Science / Microbe of the Month: Danger can lurk inside an amoeba: Protozoa may be the Trojan horses of the microbe world, harbouring disease-causing bacteria. Bernard Dixon reports

Everybody learns at school about the amoeba - that primitive blob of protoplasm that ingests bits of food and gets about by extending finger-like pseudopodia. But apart from existing, getting about, and occasionally dividing in two, what does the amoeba do? Is its existence really as boring and pointless as it seems?

Not at all. Research on the amoeba and other protozoa, living in the soil, rivers and other ecosystems, is higher up the agenda than ever before. We now know that these microbes live by eating smaller microbes - bacteria. By doing this they control bacterial populations, help to maintain soil fertility and recycle nutrients in aquatic food chains.

In addition, however, researchers are finding that some protozoa serve as reservoirs for the bacteria that are responsible for infections such as Legionnaires' Disease and cholera. Far from destroying these disease-causing bacteria, the protozoa help to protect, maintain and disseminate them.

Legionnaires' Disease - a severe, potentially fatal form of pneumonia - baffled the medical world when it first struck during an American Legion convention in Philadelphia in 1976. Today we know that it is caused by the bacterium Legionella pneumophila, which is commonly found in rivers and lakes. Human health is threatened only when this bacterium is breathed in as a mist or aerosol from cooling towers, humidifiers and showers that harbour it.

Proper maintenance and disinfection of these systems usually prevents any problem. But L pneumophila is a relentless opportunist, able to take immediate advantage of favourable conditions for airborne spread.

Now there is an extra dimension to the story of Legionnaires' Disease. L pneumophila can infect not only humans but certain species of free-living amoebae. The protozoa thereby help the bacterium to survive, especially in the presence of poisonous chemicals. In addition, they may alter its behaviour and serve as vectors (like mosquitoes ferrying malarial parasites) to transmit the bacterium to humans.

Writing in the current issue of Microbiology, John Barker of Sheffield Hallam University and Michael Brown of Aston University, Birmingham, cite this as one example of what could be a much wider role of amoebae and other protozoa as reservoirs for disease-causing bacteria. Protozoa may be the hitherto unrecognised Trojan horses of the microbe world, helping to maintain and distribute many other dangerous micro-organisms too.

When L pneumophila encounters one type of amoeba, the outcome is the opposite of that which is usually portrayed in classroom accounts. The smaller creature is certainly engulfed by the larger one, but it is not then digested in its food vacuole. Instead, the bacterium multiplies rapidly, producing up to 10,000 copies of itself, which are then liberated when the single amoeboid cell bursts.

In adverse environmental conditions, on the other hand, the bacterium can enjoy a safe haven when an amoeba forms a thick-walled cyst for its own protection.

One reason why efforts to eradicate L pneumophila from contaminated water systems sometimes fail at the first attempt is probably because the bacterium is protected in this way from disinfectants such as chlorine. The same explanation may also account for reports of L pneumophila emerging unharmed from sewage treatment processes that remove the vast majority of harmful microbes. Another potential advantage of living for a while in an amoeboid cyst is the opportunity for the bacterium to be blown away through the air to colonise new habitats.

Although, as Barker and Brown point out, the role of protozoa as reservoirs of human disease has come to light only very recently, they have already been found to harbour other harmful bacteria. One is Listeria, formerly thought to be principally found in soil but now widely recognised as a significant cause of food poisoning. It, too, can enter and proliferate within certain varieties of amoeba.

The realisation that protozoa serve as Trojan horses for disease-causing bacteria also offers a solution to the puzzling survival of Vibrio cholerae, the agent of cholera, in places that are entirely free of the disease.

A few years ago, doctors at Preston Hall Hospital, Maidstone, reported that they had found strains of V. cholerae in various parts of Kent, particularly during the summer months. Although some came from streams that could conceivably have been contaminated with sewage, others were from an agricultural drainage ditch where the chances of this occurring were negligible.

There were no cases of cholera in Kent to serve as sources of the bacterium. Cholera is so serious that, on the rare occasions when the disease is imported into the UK, it inevitably comes to the attention of the medical authorities. Intensive scrutiny of Chesapeake Bay, Maryland - an area where cholera has never been known - has also revealed the existence of V. cholerae.

Although most of the strains found there and in Kent did not produce the poisons responsible for classical cholera, their mere presence was disquieting. It now seems more than likely that they had persisted over a long period inside amoebae, following importation from elsewere. Laboratory tests have confirmed that V. cholerae can both multiply inside amoebae and survive there when they form cysts.

Amoebae, clearly, are neither boring nor pointless. It is already clear that their activities impact far more keenly on our world than was suspected even a decade ago. Determining the full extent of that impact is a task for the decade ahead.

Start your day with The Independent, sign up for daily news emails
ebooks
ebooksA special investigation by Andy McSmith
  • Get to the point
Latest stories from i100
Have you tried new the Independent Digital Edition apps?
Independent Dating
and  

By clicking 'Search' you
are agreeing to our
Terms of Use.

iJobs Job Widget
iJobs General

Recruitment Genius: QA Technician

£16000 - £18000 per annum: Recruitment Genius: This leading manufacturer of re...

Recruitment Genius: Office Administrator

£20000 per annum: Recruitment Genius: Due to expansion, an experienced and hig...

SThree: Graduate Recruitment Resourcer

£20000 per annum + commission: SThree: Sthree have an exciting opportunity for...

Recruitment Genius: Plumbing & Heating / Bathroom Trade Counter Sales

£22000 - £26000 per annum: Recruitment Genius: This well established London ba...

Day In a Page

General Election 2015: The masterminds behind the scenes

The masterminds behind the election

How do you get your party leader to embrace a message and then stick to it? By employing these people
Machine Gun America: The amusement park where teenagers go to shoot a huge range of automatic weapons

Machine Gun America

The amusement park where teenagers go to shoot a huge range of automatic weapons
The ethics of pet food: Why are we are so selective in how we show animals our love?

The ethics of pet food

Why are we are so selective in how we show animals our love?
How Tansy Davies turned 9/11 into her opera 'Between Worlds'

How a composer turned 9/11 into her opera 'Between Worlds'

Tansy Davies makes her operatic debut with a work about the attack on the Twin Towers. Despite the topic, she says it is a life-affirming piece
11 best bedside tables

11 best bedside tables

It could be the first thing you see in the morning, so make it work for you. We find night stands, tables and cabinets to wake up to
Italy vs England player ratings: Did Andros Townsend's goal see him beat Harry Kane and Wayne Rooney to top marks?

Italy vs England player ratings

Did Townsend's goal see him beat Kane and Rooney to top marks?
Danny Higginbotham: An underdog's tale of making the most of it

An underdog's tale of making the most of it

Danny Higginbotham on being let go by Manchester United, annoying Gordon Strachan, utilising his talents to the full at Stoke and plunging into the world of analysis
Audley Harrison's abusers forget the debt he's due, but Errol Christie will always remember what he owes the police

Steve Bunce: Inside Boxing

Audley Harrison's abusers forget the debt he's due, but Errol Christie will always remember what he owes the police
No postcode? No vote

Floating voters

How living on a houseboat meant I didn't officially 'exist'
Louis Theroux's affable Englishman routine begins to wear thin

By Reason of Insanity

Louis Theroux's affable Englishman routine begins to wear thin
Power dressing is back – but no shoulderpads!

Power dressing is back

But banish all thoughts of Eighties shoulderpads
Spanish stone-age cave paintings 'under threat' after being re-opened to the public

Spanish stone-age cave paintings in Altamira 'under threat'

Caves were re-opened to the public
'I was the bookies’ favourite to be first to leave the Cabinet'

Vince Cable interview

'I was the bookies’ favourite to be first to leave the Cabinet'
Election 2015: How many of the Government's coalition agreement promises have been kept?

Promises, promises

But how many coalition agreement pledges have been kept?
The Gaza fisherman who built his own reef - and was shot dead there by an Israeli gunboat

The death of a Gaza fisherman

He built his own reef, and was fatally shot there by an Israeli gunboat