Probiotic Suppression of Bacillus cereus Virulence: Growth Kinetics, Cereulide Quantification, and PlcR-PapR Pathway Disruption under Lacticaseiibacillus casei, Lactiplantibacillus plantarum, and Consortia Coculture Conditions.

Bacillus cereus emetic poisoning is mediated by cereulide, a thermostable cyclic depsipeptide encoded by the cesA/cesB operon under PlcR-PapR control. Once formed, cereulide resists heat and standard food processing, making transcriptional intervention the only viable biocontrol strategy. Lacticaseibacillus casei NCIM 2125, Lactiplantibacillus plantarum NCIM 2083, and a three-strain consortia additionally containing Bifidobacterium bifidum NCIM 5697 were co-cultured with B. cereus NCIM 2217 in BHI broth (1:1 v/v; OD₆₀₀ = 0.1; 37 °C) over 24-72 h (n = 3). Viable counts were determined by standard plate count, cereulide by LC-MS following liquid-liquid extraction, and cesA/cesB transcripts by RT-qPCR (2⁻ΔΔCT, dual housekeeping-gene normalisation). L. casei inhibited growth at 24 h only (1.97 log CFU/mL; p = 0.021). The consortia achieved 18.0% area-under-curve reduction and 81.6% cereulide suppression at 48 h (p < 0.001; Cohen's d = 29.03). A protein-mRNA paradox emerged: L. plantarum suppressed cesA by 87.9% at 48 h yet cereulide rose 10.5% above control (p = 0.007), attributable to depsipeptide stability post-secretion. Consortium ces silencing at 24 h (cesA FC = 0.0031) preceded peak synthesis, yielding a 77.5 percentage-point synergy bonus. Effective cereulide biocontrol requires ces silencing before the synthesis window closes, a criterion met only by the consortia.