This study assessed the safety and probiotic properties of Niallia circulans SWUN-Y10, through both in vitro methods such as antibiotic susceptibility testing, biogenic amine testing, and virulence gene analysis, and in vivo methods including biochemical testing, routine blood index testing, bacterial displacement capacity, and histopathology analysis. The whole genome sequence of SWUN-Y10 was determined to be 5074781 bp in length, with an average G+C content of 35.41%, which encoded totally 5269 genes. Genomic analysis confirmed the absence of genes encoding critical virulence factors and identified only a single chromosomal antibiotic resistance gene (vmlR); alongside genes associated with carbohydrate metabolism and stress tolerance were also identified. In vitro phenotypic characteristics demonstrated the strain was also safety, evidenced by non-hemolytic activity, lack of biogenic amine production, and broad antibiotic susceptibility. Furthermore, SWUN-Y10 exhibited robust in vitro probiotic attributes, including high tolerance to simulated gastrointestinal stress (pH 2.0, 0.3% bile salts), significant auto-aggregation, and substantial cell surface hydrophobicity. The results of 14-day acute oral toxicity test showed that Niallia circulans SWUN-Y10 (10¹¹ CFU/kg body weight) did not cause significant effect on the normal growth, erythrocytes, white blood cells, and other routine blood parameters in mice. Furthermore, there was no bacteria migration observed in the liver, kidney, spleen, or heart during the feeding trial, which indicated that Niallia circulans SWUN-Y10 did not cause any infections in vivo. Moreover, H&E staining showed no evidence of lesion damage to the liver. To sum up, Niallia circulans SWUN-Y10 exhibits a promising safety and functional probiotic properties, supporting its potential use in future as a starter culture for food fermentation or as a probiotic supplement.

Staphylococcus aureus is one of the important pathogens that cause food contamination worldwide, and poses a great danger to people. The common treatment is antibiotic therapy; however, the misuse of antibiotics has led to the continuous emergence of drug-resistant strains. Therefore, new antibacterial methods need to be explored. In this study, a lytic S. aureus phage (named SP-CmSa-11) was isolated from dairy farms which belongs to the Myoviridae phage family. The optimal multiple of infection of SP-CmSa-11 is 0.1, and the host range included 7 strains of mastitis cow-derived S. aureus, 12 strains of animal-origin methicillin-resistant S. aureus, 1 strain of S. xylosus, 1 strain of S. epidermidis and 1 strain of Enterococcus faecalis. SP-CmSa-11’s burst period was 40 min, and the burst size was about 130 PFU/cell. SP-CmSa-11 was inactivated after 10 min in a water bath at 70 °C, and the pH tolerance of SP-CmSa-11 ranges from 4 to 10 in 2 h. It’s insensitive to chloroform and ultraviolet radiation. The total genome length of SP-CmSa-11 is 45 816 bp. The G + C content is 27.23%, and has 59 putative open reading frames. SP-CmSa-11 showed good antibacterial effects in vitro and milk. These results suggest that SP-CmSa-11 may be a promising alternative therapy for food contamination caused by S. aureus.