Changes in the Microbial Community of the Mottled Skate (Beringraja pulchra) During Alkaline Fermentation

Abstract

Beringraja pulchra (mottled skate), a cartilaginous fish belonging to the family Rajidae, is found predominantly in the south sea of Korea, southern Japan, and East China Sea [1]. Skate have specific features that producing ammonia in their muscle during fermentation period in the low temperature [2]. During the fermentation, trimethylamine oxide (TMAO) and urea in the skate muscle are converted to trimethylamine (TMA) and ammonia, respectively [3]. Also, they produce mucus containing proteins, antibacterial enzymes, and proteolytic enzymes on their skins to prevent infectious pathogens and maintain innate immunity [4]. Certain strains of marine bacteria use these chemical compounds and producing antibacterial products which have probiotic potential found in the skate skin. Because of its specific features and flavors during fermentation, fermented skate is a popular seafood in South Korea and Iceland, with a high commercial value [5]. Especially, in Jeolla province of Korea, many consider it to be an irreplaceable food item for ceremonial occasions [6]. Continued skate ingestion can improve brain growth and cognitive function, alleviate symptoms of arthritis, improve skin health, prevent obesity, and it is also beneficial for the elderly [7, 8]. However, there are several safety concerns due to high concentrations of ammonia and bacterial contamination of fermented skate [9]. Thus, several studies on fermented skate have focused on the physicochemical and microbiological quality characteristics [10, 11]. According to a previous study, the pH of fermented skate ranged from 8.75-9.43 and the total number of microorganisms present in the skate was found to be between 4.8 log CFU/g and 7.5 log CFU/g [11]. In a study of prokaryotic community composition in alkaline fermented skate, the major phylum observed in the fermented skate was Firmicutes, whereas that in the fresh skate was Gammaproteobacteria [12]. However, only a small number of samples were analyzed and no repetition even using different conditions of fermentation period; therefore, limited information regarding the detailed bacterial distribution, interactions, or changes in microbial composition of the alkaline fermented skate is available. Furthermore, no previous studies Beringraja pulchra, Cham-hong-eo in Korean, is a mottled skate which is belonging to the cartilaginous fish. Although this species is economically valuable in South Korea as an alkalinefermented food, there are few microbial studies on such fermentation. Here, we analyzed microbial changes and pH before, during, and after fermentation and examined the effect of inoculation by a skin microbiota mixture on the skate fermentation (control vs. treatment). To analyze microbial community, the V4 regions of bacterial 16S rRNA genes from the skates were amplified, sequenced and analyzed. During the skate fermentation, pH and total number of marine bacteria increased in both groups, while microbial diversity decreased after fermentation. Pseudomonas, which was predominant in the initial skate, d...