Amoebae pose a unique threat to the healthcare environment. Not only are they pathogenic in their own right, but they can also function as “Trojan horses” that can harbor and safely transport Amoeba-resistant strains of bacteria, viruses, and occasionally fungi⁽¹⁾. Amoeba-resistant organisms are defined as those microbes that have evolved to resist destruction by free-living amoeba⁽¹⁾. Some Amoebaresistant microbes cannot be engulfed by amoeba, while others can survive and grow after internalization and subsequently return safely to the environment⁽¹⁾. Amoebae have been recovered from drinking water, cooling towers, and hospital water networks⁽¹⁾. They often live on biofilm and other water-soil, water-air, and water-plant interfaces⁽³⁾. When they are confronted with harsh environmental conditions that threaten their survival, they convert to a resting form known as a cyst⁽¹⁾. Amoeba cysts are resistant to chlorination, adverse pH, osmotic pressure, and temperature⁽¹⁾. Free-living amoebae have demonstrated resistance to hyperchlorination treatment of complex water delivery systems⁽²⁾.

Amoeba-Resistant Microbe Facts:

• Studies have confirmed that free-living amoebae are necessary for Legionella multiplication in water biofilm, although they may survive in a latent state in biofilm without amoebae⁽⁴⁾
• Microbes that are engulfed by amoebae are encased in internal structures called vesicles. A single vesicle inside an amoeba may harbor thousands of Legionella bacteria^(5,6).
• When an amoeba containing L. pneumophila encysts to protect itself from adverse environmental conditions, it may protect L. pneumophila from the effects of chlorine⁽⁷⁾.
• The survival of Mycobacterium avium within amoeba cysts has been well documented^(8,9).
• Contamination of intensive care unit tap water led to a cryptic epidemic of ventilatoracquired pneumonia caused by L. anisa⁽¹⁰⁾.

Amoeba-Resistant Microbes and Point-Of-Use Filtration:

• Free-living Acanthamoeba spp. measure 15-45 microns in diameter, while their cysts measure 10-25 microns in diameter⁽¹¹⁾. They and the Amoeba-resistant microbes that they might harbor are therefore easily retained by point-of-use 0.2 micron filters.

Some waterborne amoeba-resistant microorganisms include the following:

• Legionella pneumophila
• Other Legionella spp.
• Pseudomonas aeruginosa
• Burkholderia cepacia
• Burkholderia pseudomallei
• Mycobacterium avium
• Mycobacterium fortuitum
• Mycobacterium simiae
• Flavobacterium spp.

References:
1. Greub, G. and D. Raoult. 2004. Microorganisms resistant to free-living amoebae. Clin. Micro. Reviews. 17(2):413-433.
2. Ortolano, G.A., M.B. McAlister, J.A. Angelbeck, J. Scaffer, R.L. Russell, E. Maynard, and B. Wenz. 2004. Hospital water point-of-use filtration: a complementary strategy to reduce the risk ofnosocomial infection. Filtration Supplement 1:3-26.
3. Rodriguez-Zaragoza. S. 1994. Ecology of free-living amoebae. Crit. Rev. Microbiol. 20:225-241.
4. Murga, R., T.S. Forster, E. Brown, J.M. Pruckler, B.S. fields, and R.M. donlan. 2001. Role of biofilms in the survival of Legionella pneumophila in a model potable water system. Microbiology 147:3121-3126.
5. Anand, C.M., A.R. Skinner, A. Malic, and J.B. Kurtz. 1983. Interaction of L. pneumophila and a free-living amoeba (Acanthamoeba palestinensis). J. Hyg. 91:167-178.
6. Berk, S.G., R.S. Ting, G.W. Turner, and R.J. Ashburn. 1998. Production of respirable vesicles containing live Legionella pneumophila cells by two Acanthamoeba spp. Appl. Environ. Microbiol. 64:279-286.
7. Kilvington, S., and J. Price. 1990. Survival of Legionella pneumophila within cysts of Acanthamoeba polyphaga following chlorine exposure. J. Appl. Bacteriol. 68:519-525.
8. Primm, T.P., C.A. Lucero, and J.O. Falkinham III. 2004. Health impacts of environmental mycobacteria. Clin. Micro. Rev. (17)1:98-106.
9. Steinert, M., K. Birkness, E. White, B. Fields, and F. Quinn. 1998. Mycobacterium avium bacilli grow saprozoically in coculture with Acanthamoeba polyphaga and survive within cyst walls. Appl. Environ. Microbiol. 64:2256-2261.
10. LaScola, B., I. Boyadjiev, G. Greub, A, Khemis, C. Martin, and D. Raoult. 2003. Amoeba-resisting bacteria and ventilator-associated pneumonis. Emerg. Infect. Dis. 9:815-821.
11. Visvesvara, G.S. 2003. Pathogenic and opportunistic free-living amebae, p.1984. In (ed.) Baron, E.J., J.H. Jorgensen, M.A. Pfaller, and R.H. Yolken. Manual of Clinical Microbiology, 8th ed. ASM Press, Washington, D.C.