Aggregation in lactobacilli: an unexplored dimension

Autoaggregation is an often-overlooked but critical phenotypic trait in Lactobacillus species that plays a pivotal role in host colonisation, pathogen exclusion, and probiotic functionality. This review explores the molecular mechanisms, surface factors, and environmental cues influencing aggregation, distinguishing it from but also linking it to biofilm formation. While traditionally associated with initial steps in biofilm development, autoaggregation in lactobacilli can occur independently of and sometimes conversely to biofilm production. We assess the contributions of surface proteins, such as S-layer proteins and aggregation-promoting factors, and those of exopolysaccharides, pili, and environmental modulators in shaping aggregation behaviour. In addition, we discuss how aggregation enhances mucosal adhesion, immune modulation, and competitive exclusion of pathogens, making it a promising selection marker for next-generation probiotics and live biotherapeutics. Finally, we stress the need for standardised methods and advanced tools to elucidate the complex interplay between bacterial surface architecture and lifestyle strategies like aggregation and biofilm formation.