IgG-dependent aggregation of Staphylococcus aureus inhibits bacteriophage attack

• Identification of milk component which facilitates S. aureus aggregation.
• Polyclonal bovine IgG enhanced aggregation of S. aureus.
• IgG-dependent aggregation delayed S. aureus lysis by the specific bacteriophage.
• IgG lowered the phage–host binding constant.
• IgG increased the minimum inhibitory concentration of bacteriophage.

Staphylococcus aureus causes a variety of diseases, including bovine mastitis. The standard treatment regimen is treating it with antibiotics. However this approach has drawbacks, including high cost and the incidence of antibiotic-resistant bacteria. As a result, there has been renewed interest in the use of bacteriophages (phages) to control S. aureus. However, aggregation facilitates this bacterium’s persistence in the host, and the ability to aggregate has been increasingly recognized as an important Staphylococcus virulence factor. This study revealed that polyclonal bovine IgG enhanced aggregation of S. aureus (SA003), isolated from raw-milk samples from cows with mastitis. IgG-dependent aggregation delayed lysis of SA003 by the specific bacteriophage (ΦSA012), originally isolated from sewage influent. Addition of IgG also lowered the phage–host adsorption rate constant (ka). The ka value of the IgG-containing condition was approximately 1/8 of that of the IgG-free condition. In addition to the decrease of ka value, increase of the minimum inhibitory concentration (MIC) of ΦSA012 toward SA003 was observed. Fluorescence microscopical and confocal laser microscopical observations revealed that phages were mainly localized on the surfaces of cell aggregates. IgG-dependent S. aureus aggregation in crude milk is a problem that must be solved before phage therapy can be successfully used to treat bovine mastitis caused by this bacterium.