Bugs in the brain

By Phyllida Brown A COMMON microbe could help to trigger Alzheimer’s disease, say researchers in the US. If true, their controversial claim could turn the multimillion-dollar field of Alzheimer’s research on its head and force a rethink on how to prevent the disease. The microbe in question is Chlamydia pneumoniae, which is spread by coughs and sneezes. By the age of 20, half the population have been infected with C. pneumoniae, and the likelihood of being infected increases with age. The bacterium has already been accused of triggering atherosclerosis—blocked arteries that can lead to heart attacks (“Can you catch a heart attack?”, New Scientist, 8 June 1996, p 38). Alan Hudson at Wayne State University in Detroit and his colleagues did postmortems on the brains of 19 Alzheimer’s patients and 19 people of the same age who had died of other causes. They found signs of C. pneumoniae in 17 of the Alzheimer’s sufferers, in the hippocampus and temporal cortex. These are the parts of the brain which usually sustain most damage in Alzheimer’s disease. Unaffected areas of the brain were much less likely to harbour the bacterium. The bacterium turned up in the brain of only one of the non-Alzheimer’s patients. The team also managed to culture the microbe from two of the affected brains, showing that the organism was still alive rather than a long-dead bystander (Medical Microbiology and Immunology, vol 187, p 23). C. pneumoniae’s presence in the diseased brains does not mean that it cause Alzheimer’s, the scientists stress. But they think the bacterium may at least be a risk factor. Chlamydia bacteria do cause inflammation when they attack other parts of the body. And the brains of people with Alzheimer’s are inflamed and contain high levels of messenger chemicals called cytokines, which trigger inflammation. Hudson says the bacterium infects microglia and astroglia, the cerebral cousins of scavenger cells called macrophages, and this produces inflammatory cytokines. “It seems reasonably likely that C. pneumoniae could be causing the inflammation,” says Hudson. No one knows why a bacterium that infects most of us should help trigger Alzheimer’s in only a few. But Hudson and his colleagues suspect that the brains of people with a genetic predisposition to Alzheimer’s disease may be more vulnerable to infection. People carrying a particular variant of a gene for a protein called ApoE are known to have an increased risk of developing Alzheimer’s in old age. If chlamydia does play a role in Alzheimer’s, scientists will have to find out if the disease could be prevented by simple antibiotics. But because C. pneumoniae lives inside cells, it may not respond to treatment. In any case, in the absence of evidence for a causal link, it is too soon to speculate about treatment, Hudson says. Gareth Roberts, a neuroscientist who runs an independent consultancy, Opine, in Cambridge, says the results are interesting but need further analysis. He points out that pneumonia is a frequent cause of death among older people,
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