Harmless Naegleria amoebas push into micrometre-scale channels unprompted

The useful part of the design is the comparison species. Alongside the harmless Naegleria gruberi, the team tested Dictyostelium, a standard laboratory slime mold ^[5]. Both went through four settings: open glass coverslips, a 5-micrometre gap between two coverslips held apart by silicone pillars, microchannels 2 to 8 micrometres wide and 5 micrometres high, and an artificial mud of polymer microparticles that swell into a soft mesh ^[6]. If any crawling cell entered whatever slot it bumped into, both species would go in. Dictyostelium often disengaged and turned away ^[10].

The cells were filmed for 20 to 60 minutes under phase-contrast microscopy, and confocal microscopes followed the burrowing in the mud ^[7]. Nothing was used to attract Naegleria into the channels ^[8]. That means the entries cannot be explained by cells following a chemical gradient. Between 65% and 100% of cells entered the 8-micrometre channels voluntarily on contact ^[9]. Even the low end is roughly two cells in three ^[17]. The account of the work does not say how many cells were tracked, or which conditions produced each end of that range ^[9].

Once confined, the amoeba changed how it moved. On flat glass it used a mix of broad sheetlike extensions and blebs, which are fluid-filled membrane bubbles ^[14]. In narrow channels it crawled by blebs alone ^[11] and nearly doubled its speed ^[12]. It also held an almost perfectly straight line for more than a millimetre and rarely turned ^[13]. In the mud mimic it burrowed in all directions, using both kinds of protrusion ^[15].

The evidence is weaker on the medical side. Every measurement here comes from N. gruberi. Its relative N. fowleri lives harmlessly in warm freshwater, eating bacteria ^[2], but in humans it causes a brain infection with a 95% fatality rate, according to phys.org ^[1]. To get there it has to squeeze along bundles of olfactory nerves and slip through small openings in the cribriform plate, the bone between the nasal cavity and the brain ^[3]. A cell that seeks out tight spaces and speeds up inside them suits that route. The thing this doesn't tell you is whether the pathogen behaves the same way. The researchers chose the harmless relative precisely because N. fowleri is so dangerous, so the deadly species never went through the course ^[5].

I think the behavioural result is solid within its design. The link to invasion is a well-motivated hypothesis. The researchers' own explanation is ecological: they proposed that these tactics evolved for hunting bacteria hidden in muddy pond sediments ^[18]. If they are right, the pathogen reaches the brain using a trait shaped for mud. A drug would need something to aim at, such as the protein or pathway that lets a cell sense a narrow gap and switch to blebs. How N. fowleri actually makes the journey into the brain remains unexplained ^[4].