As an intracellular pathogen, Listeria monocytogenes uses host cell protrusions to spread between cells, for example in the intestinal epithelium during food-borne infection. The underlying molecular mechanisms, such as the formation of host actin ‘comet tails’ that propel the bacteria, have been studied in detail. However, little is known about the dynamics of L. monocytogenes populations during tissue spread. A new study that combines video microscopy and computational modelling shows that rare pioneer bacteria move beyond the original infectious focus, which might help maintain persistent infection.
To observe the dynamics of L. monocytogenes spread, the authors infected monolayers of a canine kidney epithelial cell line with a bacterial strain that expresses red fluorescent protein when inside the host cytosol and treated the cultures with gentamicin, which is an antibiotic that cannot penetrate the host cells. This experimental setup enables tracking of intracellular bacteria as they move between cells. Because invasion of host cells is rare, most infections likely started with a single bacterium. Despite the clonal nature of the infectious foci, the bacterial cells showed heterogeneous behaviour. A few bacteria spread much further beyond the initial roughly circular focus.
Indeed, when the authors ran computer simulations, an infection arising from a single cell resulted in roughly circular foci if the cells carried out random walks. Even if the model took into account some persistence of directional movement, which could occur due to the comet tails, and the influence of cell boundaries, from which bacteria could ‘bounce off’, the simulated foci were more circular than the experimentally observed ones.
Only when the models included pioneer bacteria, that is, a subpopulation of cells that moved further than the average cells, irregular foci arose. Importantly, the simulations matched the experimental foci best when pioneer rates were relatively low, which translated to 1.4–12% of the bacterial population being or descending from pioneers.
“rare pioneer bacteria move beyond the original infectious focus, which might help maintain persistent infection”
The mechanisms underlying ‘pioneering’ motility are unclear; however, when the authors used a mutant L. monocytogenes strain that forms shorter actin comets and is less likely to form and enter long protrusions, the observed foci were more circular than the foci produced by the wild-type strain. This suggests that the pioneers use long protrusions to spread to cells that are not directly adjacent. Such behaviour might help sustain infection in the gut, as the epithelium, and with it the intracellular bacteria, is continuously shed at the tip of the villi. Maintaining infection in such an environment requires a fine balance between vigorous growth, which potentially overwhelms the host, and local growth, which risks elimination. Indeed, if the models took into account shedding, bacteria were more likely to persist in a steady state if pioneers were present.
Ortega, F. E., Koslover, E. F. & Theriot, J. A. Listeria monocytogenes cell-to-cell spread in epithelia is heterogeneous and dominated by rare pioneer bacteria. eLife 8, e40032 (2019)
Radoshevich, L. & Cossart, P. Listeria monocytogenes: towards a complete picture of its physiology and pathogenesis. Nat. Rev. Microbiol. 16, 32–46 (2018)