BY ALTERING the balance of gut microbes, antibiotics can disrupt the immune system鈥檚 ability to distinguish between innocuous substances and harmful microbes. This finding, from experiments on mice, adds weight to the notion that antibiotics could be at least partly responsible for the rise in allergies and asthma in children.
The antibiotic hypothesis is one of many competing theories for the worldwide rise in asthma. Its proponents argue that the increase has tracked widespread antibiotic use. For instance, research in Berlin found that both antibiotic treatment and asthma were low in the east compared with the west before the wall came down. But as antibiotic use has gone up in the east, so has asthma.
And last year a team in Detroit that monitored 500 children for seven years reported that those given antibiotics before they were 6 months old were twice as likely to develop allergic asthma. Those given broad-spectrum antibiotics were up to 9 times as likely to suffer from the condition (New Scientist, 24 October 2003, p 16).
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But critics pointed out that as most of these children had been given antibiotics to combat respiratory infections, it may be the infections that are to blame rather than the antibiotics.
Now Gary Huffnagle鈥檚 group at the University of Michigan in Ann Arbor has provided the first experimental evidence that upsetting the gut flora can increase the risk of an allergic response. The researchers altered the balance of gut microbes in some mice by giving them a course of antibiotics before feeding them a yeast found on human skin. With most bacteria killed, the yeast quickly colonised their guts.
Then the team exposed all the mice to fungal spores that can trigger allergies in people. The immune response was much stronger in the mice whose gut flora had been altered. 鈥淪uddenly, the ability to ignore a mould spore has gone,鈥� Huffnagle says.
The team also exposed some mice to ovalbumin, a chicken egg protein commonly used in allergy research. In this case, the effect on the animals鈥� lung linings was striking. 鈥淭hey are shredded, absolutely shredded. I鈥檓 sure they can鈥檛 breathe,鈥� says Huffnagle.
鈥淗e鈥檚 onto a very special track,鈥� says Juneann Murphy, an expert in stomach bacteria at the University of Oklahoma in Oklahoma City. 鈥淣o one else has been able to make the connections before.鈥� Doctors are already being urged to use antibiotics only when absolutely necessary, to minimise the spread of resistant strains, and Murphy says these results simply reinforce this message.
Precisely why altering the gut flora should affect the immune system of mice remains a mystery. But if the same happens in humans, there might be a simple solution, Murphy suggests. People on antibiotics could also be given 鈥減robiotic鈥� tablets or drinks containing gut bacteria. Some people already take probiotics to prevent side effects like diarrhoea.