The figure jumps to three-quarters of all children by the age of two, report researchers from the University of Dundee and the University of St. Andrews.

The findings, published in the Journal of Antimicrobial Chemotherapy, shed light on the spread of antibiotic resistance by revealing that children from economically deprived backgrounds are much more likely to be prescribed the medicines than those from more affluent families.

"Most of the children in our cohort were exposed to anti-bacterials very rapidly after birth," reported the researchers, led by Dr. Peter Davey from the Medicines Monitoring Unit at the University of Dundee.

Boys were exposed to antibacterials earlier than girls, they found.

Resistance to common antibiotics has been a major concern for National Health Service hospitals in the UK in recent years.

Earlier this month, the Public Health Laboratory Service--which gathers data on infectious diseases--reported a rise in the number of deaths due to methicillin-resistant Staphylococcus aureus, the so-called hospital superbug that is resistant to most antibiotics.

However, it remains unclear whether exposure to the drugs early in life is an important factor in the development of resistance.

The Dundee researchers wanted to assess how soon in life young children come into contact with antibiotics and to investigate patterns of resistance among patients aged from zero to 40 who had given urine samples at the hospital.

They studied records for all children born in Tayside--a large area in eastern Scotland--during 1993 to see how many had needed treatment for infections in the two years following their birth.

The results showed 63% had needed antibiotics by the age of one and 75% by the age of two.

Sixty-seven percent of boys were exposed in their first year, compared to 60% of girls. Children from poorer areas were significantly more likely to be treated earlier than those from less deprived backgrounds.

Davey and his colleagues said in a report on their findings that this contradicted earlier research.

"Two previous studies reported that children from affluent areas were more likely to be exposed to antibacterials than children from deprived areas. However, both these studies measured total exposure--for example, the number of prescriptions per 1,000 inhabitants--rather than time to first exposure."

They said deprivation might be a factor because it is associated with higher rates of respiratory illness due to over-crowding, smoking and lower levels of breast-feeding.

But tests on urine samples showed resistance acquired through exposure to drugs does not necessarily remain constant as the children get older.

"Resistance increased with age up to six years, then declined with age until 20 years, then increased with age," the researchers said.

"Antibacterial exposure was highest in the first two years of life and decreased steadily thereafter."

SOURCE: Journal of Antimicrobial Chemotherapy 2002;50:1085-1088.