The oral care probiotic strain Weissella cibaria CMU (oraCMU) inhibits volatile sulphur compounds associated with halitosis, presumably by inhibiting the growth of associated oral pathogens. In the present study, we investigated whether oraCMU inhibits the production of these compounds by suppressing the expression of mgl . This gene encodes L-methionine-α-deamino-γ-mercaptomethane-lyase (METase) and is involved in the production of methyl mercaptan (CH3SH) by Porphyromonas gingivalis . Therefore, we specifically investigated the effects of oraCMU on the growth, CH3SH production, METase activity, and mgl expression of P. gingivalis . The minimum inhibitory concentrations of cell-free supernatant and secreted proteins from oraCMU were 125 mg/mL and 800 µg/mL, respectively. At sub-minimum inhibitory concentration levels, these metabolites inhibited CH3SH production, but they also reduced P. gingivalis viability. Only heat-killed oraCMU decreased CH3SH production without affecting P. gingivalis viability. Heat-killed oraCMU also inhibited METase activity toward L-methionine and mgl mRNA expression (p < 0.05). In summary, we demonstrated the inhibition of volatile sulphur compounds via the antimicrobial action of oraCMU and, for the first time, the inhibition of such compounds by heat-killed oraCMU, which occurred at the molecular level.