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LF3872 was isolated from the milk of a healthy lactating and breastfeeding woman. Earlier, the genome of LF3872 was sequenced, and a gene encoding unique bacteriocin was discovered. We have shown here that the LF3872 strain produces a novel thermolabile class III bacteriolysin (BLF3872), exhibiting antimicrobial activity against antibiotic-resistant <i>Staphylococcus aureus</i> strains. Sequence analysis revealed the two-domain structural (lysozyme-like domain and peptidase M23 domain) organization of BLF3872. At least 25% residues of this protein are expected to be intrinsically disordered. Furthermore, BLF3872 is predicted to have a very high liquid-liquid phase separation. According to the electron microscopy data, the bacterial cells of LF3872 strain form co-aggregates with the <i>S. aureus</i> 8325-4 bacterial cells. LF3872 produced bacteriolysin BLF3872 that lyses the cells of the <i>S. aureus</i> 8325-4 mastitis-inducing strain. The sensitivity of the antibiotic-resistant <i>S. aureus</i> collection strains and freshly isolated antibiotic-resistant strains was tested using samples from women with lactation mastitis; the human nasopharynx and oral cavity; the oropharynx of pigs; and the cows with a diagnosis of clinical mastitis sensitive to the lytic action of the LF3872 strain producing BLF3872. The co-cultivation of LF3872 strain with various antibiotic-resistant <i>S. aureus</i> strains for 24 h reduced the level of living cells of these pathogens by six log. The LF3872 strain was found to be able to co-aggregate with all studied <i>S. aureus</i> strains. The cell-free culture supernatant of LF3872 (CSLF3872) induced <i>S. aureus</i> cell damage and ATP leakage. The effectiveness of the bacteriolytic action of LF3872 strain did not depend on the origin of the <i>S. aureus</i> strains. The results reported here are important for the creation of new effective drugs against antibiotic-resistant strains of <i>S. aureus</i> circulating in humans and animals.