, was unclear.

The study in China involved 326 children, ages 3 to 5 years, who were randomly assigned to three different groups: one given milk with a bacterium called Lactobacillus acidophilus mixed in, another that received the same organism along with a strain of another bacterium, Bifidobacterium animalis, and a third that received just milk with placebo.

The test formulas were given twice daily, from November 2005 to May 2006, by school officials during the week and by parents or guardians on weekends.

In the journal Pediatrics, the researchers report that compared to the placebo group, the Lactobacillus group had 53 percent fewer fevers, 41 percent fewer cough episodes, and 28 percent fewer runny noses.

The Lactobacillus/Bifidobacterium group had even larger reductions in symptom rates: 72 percent fewer fevers, 62 percent fewer coughs, and 59 percent fewer runny noses.

For example, that meant 66 fevers in the plain milk, 31 in the Lactobacillus group, and 18 in the combined probiotic group. All of the groups were comparable in size, at about 100 children.

Furthermore, when children in the test groups did get fevers, coughs or runny noses, they recovered significantly faster. Compared to placebo, the length of illness was decreased by 32 percent with Lactobacillus and by 48 percent with Lactobacillus/Bifidobacterium.

The investigators also note that antibiotic use was 68 percent lower in the Lactobacillus group and 84 percent lower in the Lactobacillus/Bifidobacterium group, compared to the placebo group.

Finally, they say, children who received the probiotics were absent from day care 28 to 32 percent less often than children in the placebo group.

The combination probiotic product used in this study is now marketed as "HOWARU Protect" by Danisco in Madison, Wisconsin. Danisco's Dr. Gregory J. Leyer, who headed the study, and his colleagues believe that probiotics may limit the need for medications. The use of medications in children has come under particular scrutiny, Leyer and his co-authors note in the study.

Still, Leyer noted some caveats. "Not all probiotics are the same...not all do everything," he told Reuters Health. Some probiotic strains, for example, are effective against allergies or at preventing diarrhea.

For this study, he said, he and his colleagues used "very carefully selected organisms" and doses, with the specific intention of "warding off cold and flu." That suggests the results cannot be widely applied to other bacteria or other illnesses.