Probiotic suppression of Bacillus cereus virulence: growth kinetics, cereulide quantification, and PlcR-PapR pathway disruption under Lacticaseiibacillus casei, Lactiplantibacillus plantarum, and consortia co-culture 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 National Collection of Industrial Microorganisms (NCIM) 2125, Lactiplantibacillus plantarum NCIM 2083, and a three-strain consortium additionally containing Bifidobacterium bifidum NCIM 5697 were co-cultured with B. cereus NCIM 2217 in brain heart infusion 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). Lacticaseibacillus 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.