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The composition of the compost may play a role in its bacteria-bearing ability. The study authors noted that UK compost used to be made mostly from peat, but now contains more wood residues, such as sawdust. The contaminated Australia compost from 1989 was made from wood scraps.
"It may be that the change in composition of composts in the UK, moving away from peat based products, could be resulting in species such as Legionella longbeachae being present in compost and therefore more cases of infection could occur," Beattie added.
However, 12 out of 18 peat-based composts contained Legionella as well. Hence, just because a compost contains peat doesn't mean it is Legionella free. The bacteria may have been introduced by other ingredients in the compost mixture, suggested the study authors.
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Most healthy people face little threat from Legionella or Legionnaire's disease. A healthy body can usually fight off the bacteria. However, the CDC warns that Legionella infection risk is greater for older individuals, former smokers, chronic lung disease patients and those with weakened immune systems.