Characteristics of biofilm-state Streptococcus thermophilus and its impact on bacterial tolerance to drying and storage

Abstract

Direct-added lactic acid bacteria starter (DVS) has been widely used in the food fermentation industry due to its high activity advantage, but its activity is often threatened by processing and storage conditions. The bacterial biofilm formation can effectively resist adverse environments, but whether the widely used food starter Streptococcus thermophilus has a biofilm state and its stress resistance are unknown. In this study, Streptococcus thermophilus S6–13 (S. thermophilus S6–13) with good fermentation performance was selected as the research object. S. thermophilus S6–13 was found to have weak biofilm formation ability by crystal violet staining. Scanning electron microscopy and laser confocal scanning microscopy observations showed that the biofilm-state S. thermophilus S6–13 had a small amount of extracellular matrix and displayed a loose network structure. Fourier transform infrared spectroscopy showed that the main components of biofilm were extracellular polysaccharides and extracellular proteins. The results of comparing the drying tolerance of planktonic and biofilm-state S. thermophilus S6–13 showed that the formation of biofilm increased the freeze-drying survival rate by 11 % and increased the spray-drying survival rate to more than 1.5 times that of planktonic. The survival rates of biofilm-state freeze and spray dried powders when stored at 25 °C for 3 months were more than twice that of the planktonic, and biofilm-state dried powders maintained higher acidification activity after storage; that is, biofilm-state dried powders showed higher storage stability. Therefore, the formation of biofilm is expected to be a new strategy to improve the viability of Streptococcus thermophilus during processing and storage.