IN BRIEF
- Most gardeners face little risk, but older people and those with weakened immunity can develop serious infections.
- To reduce risk, wet potting mix before use, wear a mask and gloves, open bags away from your face and wash your hands afterwards.
For millions of Australians, gardening is a weekend ritual, but experts warn there can be more than nutrients and organic matter in a bag of potting mix.
Bacteria and fungi are a natural part of the material, and while most pose no threat to healthy people, some can cause serious infections in those who are more susceptible.
One of the best-known examples is Legionella longbeachae, a species of bacteria associated with potting mix and soil that can cause Legionnaires' disease.
'I didn't think twice about it'
For 72-year-old Sydney gardener Robert, that risk became a reality after routinely handling potting mix without taking the precautions recommended on the packaging.
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He told SBS News that he ended up in hospital after developing Pontiac fever, a milder form of illness caused by exposure to Legionella bacteria.
Unlike Legionnaires' disease, Pontiac fever does not cause pneumonia and usually resolves on its own.
But Robert's experience was a reminder that something as ordinary as opening a bag of potting mix can expose people to bacteria capable of causing illness.
While he recovered after a trip to the emergency department, he says the experience has changed how he approaches gardening.
"I never really thought twice about handling potting mix. You just open the bag and get stuck into it," he said.
"After getting sick from it, I'll definitely be wearing a mask and gloves because it's just not worth taking the risk of getting something worse next time."
His case also highlights the spectrum of illness associated with Legionella, from a relatively mild, self-limiting infection to potentially fatal pneumonia.
A microscopic ecosystem
Richard Bentham, adjunct associate professor at Flinders University, says potting mix is effectively a living ecosystem.
"Potting mix is made by microorganisms, so it's very, very rich in microorganisms," he says.
He told SBS News that some microbes can cause infection when they encounter someone who is particularly susceptible.
Legionella longbeachae is one example, but Bentham says other bacteria, including mycobacteria, can also be found in potting mix and potentially cause infection.
But he insists that the presence of microorganisms does not mean potting mix is inherently dangerous.
Bentham said millions of bags are used safely every year and that the risk of serious infection remains low, but the consequences can be severe for people who are more vulnerable.
From potting mix to the lungs
Experts say the main concern is what happens when contaminated material is disturbed, and fine particles can become airborne when dry potting mix is opened or handled, allowing microorganisms to be inhaled.
Bentham said inhalation is considered an important route of infection, although researchers aren't certain it's the only one. Contaminated material could also potentially be transferred from the hands to the mouth.
Legionella longbeachae can reach the lungs and, in susceptible people, develop into serious pneumonia.
The Australian Centre for Disease Control says L longbeachae is most commonly associated with potting mix, compost and garden soil, while other Legionella species are more commonly associated with water systems such as cooling towers.
"This Legionella longbeachae has nothing to do with air conditioning or water systems," Bentham says.
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