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Udder suicide

E. coli kill off milk-making mammary cells.

Udder infections cost dairy farmers millions. Credit: © Corbis

Milk loss is the crime; Escherichia coli is the culprit. By infecting cows' mammary glands, these bacteria are driving milk-making cells to suicide, according to ongoing investigations into costly udder troubles1.

Mastitis - inflammation of the mammary glands - cuts milk yield, costing the US dairy industry alone $1.7 billion a year - $180 per cow. E. coli is one of the chief bacterial offenders.

E. coli trigger cell death in udders, Xin Zhao of McGill University in Quebec, Canada and his team now show. "Cell death contributes to decreased milk production," he says. Conversely, the bugs also increase cell division, the researchers found.

This result is potentially exciting, thinks vet Andrew Bradley of the University of Bristol, UK. Some E.coli infections are highly persistent, suggesting they adapt to life inside mammary glands. If the bacteria are invading cells and stimulating division, it may be "a way of staying in, and hiding from, the host," Bradley speculates.

Milk bar

Around 150 different bacteria enter cows' teats and cause mastitis. In the early 1960s, infection by contagious Streptococcus and Staphylococcus was rife. Faster treatment, improved dairy hygiene and regular antibiotics cut cases significantly, in the UK and worldwide. Currently, around 12 per cent of cows carry an infection.

E. coli and other bacteria picked up from the environment are now the most common cause of mastitis, says Bradley, and research focuses on these. Milk is routinely spot-checked for 'somatic cell count', which rises during infection as cows produce extra disease-fighting blood cells. The quality of such milk is too poor to be used for dairy products.

Antibiotics are farmers' main defence. To allay "consumer concern" about the drugs passing into human consumption, cows are not milked during treatment or for a few days afterwards, until the drug clears from the body, explains Michelle Stephens of the UK National Dairy Council.

"It's a highly contentious area," agrees Eric Hillerton of the Institute of Animal Health in Compton, UK. Most cows also receive long-lasting antibiotics between milking periods to wipe out latent infections, and many cases clear up without intervention. But even with treatment, bacteria may lie low, ready to flare up again; cows with chronic infections are culled.

The first vaccine against E. coli recently went on the market. But with 150 strains to tackle, "it's a complicated product," says Hillerton. Understanding the bugs' mode of action may expose further targets, he thinks.

Infection causes huge changes in mammary glands: "Milk-secretory cells are nobbled," says Hillerton, and milk yield may never recover. By interfering with cell death, "you could reduce the amount of long-term damage or even reverse it," he speculates.

"We're not just talking about cows," he adds. Women's mammary glands are also susceptible to the same painful infection - mastitis affects about 40 per cent of breastfeeding mothers. Hillerton is looking at how HIV transmission might be affected by mastitis. "Maybe we could use knowledge in cows to apply to humans," he hopes.


  1. Long, E. et al. Escherichia coli induces apoptosis and proliferation of mammary cells. Cell Death and Differentiation 8, 808 - 816 (2001).

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Pearson, H. Udder suicide. Nature (2001).

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