Several bacteria can adapt to stress by entering a viable but non-culturable (VBNC) state (1), where they remain alive and metabolically active but cannot grow in standard culture media (Fig. 1).

Listeria monocytogenes (Lm), a deadly foodborne pathogen for immunocompromised or elderly individuals and newborns, can survive in various stressful environments by transitioning to a VBNC state (2, 3). However, the molecular mechanisms governing this transition are not well understood.

We have recently shown that during starvation in mineral water, Lm progressively transitions to a VBNC state (Fig. 2A-B) (4). Phase contrast microscopy observation revealed that Lm rods become progressively shorter to transform into coccoid forms after 2-3 weeks (Fig. 2C). Cryo-electron tomography (Fig. 2D) and peptidoglycan profiling (not shown) indicated that this transformation is due to a shedding process of the peptidoglycan, resulting in cell wall-deficient bacteria.