Isolation and Characterizations of Histamine- and Tyramine-Producing Strains Isolated from Fermented Soybean Food: Soy Sauce and Soybean Paste

Histamine (HIM) and tyramine (TYM) are among the most toxic biogenic amines (BAs) commonly found in various fermented soybean foods, yet the crucial BAs-producing strains are ignored. This study discussed and compared the effectiveness of two methods based on medium pH screening and target gene amplification for identifying HIM- and TYM-producing strains from two fermented soybean foods. The crucial strains responsible for HIM and TYM formation were identified and then characterized. It was found that the strains forming large amounts of total BAs promoted a high pH at the final medium, but there was no correlation between TYM/HIM formation and the pH value among the isolates. Furthermore, a large portion of isolates that produce TYM/HIM cannot be amplified. The hdc and tdc genes utilized reported universal pairs of primers, resulting in false negative results. Following two rounds of screening, most TYM/HIM-producing strains were found to belong to Bacillus. Bacillus cereus-HT-31-2 and Millerozyma farinosa-HT-42-1 were identified as crucial producers of TYM and HIM in soy sauce during the fermentation stage, while Proteus mirabilis-T-24-2 was found to be the key producer of TYM in thua nao. Moreover, the simulated medium was found to be beneficial for the formation of TYM/HIM by B. cereus-HT-31-2 and P. mirabilis-T-24-2, but not for M. farinosa-HT-42-1. The formation of TYM/HIM was not synchronized under different conditions. This study provides insights into the key strain responsible for the formation of HIM and TYM in fermented soybean foods.