Isolation and characterization of psychrotolerant endospore-forming Sporosarcina species associated with minced fish meat (surimi)

Abstract

We studied the changes of resident microbiota in surimi—minced fish meat—during heat-treatment and subsequent cold-storage via the sequencing of partial 16S rRNA gene. Raw surimi made from Alaska pollock, pike conger, and white croaker was contaminated with 104 to 106CFU/g of various non-endospore-forming bacteria. Immediately after heat-treatment, the bacterial counts were significantly reduced to less than 1CFU/g, and only endospore-forming bacteria, identified as Bacillus species were retrieved. Subsequently, the bacterial counts increased up to 10 to 105CFU/g in the heated surimi after refrigerated storage at 5°C for 2weeks or at 10°C for 1week. Most of the isolates from the refrigerated surimi were identified as Sporosarcina species. The Sporosarcina isolates have an increased ability to grow at 10°C than the isolates related to the other endospore-forming bacteria, such as Bacillus, Lysinibacillus, and Paenibacillus species. Endospores of the Sporosarcina isolates were able to germinate and proliferate in a fish-paste product model system stored at 10°C within 8days. In order to study the cold-adaptation mechanism of Sporosarcina species, the fatty acid composition of the isolates was analyzed. At the growth temperature of 10°C, the proportions of unsaturated to saturated fatty acids and anteiso to iso fatty acids were higher than those at 28°C. The alteration of the fatty acid composition suggests that Sporosarcina species adapt to cold by maintaining the fluidity of the cell membrane because unsaturated and anteiso fatty acids have lower melting points than saturated and iso fatty acids, respectively. We concluded that the endospores of Sporosarcina species are widely distributed in surimi, and that they can survive heat-treatment and proliferate during cold-storage in fish-paste products. Controlling Sporosarcina species would contribute to improving the quality of surimi product.