Andrew Rogerson

The aim of this study was to determine the prevalence of Acanthamoeba in the domestic water supply in Huntington, West Virginia (U.S.A.) and the factors that may contribute to their presence or absence. One hundred sixty-two one liter tap water samples were collected over eight months. Amoebae in the samples (cysts or trophozoites) were harvested by passively filtering onto 5 μm pore size filters and enriching for amoebae on non-nutrient amoeba saline agar plates seeded with Escherichia coli for cultivation. Thirteen percent of all samples were positive for amoebae and 9.3% were positive for the amoeba of interest, Acanthamoeba. Chlorine levels were determined for samples at the time of collection, yielding a mean level of 1.56 mg l–1 chlorine in the distribution system ca. 8 kilometers from the water treatment plant. Cysts and trophozoites of Acanthamoeba clonal isolates were found to tolerate up to 50 mg l–1 and 4 mg l–1 chlorine respectively. This study showed that Acanthamoeba were present in the domestic water supply in Huntington, WV and although no attempt was made to count cells in liter samples, their frequency of occurrence (9.3%) and failure to be present in all replicates, suggests they were present at background levels of perhaps a few cells per five liters. This is only the second U.S. study to consider amoebae in tap water and is unique since the source water was river water. Acanthamoeba trophozoites and cysts were able to withstand levels of chlorine higher than those typically found in tap water suggesting they may be present in either form in the distribution system. Acanthamoeba are opportunistic pathogens capable of causing eye infections and their presence in tap water is a potential risk factor for susceptible individuals, particularly contact lens wearers who may use tap water to clean lenses and storage cases.

Amoebae feed on attached bacteria within, and below, bacterial biofilms where they experience reduced oxygen levels. The implications of this were examined by comparing the migration (an index of growth) of thirteen strains of Acanthamoeba and five species of naked amoebae grown under microaerophilic and aerobic conditions. All amoebae replicated well under both conditions and twelve isolates migrated significantly faster under low oxygen. Only one isolate, Vannella sp., migrated further (presumably grew faster) under aerobic conditions. The data show most amoebae prefer low oxygen as befits the biofilm habitat. Interestingly, the eleven acanthamoeba strains that replicated faster under microaerophilic conditions were all T4 genotypes and included four strains isolated from patients with amoeba keratitis (AK) infections. This genotype is most frequently found in AK cases and it is suggested that strains of Acanthamoeba capable of rapid growth in a biofilm of a poorly cleansed contact lens may enable amoebae to multiply and provide an infective dos e when placed onthe cornea.