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How I Learned to Stop Worrying and Love the Bug

Legionella causes about 300 infections per year in Australia.

Legionella causes about 300 infections per year in Australia.

By Michael Taylor

The public may think of Legionella as a deadly disease, but Michael Taylor sees parallels with the structure of the universe in the slimes he examines under the microscope.

When I tell people that I grow disease-ridden slime for a living, I usually get an interesting reaction. Sometimes it involves them sneaking away to wash their hands, but most often people just ask: “Why?”.

My enthusiasm often gets the better of me and I start by telling them about my research. I dive right into explaining the system I built to grow the dangerous bacteria Legionella, a one-of-a-kind set-up designed to study this organism under controlled circumstances. This device took the first high resolution images of Legionella growing in its natural environment, and found it living as both a parasite and independent of a host – a finding that challenges some long-held theories about this pathogen.

But that’s not really the best place to start, and sometimes I wonder how I ended up here myself. I didn’t start out with a passion for slime, but somewhere along the way it just happened. So usually I try to explain it like this…

No one likes the thought that through no fault of their own they could contract a disease: just by doing the weekly shopping, taking the kids to the aquarium, or going with grandma to see a flower show. On the other hand, we’ll admit that the reason we’ve been vomiting for six straight hours is because lunch consisted of left-over chicken sausages that had been fermenting in the bottom of the fridge. They should have been binned after we smelt them, but ultimately we did this to ourselves.

This random, bolt out of nowhere is what makes airborne pathogens so unnerving. Each of the activities mentioned earlier have been the source of Legionnaires’ disease outbreaks: 125 cases at the Melbourne Aquarium in 2000, 188 at a flower show in The Netherlands in 1999, and 12 at a supermarket in Spain in 2006. The people involved were simply going about their everyday business and by simply being in the wrong place, or because the wind was blowing in an unfortunate direction, they ended up inhaling a lung full of bacteria.

In terms of large public health problems, Legionella ranks fairly low, on average causing about 300 infections per year in Australia. However, the fear of being struck down randomly causes a strong reaction. For a bacteria that affects so few, we certainly hear more about it than Salmonella, which causes more than ten times as many infections but somehow seems so much less frightening.

Legionnaires’ disease is caused by bacteria from the genus Legionella, and has been the focus of my research at Flinders University for the past 6 years. The bacterium received its name and identity after being recognised as the source of a mysterious illness that struck in 1976 at a convention of the American Legion. This outbreak caused 29 deaths among the 180 people attending the convention, and Legionella received its name and identity the year after. While Legionella may well have been the cause of similar illnesses before 1976, this was the first time that such a large concentration of people had been so rapidly infected.

So what happens if you’re unfortunate enough to inhale a load of Legionella? First off, not everyone will get Legionnaires’ disease. Often a selective disease, infections tend to occur in people who are already unwell with ongoing lung and heart conditions, particularly the elderly. Another group at risk is people who have weakened immune systems. Hospitals closely monitor people who are undergoing chemotherapy or have received organ transplants because their compromised immune systems make them extremely susceptible to diseases like Legionnaires’.

The disease itself is a type of severe bacterial pneumonia, a lung infection that causes an accumulation of fluid in the chest, a raging fever, diarrhoea, and has a mortality rate of 15–45% depending on the original health of the patient. For someone fit and healthy, coming in contact with Legionella may only produce flu-like symptoms that do not even warrant a trip to the doctor, or possibly no illness at all. This less serious version of the disease is called Pontiac fever, and is more commonly associated with poorly maintained spa pools, where bathers breathe in the mist and steam that floats around the surface of the tub.

The particular species of Legionella that causes the greatest number of illnesses worldwide is Legionella pneumophila. Oddly, “pneumophila” is Greek for “lung-loving”, which doesn’t quite seem accurate considering its symptoms.

In Australia, particularly South Australia, we buck the global trend. Rather than 90% of our Legionnaires’ disease cases being caused by L. pneumophila, at least 40% of our Legionella infections are caused by a related species called Legionella longbeachae, which is named after Long Beach in California.

If you’re someone who likes to garden you’ve probably seen the warnings on bags of potting mix which read something like: “This bag contains microorganisms that can cause illness”.

L. longbeachae is one of the organisms they’re warning you about. Occasionally the soil and dust from these bags can contain Legionella, and if you get a lung full of it you can end up in hospital. So next time you head out into the garden make sure you lightly wet your potting mix to keep the dust down, or wear a mask capable of excluding any floating soil particles. If you want to be extra vigilant, try not to poke a dirt-caked finger into your nose or mouth. Bacteria aren’t fussy about how they get into you and snorting them up works just as well as breathing them in.

There are a few theories as to why L. longbeachae is a particularly South Australian phenomenon, and it may have something to do with the kind of materials we put into our potting mix. While Europeans tend to use a lot of peat moss, we use a higher portion of pine mulch. Or it may be a perfect storm of other factors: we have an ageing population combined with a climate that is perfect for pottering about in the garden for quite a few months of the year. We simply don’t have a definitive answer yet, but a few of my colleagues have begun to put their heads together and we may soon have some leads to follow.

The common link between all Legionella and where to find them, be it moist soil or the inside of a pipe, is their love of warm, wet environments. Unfortunately humans tend to build a lot of these, usually right next to where we live and work. Places like large hot water systems in hotels or air-conditioner cooling towers and heat exchange systems provide the perfect conditions for bacterial growth. Legionella doesn’t work alone though, and only inhabits these systems when other bacteria have already moved in and built a nice slimy home.

Microorganisms working together can create vast communities underneath this layer of slime. If you’ve ever had to clean a neglected fish tank you’ll know the exact slime layer I’m talking about. These complex microbial communities are described as biofilms, and can contain a huge diversity of life: not just the bacteria that make the slime but also fungi, algae, nematodes (microscopic worms) and a plethora of amoebae and protozoa that can be thought of as a bit like microscopic pigs, grazing on any organic waste and bacteria they find. It’s the presence of these protozoa and Legionella’s relationship to them that tends to cause the most arguments among Legionella researchers.

There are currently two prevailing ideas about the lifecycle of Legionella in the environment: either that it requires a protozoan host to parasitise in order to survive, or that it can get by on its own using nutrients caught in the biofilm. Normally a protozoan ingests bacteria for food, but Legionella not only survive being swallowed they reproduce within their host until it bursts, releasing them back into the environment. The reason it’s so important to know how Legionella lives is so that we can understand how to control and remove it from places where it could cause harm.

When I started my research I wanted to try and shed some light on how Legionella survived in human constructed environments. I couldn’t deliberately contaminate a working cooling tower, and sticking a contaminated cooling tower inside a lab isn’t really a great idea either. So I needed to build something that worked like a cooling tower, fit in a lab, and didn’t belch bacteria out over my lab-mates. What I ended up putting together looked like a plumbing pipe-organ, but despite its appearance it worked fairly well, didn’t leak any Legionella, and soon I had more slime than I knew what do to with.

A lot of the biofilm I grew ended up dead, dosed with chlorine and other chemicals that are added to cooling towers to control bacteria. I was looking for a way to detect the number of living Legionella left hiding in the slime after I tried to disinfect it. I used a method called the polymerase chain reaction to detect the amount of Legionella DNA left in a sample, and attempted to tease out only the DNA contained in living cells. I came fairly close, and with some further work we may soon have a rapid way of detecting only the living Legionella in a water sample.

What I really wanted, though, was to actually see what was going on inside my biofilm. Using high-powered microscopes I peered into the inner workings of the bacterial community and photographed what I saw. There were plenty of bacteria spread about through my slime, but only little pockets of Legionella tucked into tightly packed clusters. I also saw Legionella living inside protozoa as well as Legionella dotted around the place on their own without a host. Unfortunately, to prepare my samples meant that I had to kill them before sticking them under a microscope, but it certainly appeared that my bugs were living outside as well as inside protozoa.

What amazed me most became apparent when I started to peer as close as I could get into the slime using microscopes. The patterns and shapes that appeared in the biofilm looked like images of galaxies, arrangements of planets teeming with life that could have been captured by the Hubble telescope. It never ceases to astonish me that it doesn’t matter if we look up at huge objects that are very far away or down at miniscule things up close: it all starts to look very familiar after a while.

Eventually I realised that each little section of biofilm is almost a universe of life in its own right, containing billions of interconnected organisms working together, feeding on each other and fighting to survive. A little like us.

And that’s usually what I answer when people ask why I play with slime.

As for what’s next, there’s always work to be done with public health. I’d like to continue to improve our Legionella detection methods to be even more rapid and accurate, hopefully finding and eliminating the problem before an outbreak occurs. It would also be fascinating to dig deeper into Legionella ecology in biofilms and observe more closely how it can survive without a host.

Or maybe I’ll pick a research direction that I can bring up in conversation more easily. Maybe something like mould…

Michael Taylor has been investigating the ecology of Legionella in potting mix and human-constructed environments for his PhD, which he completed at Flinders University’s School of the Environment under the supervision of Kirstin Ross and Richard Bentham.