RelBE-mediated stress responses reshape cellular morphology of <i>Weissella cibaria</i> under chloramphenicol exposure

ABSTRACT Morphological remodeling accompanies the emergence of persister states in some bacteria, yet the regulatory mechanisms underlying these changes remain poorly defined. Weissella cibaria is an emerging probiotic bacterium that exhibits pronounced phenotypic heterogeneity under antibiotic stress. Here, we show that exposure to chloramphenicol (CAP) activates the type II toxin-antitoxin module RelBE in W. cibaria and is associated with growth arrest and marked cellular elongation in a subpopulation of cells. RelBE consists of the mRNA endonuclease RelE and its cognate antitoxin RelB. Under non-stress conditions, RelBE is maintained at low levels as a non-toxic complex. CAP treatment induces the ClpXP protease, promoting RelB degradation, and transiently increasing free RelE. RelE activity coincides with coordinated transcriptional remodeling of pathways governing cell wall and membrane biogenesis, suppression of core cell division genes, and induction of biofilm-associated programs. These changes are accompanied by compromised cell wall integrity, membrane depolarization, enhanced aggregation, and inhibition of septation, collectively providing a mechanistic basis for the observed elongation phenotype. Rather than establishing a universal role for RelBE in persister formation, our results define how RelE activity reshapes cellular morphological architecture under antibiotic stress. This work links toxin-mediated mRNA cleavage to envelope remodeling, division blockade, and surface-associated adaptations, offering mechanistic insight into stress-induced morphological plasticity in a probiotic bacterium. IMPORTANCE Probiotic bacteria frequently encounter antibiotic and industrial stresses that challenge their viability and functional stability. Understanding how these organisms adapt at the cellular level is therefore critical for both microbiology and applied biotechnology. This study reveals how the RelBE toxin-antitoxin (TA) module reshapes cellular architecture in the probiotic Weissella cibaria under chloramphenicol stress. We show that controlled activation of the RelE toxin coincided with coordinated remodeling of envelope biogenesis, repression of cell division programs, and enhancement of surface-associated behaviors, culminating in pronounced cell elongation. Rather than acting through a single pathway, RelBE engaged multiple cellular systems to generate stress-adaptive morphology. These findings provide a mechanistic insight into stress-induced phenotypic plasticity in beneficial bacteria and suggest that rational modulation of TA systems could be exploited to improve probiotic robustness and performance under adverse industrial and therapeutic conditions.