Mycobacterium aviumBacilli Grow Saprozoically in Coculture withAcanthamoeba polyphagaand Survive within Cyst Walls

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Abstract
Protozoans are gaining recognition as environmental hosts for a variety of waterborne pathogens. We compared the growth ofMycobacterium avium, a human pathogen associated with domestic water supplies, in coculture with the free-living amoebaAcanthamoeba polyphagawith the growth ofM. aviumwhen it was separated from amoebae by a 0.1-μm-pore-size polycarbonate membrane (in a parachamber). Although viable mycobacteria were observed within amoebal vacuoles, there was no significant difference between bacterial growth in coculture and bacterial growth in the parachamber. This suggests thatM. aviumis able to grow saprozoically on products secreted by the amoebae. In contrast,Legionella pneumophila, a well-studied intracellular parasite of amoebae, multiplied only in coculture. A comparison of amoebae infected withL. pneumophilaand amoebae infected withM. aviumby electron microscopy demonstrated that there were striking differences in the locations of the bacteria within amoebal cysts. WhileL. pneumophilaresided within the cysts,M. aviumwas found within the outer walls of the double-walled cysts ofA. polyphaga. These locations may provide a reservoir for the bacteria when environmental conditions become unfavorable.