Protozoa: The predators, packagers, and prey of Legionella.

An emerging body of evidence demonstrates that poorly managed water supplies can result in a wide spectrum of microbial disease in health and aged care premises. These organisms including protozoa and bacteria are abundant and persistent in the built environment.

Legionella contamination in building water systems has been a major focus of public health legislation and guidelines both nationally and internationally. Less focus has been placed on a wide range of other microorganisms that also pose a significant threat to human health. The effects of some of these organisms is indirect. Protozoa play a significant part in the life cycle of Legionella as the predators, packagers and the prey. So how do they fulfil all three roles? Read on!

The Protozoa Clan

Protozoa are single cell organisms that feed on bacteria and other organic material. Sometimes this is by active ingestion other times it is by diffusion. There are five different groups of protozoa. their behaviours are very different and so are their health impacts. Diseases such as malaria, dysentery, sleeping sickness, meningitis, and keratitis are all protozoan infections. Some only survive as parasites and others never cause disease. In the middle are opportunists that may cause or contribute to disease in some situations. Of the five different groups the major problem families in potable water systems are Sarcodina (Sark-o-dina) and Ciliophora (Silly-o-fora).


The Sarcodina, often known as amoeba, have very similar characteristics, they are mobile and actively crawl around hunting food. The cells are called trophozoites (troffo-zo-ites) and multiply by simple division. One sub-group of the sarcodina are particularly important to us: the cyst-forming amoebae. The cyst forming amoebae exist in two forms; a slug-like trophozoite that actively grazes surfaces for food, and a dormant hard walled cell called a cyst. There is also a small group that can form a third ‘swimming’ cell but let’s not get too ahead of ourselves!

Trophozoites have only a single outer membrane for protection which leaves them vulnerable to disinfectants. Cysts have a hard exterior cell wall (shell) that protects the organism from disinfectants, heat and drying out. In fact they will survive over 30 mg /L chlorine for hours! The growth range for cyst forming amoebae, depending on the species is between 25 and 44⁰C, very similar to the growth range for Legionella. Also very similar to common potable water temperature ranges.



Left: Three protozoan cysts in a sea of bacteria.




The ciliophora are often known as ‘ciliates’. They are mostly mobile cells that use fine hairs (cilia) to propel themselves around and to flush food into a primitive stomach. Usually they are smaller than amoebae (plural of amoeba) and move more quickly. Ciliates lead more simple lies than amoeba as they only have one form. However they are structurally more complex than amoebae.They have a diverse range of shapes and sizes that make them more easily identifiable to the trained eye. A ‘typical’ ciliate is paramecium (para-mee-see-em) though truly there are no typical forms.






Far left: paramecium

Left: rotifers in waste water



In the normal cycle amoebae are the predators. They graze biofilms for bacteria ingest and digest them and multiply. If conditions are unfavourable they enter the cyst form. Once in this form they can survive heat, drying out, and disinfection until more favourable conditions return. They play an important role in keeping the microbial community happy. They perform a similar role to animal predators ensuring the survival of the fittest. For example remove the protozoans from soil and it loses fertility.





Legionella and a number of other bacteria, including Mycobacteria and others listed below, can infect and multiply within cyst forming amoebae. Once inside the amoebae they multiply and eventually kill the host releasing more bacteria into the surrounding environment. As well as providing food for the bacteria the amoebae provide protection from disinfectants, even more so if they are inside the cysts.


Ciliates are less susceptible to Legionella attack. Often they cannot digest the Legionella when it infects them. In some species instead they wrap the bacteria in sticky membranes (like cling wrap) and then expel them from the cell. These sticky parcels of Legionella cause two problems. Firstly they are small enough to be inhaled by humans. So tiny Legionella bombs can be breathed in. Secondly the sticky membranes preserve the Legionella from disinfectant and from drying out. In this way the ciliates can assist the survival of Legionella for long periods.

A problem in their own right

Because cyst-forming amoebae graze surfaces for organic matter, typically biofilms, they may sometimes cause human infection. Some delicate tissues such as membranes covering the eye or surrounding the brain are the sites of infection and disease can be life threatening. This usually occurs in young children or immune compromised individuals. These infections are not a normal part of their life cycle which means they are opportunist pathogens. Unfortunately a sad example is keratitis. Contact lenses that are not placed in clean (sterile) water between uses can allow growth of amoebae on the lens surface. So if one cell gets between the lens and the cornea when it is on the eye it will eat what is available causing hard to treat keratitis.


Control of these organism will help to control Legionella. A Water Safety Plan needs to consider ways of keeping protozoa under control. So reducing nutrients, safely filtering water, residual disinfection, good temperature control and regular servicing of plumbing will go along way to solving the problem.

Further Reading

World Health Organisation (2007) Legionella and Prevention of Legionellosis

Carter, A. (2009) The hunt for microbial ‘Trojan horses’ .Oceanus Magazine Vol. 47, No. 3.