A novel type of autoaggregation in lactic acid bacteria promoted by new AggS aggregation factor from Streptococcus thermophilus CC40-4S.

Autoaggregation in lactic acid bacteria is considered a beneficial probiotic trait and can be used in the food and medical industries to enhance the properties of utilized microorganisms. Currently, a group of aggregation-promoting factors (APFs) in lactic acid bacteria, known as Snowflake Forming Collagen Binding Aggregation Factors (SFCBAFs), is well described. These are large proteins with a molecular mass of over 170 kDa, containing collagen-binding domains and a repeat region, forming a unique autoaggregation phenotype. Here we describe a new type of autoaggregation in lactic acid bacteria found in Streptococcus thermophilus CC40-4S. The whole genome of the autoaggregation-positive strain S. thermophilus CC40-4S was sequenced, and bioinformatic analysis predicted a putative gene aggS involved in autoaggregation, located on the chromosome and flanked by insertion sequences. The aggS gene disruption by homologous recombination using the temperature-sensitive vector pSC led to the loss of the aggregation phenotype. Cloning and heterologous expression in Lactococcus lactis subsp. cremoris MG1363 confirmed the role of the AggS protein in autoaggregation, given that a strong aggregation phenotype was obtained. Like SFCBAF-type APFs, AggS is a large protein (237 kDa) with a repeat region, but it does not contain collagen-binding domains and forms an autoaggregation phenotype with small aggregates. To our knowledge, this is the first report on a gene coding for an aggregation-promoting factor in S. thermophilus.