Nisin-bacteriophage crossresistance in Staphylococcus aureus

The combined effect of nisin and two lytic phages against Staphylococcus aureus was assessed. In short-time challenge experiments performed in pasteurized milk, a synergistic effect was observed. However, the development of nisin-adapted cells seriously compromised bacteriophage activity. A nisin-adapted strain became partially resistant to both phages. Efficiency of plaquing as well as adsorption values differed. Changes on the bacterial surface, often linked to nisin resistance, could account for the phenotypes observed, most likely by interfering with binding/recognition of phage receptors. The nisin-adapted strain was significantly less hydrophobic and with a higher positive net charge as shown by the lack of binding of cytochrome c and nisin. Loss of the nisin resistant phenotype restored phage susceptibility. In contrast, bacteriophage insensitive mutants were not nisin resistant. The results indicate that careful use of nisin and bacteriophages in combination is required for control of S. aureus in dairy products.