Comparative study of the effects of citral on the growth and injury of Listeria innocua and Listeria monocytogenes cells

Abstract

This study investigates the effect of citral on growth and on the occurrence of sublethal damage in Listeria innocua Serovar 6a (CECT 910) and Listeria monocytogenes Serovar 4b (CECT 4032) cells that were exposed to citral as a natural antimicrobial agent. Two initial inoculum concentrations were considered in this investigation: 10(2) and 10(6) cfu/mL. Citral exhibited antilisterial activity against L. innocua and L. monocytogenes, and the observed effects were dependent on the concentration of citral present in the culture medium (0, 0.150 and 0.250 μL/mL) (p ≤ 0.05). L. innocua had a shorter lag phase than L. monocytogenes, and the two species had nearly identical maximum specific growth rates. These results indicate that L. innocua could be used as surrogate for L. monocytogenes when testing the effects of this antimicrobial. Significant differences in the lag phase and growth rate were observed between the small and large inoculum concentration (p ≤ 0.05). Citral-treated L. innocua and L. monocytogenes that were recovered on selective medium (i.e., TSA-YE-SC) had a shorter lag phase and a higher maximum specific growth rate than cells that were recovered on non-selective medium (i.e., TSA-YE) (p ≤ 0.05). This result suggests that damage occurs at sublethal concentrations of citral.

Figure 1. L. innocua growth curves in reference medium in the presence of different concentrations of citral (μL/mL) with N₀ = 10² Log(cfu/mL).

The lines represent the fit of the experimental data to the modified Gompertz model. The standard deviation associated with each average value is expressed by error bars.

Figure 2. . L. monocytogenes growth curves in reference medium in the presence of different concentrations of citral (μL/mL) with N₀ = 10² Log(cfu/mL).

The lines represent the fit of the experimental data to the modified Gompertz model. The standard deviation associated with each average value is expressed by error bars.