The objective of this in vitro study was to evaluate the potential of egg yolk immunoglobulins (IgYs) for treating mastitis caused by Staphylococcus aureus. Specific IgY against type 5 (IgY-T5), type 8 (IgY-T8) and type 336 (IgY-T336) S. aureus strains were obtained by immunizing hens with whole cell vaccines and the IgY produced were then purified to around 80% purity using a water dilution method coupled with salting out and ultra-filtration. Enzyme-linked immunosorbent assay indicated that the IgY specifically targeted the three homologous strains. A growth inhibition assay was performed in Columbia broth (non-encapsulated form) and phosphate-buffered saline (encapsulated form) for an 8h incubation. The results showed that IgY-T336 significantly inhibited (but only 1.5 log units; P<0.01) the growth of all three strains at 15 mg/ml in the Columbia broth. In contrast, the same concentrations of IgY-T5 and IgY-T8 did not show obvious bacteriostatic activity against the two homologous strains. In phosphate buffered saline, no inhibition of the two encapsulated strains was observed with IgY-T5, IgY-T8 and IgY-T336. However, IgY-T336 reduced live bacteria by 1.0 log unit against strain 336 compared with the control. An internalization test indicated that all of the specific IgY (at 5mg/ml) significantly (about 3.0 log units of the control; P<0.01) blocked the internalization of their homologous strains by bovine mammary epithelial cells (MAC-T cells) within 6h. These results suggested that research on the application of IgY as a treatment for mastitis caused by S. aureus should be focused on the internalization inhibition activity rather than on the growth inhibition activity of the IgY.
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