Naegleria amoebae seek confinement and crawl persistently through narrow spaces

The "brain-eating amoeba" Naegleria fowleri dwells in ponds where it normally feeds on bacteria, but if it enters the brain it can cause a deadly infection. To establish infection, N. fowleri must migrate through different environments--along olfactory axons, through openings in the cribriform plate, and within brain tissue--yet how it does so remains unknown. As a model for N. fowleri migration within these environments, we examine how its non-pathogenic relative, Naegleria gruberi, navigates environments of distinct geometries. We show that Naegleria uses both actin-rich protrusions and membrane blebs to crawl across or between flat surfaces. We also explore how Naegleria interact with narrow channels and find that, unlike Dictyostelium amoebae that we show frequently disengage from channel interfaces, Naegleria amoebae probe channels until they enter. Once inside, Naegleria crawls quickly (>50 m/min) and unidirectionally over long distances (>1 mm) using only bleb-based motility. We also introduced Naegleria to granular hydrogel matrices that mimic pond sediments and found that cells readily enter and migrate through these three-dimensional matrices using both blebs and lamellar protrusions. Although cells in matrices showed lower persistence at short timescales, longer time scales correlate with increased persistence, suggesting Naegleria cells may retain memory of past orientation. We propose that pond life may select for three behaviors that prime Naegleria for pathogenesis: memory-guided motility that would facilitate exploration of sinus cavities, confinement-seeking ("claustrophilia") that would promote entry into narrow passages along olfactory axons, and fast bleb-based migration that would allow rapid transit along axons to the brain. SIGNIFICANCE STATEMENTThe "brain-eating amoeba" Naegleria fowleri causes a devastating brain infection with a [~]95% fatality rate, yet how these normally harmless pond-dwellers invade the human brain remains mysterious. Using the model species N. gruberi, we show that Naegleria amoebae exhibit three key behaviors: they retain directional memory during exploration, actively enter confined spaces, and crawl rapidly and persistently through narrow channels for millimeter-scale distances. We propose these behaviors are adaptations for hunting bacteria in pond sediments that inadvertently enable pathogenesis--directional memory facilitates sinus exploration, confinement-seeking draws amoebae into spaces between olfactory axons, and persistent channel migration enables rapid transit to the brain.