A COMMON microbe could help to trigger Alzheimer鈥檚 disease, say researchers
in the US. If true, their controversial claim could turn the multimillion-dollar
field of Alzheimer鈥檚 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鈥攂locked arteries that can lead to heart attacks
(鈥淐an you catch a heart attack?鈥�, 快猫短视频,
8 June 1996, p 38).
Alan Hudson at Wayne State University in Detroit and his colleagues did
postmortems on the brains of 19 Alzheimer鈥檚 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鈥檚 sufferers, in the hippocampus and temporal cortex. These
are the parts of the brain which usually sustain most damage in Alzheimer鈥檚
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鈥檚 patients.
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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鈥檚 presence in the diseased brains does not mean that
it cause Alzheimer鈥檚, 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鈥檚 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. 鈥淚t seems reasonably likely that C. pneumoniae could be
causing the inflammation,鈥� says Hudson.
No one knows why a bacterium that infects most of us should help trigger
Alzheimer鈥檚 in only a few. But Hudson and his colleagues suspect that the brains
of people with a genetic predisposition to Alzheimer鈥檚 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鈥檚 in old age.
If chlamydia does play a role in Alzheimer鈥檚, 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,
including those with Alzheimer鈥檚 disease. So the discovery of C.
pneumoniae in the patients at postmortem might not be significant.