Distinctive proteolytic activity of cell envelope proteinase of Lactobacillus helveticus isolated from airag, a traditional Mongolian fermented mare’s milk.

This study examines the unique characteristics of a bacterium, Lactobacillus helveticus, isolated from Airag, a traditional Mongolian fermented milk. It provides insight into the bacterium’s optimal growth conditions and its unique enzymatic characteristics, specifically its cell envelope proteinase (CEP) which is important for milk protein degradation.

Study Methodology and Findings

• Six L. helveticus strains were isolated from five homemade samples of airag, a fermented milk drink made from mare’s milk, from different regions of Mongolia.
• The optimal temperature for acidification of these strains in skim milk was found to be between 30 to 35°C, lower than the reference strains isolated from European cheeses.
• All the Mongolian strains contained a prtH1-like gene that encodes for a variant type of CEP, an enzyme that breaks down proteins.
• The study examined one strain, SBT 11087, and found that its CEP enzyme shared 71% identity with the CEP from L. helveticus CNRZ32, but 98% identity with the CEP from Lactobacillus kefiranofaciens ZW3, a bacterium isolated from fermented milk in Tibet.
• The proteolytic activity, or protein-breaking ability, of the CEP was tested using an intact-cell degradation assay. This assay checks how well an enzyme can break down a specific compound.
• It was found that the CEP from the studied strain was weak and had a lower optimal temperature (40°C) compared to reference strains, which have an optimal temperature of 45-50°C.
• The enzyme showed typical specificity for αS1-casein, a type of protein in milk, aligning with previously reported L. helveticus enzymes.
• However, when it came to degradation of β-casein, another type of milk protein, the CEP from the studied strain displayed distinct characteristics. It primarily hydrolyzes the C-terminal domain and only hydrolyzed nine of the 16 cleavage sites usually targeted by the CEPs from other strains.

Implications

• The unique traits of the Mongolian L. helveticus strains, such as their lower optimal temperature, align with their adaptation to the unique environmental conditions of Mongolia.
• The distinct characteristics of the CEP from these strains, particularly its unique degradation profile of β-casein, point to its specific suitability for the natural lactic acid bacterial population in airag.
• This investigation provides valuable knowledge for the potential utilization of these strains in functional food industry or dairy fermentation processes.