## 1 Introduction  
In daily life, people always come into contact with lactic acid bacteria (LAB) and corresponding products. Due to their safety [1, 2], they are widely used as probiotic products to improve the balance of human intestinal microbiota and promote human health [3, 4, 5, 6, 7, 8]. With the ban on antibiotics in feed, LAB are also used as feed additives to substitute for antibiotics and to promote the health of Armed animals [9, 10, 11, 12, 13]. Sufficient viable bacteria are the prerequisite for LAB to function as probiotics, and in the process of production, processing, and food consumption, a large number of living LAB will die. Therefore, how to obtain high concentration of LAB by fermentation and maintain their high survival rate in processing and application has become the interesting topic for researchers and commercial companies. At present, the production of LAB is mainly divided into liquid-state fermentation (LSF) and solid-state fermentation (SSF). Liquid-state fermentation is widely used in food industry, medicine and health industry because of its high fermentation density and low contamination by miscellaneous bacteria [14, 15]. SSFcan reduce the metabolite decomposition inhibition and substrate fermentation inhibition, and it has multiple advantages, such as high enzyme production, low energy consumption, high product stability, low production cost, a wide range of raw material sources, and ecological friendliness [16; 17; 18; 19; 20]; therefore, SSF is widely used. The growth of LAB is accompanied by the production of organic acids, which seriously affects their own growth and decreases their number [21]. In the final application stage, LAB are also faced with the acidic environment of the human or animal stomach, thus resulting in a significantly decreased number of viable bacteria and a weakened performance as probiotics [22]. Therefore, improving the acid stress tolerance of LAB is of great significance for their industrial production and application. With the extensive application of genome, transcriptome, and metabolomic techniques in recent years, the understanding of the acid tolerance mechanism of LAB has been deepened [23; 24]. Existing studies have revealed that LAB can resist acid stress by changing the types and contents of fatty acids in cell membranes to reduce the cell damage caused by the external environment [25]. Acid preadaptation of _Lactobacillus plantarum_ ZDY2013 (recently reassigned as _Lactiplantiobacillus plantarum_ ZDY2013) has been reported to significantly increase the proportion of saturated fatty acids and cyclopropane [26] and to enhance the function of proton pump. When the external pH value is low, the expression of H\({}^{+}\)-ATPase gene is up-regulated, and the activity of H\({}^{+}\)-ATPase enzyme is increased, but when H\({}^{+}\)-ATPase is absent, the mutant cannot grow normally in acidic conditions [27]. LAB can improve the pH value of the surrounding environment of cells through arginine deaminase pathway or glutamate decarboxylase system so as to improve the cell living environment [28]. In addition, related mechanisms include protection and repair of nucleic acids and proteins [24; 29; 30], two-component system regulation, cell density regulation, rigorous response (ppGpp), quorum-sensing, and changes in acid stress-related cell metabolic pathways [29; 31; 32]. However, most existing studies focus on bacterial stress response after they experience a series of acid stress stimuli, and there is still a lack of studies on how growth conditions before the introduction of acid stress affect acid stress response, and on internal mechanisms of LAB produced through different culture methods in response to environmental stress. However, these absent studies have an important application value for the selection of different culture methods and the production of probiotics and feed products. In this study, the survival rate of the LAB obtained by solid and liquid fermentation methods and their tolerance difference under acid stress were investigated.