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.