## 1 Introduction  
Halophilic lactic acid bacteria (LAB) are important bacteria that are involved in the process of food fermentation [1, 2]. They play important roles in the formation of volatiles and the enhancement of the aroma of fermented foods [3, 4]. _Tetragenococcus halophilus_ is a species of halophilic or hyperosmotic lactic acid bacteria (LAB) that is present in highly osmotic food products such as fermented fish, soy sauce and sugar thick juice [5]. _T. halophilus_ was previously classified as _Pediococcus_, known as _Pediococcus halophilus_ or _Pediococcus squea_. It has recently been reclassified and designated as _Tetragenococcus halophilus_[6]. _T. halophilus_ hydrolyzes proteins even in high-salt environments, and the addition of _T. halophilus_ in fish sauce fermentation significantly increases the total amino acids and free amino acids, especially glutamic acid, contents in fish sauce [7]. The combination of _T. halophilus_ with alkaline protease or flavor protease in fermented foods helps to increase the content of both volatiles and free amino acids, thus improves the flavor of food [7; 8]. Cooperation with yeasts such as _Zygosaccharomyces rouxii_ and _Candida_, _T. halophilus_ improves the quality of fermented foods and shortens the fermentation period [9; 10; 11]. _T. halophilus_ was also found to be able to degrade aflatoxin B1 (AFB1) or repress cadaverine formation during food fermentation [12; 13]. However, in the process of food fermentation, _T. halophilus_ experiences challenges including acidic, oxidative and osmotic stresses [14]. The tolerance to these stressful conditions of _T. halophilus_ has a great impact on its performance during food fermentation. The mechanisms of _T. halophilus_ in response to osmotic and acidic stresses have been investigated by transcriptomic and proteomic analyses. The production of functional molecular chaperones GroESL, DnaK, and ClpB by _T. halophilus_ promotes its adaptation to stressful conditions [15; 16]. Synthesis of functional proteins and the existence of post-transcriptional modification in _T. halophilus_ upon osmotic stress were found to be related to its salinity adaptation [17]. Moreover, _T. halophilus_ accumulates osmoprotectants through the transportation of carnitine and glycine betaine or the conversion of choline to glycine betaine, in order to resist osmotic stress [18]. In response to acidic stress, _T. halophilus_ increases the content of unsaturated fatty acids in the cell membrane [19]. Amino acids were found to be helpful for stabilizing cell membranes and protein structures and avoiding cell persecution by stress factors. It was found that glutamic acid and arginine also protect LAB from acidic stress [20; 21]. Apart from osmotic and acidic stress, _T. halophilus_ experiences oxidative stress during food fermentation as well. For instance, occasional aeration of the moromi during soy sauce fermentation is required to mix the contents and to promote the growth of functional aerobes such as yeasts. LAB are often classified as facultative anaerobic bacteria for lacking catalase and an active electron transport (ET) chain [22]. Aeration retards the growth of LAB and even leads to cells' death and DNA degradation [23]. Thus, both oxygen and peroxide are harmful to the cells of facultative anaerbes including _T. halophilus_. The antioxidant mechanism of the LAB _Oenococcus oeni_ was found to be associated with the DPPH and ROS scavenging capability, the presence of the glutathione system, inhibition of ascorbic oxidation, and linoleic acid oxidation (TLA) abilities [24]. Some LAB strains are capable of chelating iron ions, scavenging ROS or possessing reducing activity to enhance their antioxidant capacity [25]. Antioxidant enzymes also play important roles in oxidative stress tolerance in LAB. Enzymes that are related to oxidative stress in LAB including oxygen-consuming enzymes (NADH oxidases, NOX; pyruvate oxidase, POX; lactate oxidase, LOX) help to enhance oxidative stress [23].