<i>Naegleria</i> amoebae seek confinement and crawl persistently through narrow spaces.

Abstract

The "brain-eating amoeba" <i>Naegleria fowleri</i> dwells in ponds where it normally feeds on bacteria, but if it enters the brain it can cause a deadly infection. To establish infection, <i>N. fowleri</i> 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 <i>N. fowleri</i> migration within these environments, we examine how its nonpathogenic relative, <i>Naegleria gruberi</i>, navigates environments of distinct geometries. We show that <i>Naegleria</i> uses both actin-rich protrusions and membrane blebs to crawl across or between flat surfaces. We also explore how <i>Naegleria</i> interact with narrow channels and find that, unlike <i>Dictyostelium</i> amoebae that we show frequently disengage from channel interfaces, <i>Naegleria</i> amoebae probe channels until they enter. Once inside, <i>Naegleria</i> crawls quickly (>50 μm/min) and unidirectionally over long distances (>1 mm) using only bleb-based motility. We also introduced <i>Naegleria</i> 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 <i>Naegleria</i> cells may retain memory of past orientation. We propose that pond life may select for three behaviors that prime <i>Naegleria</i> 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 persistent bleb-based migration that would allow rapid transit along axons to the brain.