The antimicrobial activity of cationic proteins isolated from the cells in bulk milk samples.

Cationic proteins isolated from the cells in bulk milk samples were shown to inhibit the growth of two pathogenic strains of staphylococci and also Streptococcus agalactiae S 13. Polyacrylamide gel disk electrophoresis studies on these proteins revealed the presence of at least 9 components some of which had isoelectric pH's between 7.0 and 9.0. Trace amounts of the isolated protein had isoelectric pH's greater than 9.0. Staphylococci incubated with milk-cell cationic proteins absorbed the protein, thereby allowing the organism to be stained with the anionic dye Fast Green FCF. Protein-treated staphylococci in isotonic solutions autoagglutinated. This autoagglutination was more marked in hypo- and hypertonic solutions. Lysozyme was not demonstrated in the isolated protein fractions in assays involving incubation with Micrococcus lysodeikticus for 90 min. The antimicrobial activity of the cationic proteins isolated from the bulk milk samples was not destroyed after heating to temperatures up to 70 degrees C for 30 min., whereas at higher temperatures the activity diminished and was almost completely lost at 100 degrees C.