Influence of lactic acid and post-treatment recovery time on the heat resistance of Listeria monocytogenes

- Organic matter greatly attenuates the listericidal effect of lactic acid.
- The listericidal effect of lactic acid is strain-dependent.
- Heat resistance of Listeria increases during the post-lactic acid treatment period.
- Increased resistance involves several chaperones and SOS genes and proteins.
- The involvement of SigB-regulated genes in increased resistance is unlikely.

The aim of this study was to evaluate the effect of lactic acid (LA) with and without organic material at various post-treatment recovery times on the heat resistance of Listeria monocytogenes (Lm). LA decreased Lm numbers; however, the effect was remarkably attenuated by the presence of organic matter. Five strains of Lm were treated with LA and the listericidal effects were compared. The effect of LA varied depending on the strain, with ≥ 3.0% (w/w) LA required to kill the Lm strains in a short time. The heat resistance of Lm treated with LA was examined with respect to the time interval between the acid treatment and the subsequent manufacturing step. The heat resistance of Lm was shown to significantly increase during the post-treatment period. Heat tolerance (D value) increased up to 3.4-fold compared with the non-treated control bacteria. RNA sequencing and RT-PCR analyses suggested that several stress chaperones, proteins controlled by RecA and associated with high-temperature survival, were involved in the mechanism of enhanced heat resistance. These results are applicable to manufacturers when LA and heat treatment methods are utilized for the effective control of Lm in foods.