# Resistance of \(L\). _monocytogenes_ and \(S\). Typhimurium
## 3 Results and Discussion
### Influence of the Biofilm Age on the Inactivation Kinetics  
Based on the results obtained for the cell densities prior to CAP treatment (Figure 3a,b) and the log-reductions following 10 min of CAP treatment (Figure 4a,b), it has been decided to investigate the influence of the biofilm age on the CAP inactivation kinetics. The kinetics for the one-day old _L. monocytogenes_ and _S._ Typhimurium biofilms have been discussed in detail in the research of [PERSON] et al. [17] and will be compared in the presented research with the inactivation kinetics of the 7 days old biofilms. The latter biofilm age has been selected based on (i) the difference in total biofilm mass compared to the one-day old biofilm (for _L. monocytogenes_), (ii) the lower cell densities (prior to CAP treatment) obtained following this incubation time (for both species), and (iii) the significantly lower log-reductions that were obtained following 10 min of CAP treatment (for both species). The cell densities of the one and seven day(s) old biofilms as function of the CAP treatment time have been plotted in Figures 5a and 5b for _L. monocytogenes_ and _S._ Typhimurium, respectively. The percentage of sub-lethally injured cells as function of the CAP treatment time can be observed as well in these figures. For each of the microorganisms and each of the biofilm ages, the estimated model parameters of the [PERSON] et al. [27] model have been presented in Table 1.  
Figure 5: Cell density (log(CFU/cm\({}^{2}\))) as function of the treatment time following CAP treatment of the one and seven day(s) old biofilms at optimal CAP conditions for biofilm inactivation. Experimental data (symbols) and global fit (line) of the [PERSON] et al. [27] model: total viable population on general medium (o, solid line) and uninjured viable population on selective medium (x, dashed line) (left). Percentage (%) of sub-lethally injured cells as function of the treatment time for both investigated biofilm ages (i.e., one and seven day(s)) (right). (**a**) Results obtained following CAP treatment of the _L. monocytogenes_ biofilms; (**b**) Results obtained following CAP treatment of the _S._ Typhimurium biofilms. Based on the inactivation curves in Figure 5a and the estimated model parameters in Table 1, several conclusions can be drawn with respect to the influence of the biofilm age on the inactivation kinetics of CAP treatment for _L. monocytogenes_ biofilms. First of all, the initial cell density (prior to CAP treatment) of the seven days old _L._ monocytogenes biofilm is significantly lower than the one obtained for the one-day old biofilm (on both media), which has been discussed already in Section 3.1 (Figure 2a). In addition, the cells that are present in the one and seven day(s) old biofilms are not sub-lethally injured prior to treatment, which can be observed as well in previously mentioned Section 3.1, i.e., initial cell densities (prior to CAP treatment) are similar on both media. Secondly, it can be observed as well that the inactivation curves for the seven days old biofilms (on both the general and the selective medium) are differently shaped than the ones obtained for the one-day old biofilms. In the former case, there is only a log-linear inactivation phase, while in the latter case, a shorter log-linear inactivation phase is followed by a tail. This means that further inactivation of the seven days old biofilms would still be possible if the treatment time would be prolonged. This is not possible for the one-day old biofilms, since a residual cell density is reached due to the presence of a more resistant sub-population within the biofilm. Nevertheless, the inactivation rate for the seven days old biofilms is significantly lower than the one for the one-day old biofilms, meaning that a longer treatment time is required for the former biofilm age in order to obtain a similar log-reduction as for the latter biofilm age.