tStaphylococcus aureus causes a variety of diseases, including bovine mastitis. The standard treatmentregimen is treating it with antibiotics. However this approach has drawbacks, including high cost andthe incidence of antibiotic-resistant bacteria. As a result, there has been renewed interest in the use ofbacteriophages (phages) to control S. aureus. However, aggregation facilitates this bacterium’s persistencein the host, and the ability to aggregate has been increasingly recognized as an important Staphylococcusvirulence 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 ofSA003 by the specific bacteriophage (SA012), originally isolated from sewage influent. Addition of IgGalso lowered the phage–host adsorption rate constant (ka). The kavalue of the IgG-containing conditionwas approximately 1/8 of that of the IgG-free condition. In addition to the decrease of kavalue, increaseof the minimum inhibitory concentration (MIC) of SA012 toward SA003 was observed. Fluorescencemicroscopical and confocal laser microscopical observations revealed that phages were mainly localizedon the surfaces of cell aggregates. IgG-dependent S. aureus aggregation in crude milk is a problem thatmust be solved before phage therapy can be successfully used to treat bovine mastitis caused by thisbacterium.
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