Isolation and habitat analysis of Balamuthia mandrillaris from soil

Balamuthia mandrillaris is a free-living amoeba that causes meningoencephalitis in mammals. Over 200 cases of infection were reported worldwide, with a fatality rate of over 95%. A clear route of infection was unknown for a long time until a girl died of granulomatous amoebic encephalitis (GAE) in California, USA, in 2003 due to infection with B. mandrillaris detected in a potted plant. Since then, epidemiological studies were conducted worldwide to detect B. mandrillaris in soil and other environmental samples. We previously reported the isolation of B. mandrillaris from the soil in Japan; however, the existing B. mandrillaris culture method with BM3 medium and COS-7 cells was unsuccessful. Therefore, in this study, we aimed to conduct soil analysis to determine the growth conditions of B. mandrillaris. B. mandrillaris-positive soils were defined as soils from which B. mandrillaris was isolated and environmental DNA was PCR-positive. Soils inhabited by B. mandrillaris were alkaline, with high electrical conductivity and characteristics of nutrient-rich soils of loam and clay loam. The results of this study suggest a possible reason for the high prevalence of GAE caused by B. mandrillaris among individuals employed in agriculture-related occupations.