Abstract:

【Objective】 The objective of the study was to screen an attenuated strain and immunogenicity in mice of a bovine mastitis Staphylococcus aureus mutant. 【Method】 An attenuated mutant, designated Hx, was derived from Staphylococcus aureus bovine mastitis strain SA zfb after N-methyl-N'-nitro-N-nitrosoguanidine mutagenesis. The characteristics were tested and the protection experiments were carried out by intraperitoneal administration in mice and challenged with homologous and heterologous strains. 【Result】 Mutant Hx, which was isolated on the basis of hypotoxicity, showed diminished virulence in mice (LD50 of SA Hx: 10-1.83•mL-1, LD50 of SA zfb: 10-4.33•mL-1). Mutant Hx grew more slowly than its parental strain and showed decreased production of several exoproteins, such as a- and β-hemolysin, DNAse and coagulase. The production of its capsule was induced only under in vivo growth conditions. In homologous challenge, the LD50 of SA zfb in mice immunized with different doses of the mutant was about 6 to 50 times higher than that of untreated mice. In heterologous challenge, the LD50 of SA 25923 and the LD50 of SA L20-2 in mice immunized with different doses of the mutant was about 5 and 60 times higher than that of untreated mice. 【Conclusion】 The fact shows that mutant Hx is attenuated and that it confered in mice immunity against homologous and heterologous strains, so it is an interesting candidate for the preparation of a live-attenuated vaccine against staphylococcal infections.

Key words: Staphylococcus aureus, β-hemolysin, MNNG, mutant, LD50, protection test