## 4 Conclusions  
In our study, we observed a reduction in the population of _L. acidophilus_ in fermented mare's milk samples. This finding applied to both laboratory-prepared and commercial milk samples. The _L. acidophilus_ monoculture does not adapt well to the mare's milk environment, making the fermented beverage produced with these bacteria unsuitable for long-term storage. In contrast, the fermented beverage containing the _B. animalis_ subsp. _lactis_ BB-12 monoculture met the FAO/WHO criterion throughout the entire experiment. The bifidobacteria population remained above 6 log CFU/g up to the 21 st day of storage at 5 \({}^{\circ}\)C. Mare's milk, with its unique composition, presents a promising matrix to produce fermented probiotic beverages. The results obtained indicate the possibility of achieving a stable population of _Bifidobacterium animalis_ subsp. _lactis_ BB-12 for 21 days of cold storage, opening possibilities for the development of new products with an extended shelf life.  
However, the laboratory kumis prepared with mixed microflora and the commercially available kumis did not meet the FAO/WHO probiotic population size criterion. Interestingly, the bifidobacteria population was high and stable even in the unfermented mare's milk beverage, suggesting the potential for producing sweet mare's milk containing live probiotic bacteria with an extended shelf life. Further research is required to investigate the impact of interactions between bacterial strains, including LA-5 and BB-12, on the survival of probiotics in mare's milk. It is particularly important to examine the influence of different strain proportions on the pH environment and the mutual interaction on the metabolism of individual strains. This will enable the optimization of the microbial composition of fermented mare's milk beverages to ensure high probiotic survival and maximize their health-promoting potential.  
Our results also indicate that the addition of original kumis microflora to the LA-5 monoculture (in the K samples) led to the reduced mortality of the LA-5 bacteria, with the population decreasing from 7.3 to 5.9 log CFU/mL, compared to a decrease from 7.8 to 4.5 log CFU/mL in the A samples. The interactions between the LA-5 and kumis microflora in the K samples also benefitted the BB-12 bacteria, with bifidobacteria counts decreasing from 5.1 to 4.6 log CFU/mL, which was higher than the reduction observed in the OK samples (from 1.6 to 1.4 log CFU/mL).  
It is important to note that manipulating the natural kumis/LA-5/BB-12 microflora ratio can optimize the final product's therapeutic properties, enhancing the survival rate of all strains. Our findings confirm that, to ensure the quality of the final product (appropriate pH and high populations of the desired bacterial strains), not only are the culture conditions critical but also the selection of suitable probiotic strains and the optimization of starter concentrations. Our study suggests significant potential for further research and the commercialization of mare's milk products, such as yogurt and other dairy products.  
Conceptualization, A.M.-M. and I.D.; methodology, I.D., A.M.-M., K.S. and M.Z.; software, I.D.; validation, A.M.-M., I.D. and M.Z.; formal analysis, A.M.-M., I.D. and M.Z.; investigation, K.S.; resources, A.M.-M. and I.D.; data curation, A.M.-M.; writing--original draft preparation, A.M.-M. and I.D.; writing--review and editing, I.D.; visualization, I.D.; supervision, I.D. and A.M.-M.; project administration, A.M.-M. and I.D.; funding acquisition, I.D. and A.M.-M. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The original contributions presented in the study are included in the article; further inquiries can be directed to the corresponding author.  
The authors declare no conflicts of interest.