Identification and functional characterization of AclB, a novel cell-separating enzyme from Lactobacillus casei

• AclB was the first glycosidase-like autolysin characterized in Lb. casei.
• AclB was demonstrated to be involved in the cell separation of Lb. casei cells.
• The autolytic activity and its optimal acting conditions were investigated.
• The autolysin complement of Lb. casei was preliminarily drawn.

Autolysis of nonstarter lactic acid bacteria (NSLAB) was favorable for the development of flavor compounds during cheese manufacture. Among these bacteria, Lb. casei was regarded as the most important microbiota involved in cheese processes. In this study, a novel autolysin named AclB was identified in the genome of Lb. casei BL23 and its modular structure was predicted through bioinformatic approaches. Subsequently, its transcription profile in the exponential phase, hydrolytic activities against cell walls, enzymatic properties under different conditions, physiological function via gene inactivation and upregulation assays, as well as potential applications to NSLAB's autolysis were fully investigated. According to the results, AclB was recognized as a species-specific cell-separating enzyme, responsible for cell separation after cell division in Lb. casei BL23. The purified AclB showed considerable hydrolyzing activities towards cell walls, indicating its enzymatic nature as peptidoglycan hydrolase, or autolysin. The highest activity of AclB was determined at pH 5.0 and 37 °C, and the expression vector constructed based on AclB was shown to facilitate the controlled lysis of Lb. casei BL23 hosts. In summary, this study provided insight into the enzymatic properties of a novel autolysin involved in cell separation of Lb. casei BL23, which is promising to accelerate cheese ripening and improve cheese quality.