## 4 Discussion  
_T. halophilus_ is a functional LAB that contributes to increasing the content of volatiles and thus to improving of flavor in fermented foods [30]. _T. halophilus_ strains isolated from food origins exhibited differences in biological properties such as substrate utilization, growth performance under multiple-stress and formation of volatile compounds [5,31,32]. The outcome of the employment of this LAB in food fermentation may be attributed to their tolerance to different stress conditions. The salt tolerance and acid resistance mechanisms of _T. halophilus_ were investigated previously [19,33]. In this work, we found that heme \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Strain** & **R23** & **R21** & **C25** & **R44** & **R8** \\ \hline Catalase activity & + & + & \(-\) & \(-\) & \(-\) \\ Interruption of \(cat\) gene & No & No & Yes & Yes & Yes \\ \hline \hline \end{tabular} +, active; \(-\), inactive.  
\end{table} 
Table 1: Comparison of the catalase activity of different _T. halophilus_ strains. Figure 4: Effect of heme on growth of _T. halophilus_ strains under (**a**) static condition and (**b**) aerobic condition. \(-/+\): negative/positive activity of catalase detected under corresponding conditions. Asterisks mean significant differences among different groups (*, \(p\) < 0.05; **, \(p\) < 0.01). dependent CAT and pseudo-CAT are produced by _T. halophilus_, and they contribute to tolerance of oxidative stress and H2O2 with the induction of aerobic cultivation. The finding of their differences in resistance to oxidative stress and the elaboration of the possible mechanism provide theoretical criteria for the selection of suitable strains that can be used in food fermentation. Superoxide dismutase usually takes the front line in protecting cells from reactive oxygen species (ROS). This enzyme is important for the protection of bacteria against various types of oxidative stresses [34]. Other enzyme systems, such as the glutathione system and thioredoxin system, also detoxify hydroperoxide radicals. The glutathione system modulates the activities of _E. coli_ transcriptional regulators that respond to oxidative stress, and glutathione reductase is implicated in the response to chemical oxidative stresses [34,35]. The deletion of glutathione reductase in _Streptococcus pneumoniae_ led to an increased sensitivity to superoxide [36]. In most lactic acid bacteria, the degradation of hydrogen peroxide and other peroxides is generally catalyzed by peroxidase [37,38]. In this work, _T. halophilus_ strains showed different tolerance to oxidative stress (Figure 1), and their antioxidant capacity is associated with peroxidase activity (Figure 2). Moreover, increased resistance of _T. halophilus_ to H2O2 was achieved with an aerobic cultivation induction and presence of heme (Figure 3), indicating the function of a heme-dependent enzyme in tolerance to oxidative stress. Catalase is the enzyme catalyzing the decomposition of hydrogen peroxide. It is an important enzyme that protects the cell from oxidative damage caused by ROS. LAB are generally considered to be devoid of catalase activity, since they are not able to synthesize heme. However, some species of LAB exhibit catalase activity. Catalase from LAB generally groups into two types. The first is heme-dependent catalase, which is induced by adding heme, produced by some lactobacilli, _Pediococcus_, _Enterococcus_ and _Leucomostoc_ strains. The other one is the heme-independent catalase or pseudocatalase, which is produced by some strains of _Leuconostoc_, _Pediococcus_, _Enterococcus_ and _Lactiplantibacillus plantarum_[39]. Our results demonstrated that _T. halophilus_ produces heme-dependent CAT and pseudo-CAT (Table 1). Both of them improved the oxygen resistance and H2O2 tolerance of _T. halophilus_ strains. It was found that the presence of heme extended the growth period of _Lactococcus lactis_ during aerated growth and improved long-term survival. This indicated that the respiratory metabolism in _L.