2.2.2 The role of Protozoa in Protecting Legionella in the Environment

The association of legionellae with protozoa has implications for artificial water systems. 

The presence of amoebae may offer protection to bacteria and encourage them to grow. As well as providing physical protection, the growth of the bacteria in amoebae may alter the physiological status of bacteria. 

Strains of Legionella pneumophila grown in amoebae have been shown to exhibit phenotypic variations in growth including modifications in the lipopolysaccharide and fatty acid content of the cell exterior and were reported to be significantly more resistant to treatment with biocides when compared to bacteria grown on agar (38).

Legionella species have been detected in sewage and the number of organisms present are not appreciably reduced by primary or secondary treatment processes (39). This finding may be related to the protection provided by protozoa, which are ubiquitous inhabitants of sewage treatment plants. 

The resistance of amoebal cysts to extremes of temperature and to the effects of biocides contributes to the difficulties in eradicating Legionella from contaminated water systems using conventional disinfection procedures. 

• The amoebic cysts not only offer a mechanism for bacteria to evade hostile environmental conditions but also offer a mechanism for bacteria to colonise new habitats via airborne routes (40). 

Coliform organisms and pathogenic bacteria show an increase of between 30 - 120% in resistance to free chlorine residuals when ingested by protozoa (41). 

Acanthamoebae cysts are resistant to free chlorine levels at concentrations commonly used to disinfect water systems. 

• They are also resistant to a wide range of biocides, and therefore, amoebal cysts containing Legionella bacteria may survive cooling tower disinfection procedures (42). 

When conditions, such as light and temperature, become unfavourable for the growth of other supporting organisms (such as blue-green algae) then amoebae provide a reservoir of surviving legionellae. 

• Amoebal-grown legionellae have been shown to be resistant to certain biocides, for example 5-chloro-N-methylisothiazolone. 

• The biocide polyhexamethylene biguanide is effective not only against the amoebal-grown Legionella but also against amoebae (38).

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Format adapted by T2 Scientific© 2005	t2.200.1.v1.0
	Source: Blue Book 200 Part 1 downloaded 27 May 2005