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Evaluation of the antimicrobial activity of a dialysed cranberry extract against selected isolates of Staphylococcus aureus Leusink, Gwendolyn Jill


This research examined the antimicrobial activities of a dialysed commercial cranberry extract (CE90), by determining the minimum inhibitory concentration against Staphylococcus aureus and the effect of CE90 on morphology, adhesion to fibronectin, biofilm formation and removal and salt tolerance of Staph. aureus. The minimum inhibitory concentration was 16 mg CE90 ml⁻¹ for 10 of the 24 Staph. aureus isolates and 32 mg CE90 ml⁻¹ for the remaining 14 isolates. Phase contrast microscopy of treated Staph. aureus revealed a dose-dependent bacterial clumping effect, with higher CE90 concentrations resulting in larger clumps. There were no significant differences between CE90 concentrations (0, 2, 4, 8 and 16 mg CE90 ml⁻¹) in Staph. aureus binding to fibronectin, for a parental strain and two mutant strains deficient in either fibronectin-binding proteins A and B or clumping factor A, with the exception of reduced adhesion for the fnbA-/fnbB- mutant at 4 mg CE90 ml⁻¹. Biofilm formation by four isolates of Staph. aureus (PC1, SA 113, 8325-4, 2076) incubated with CE90 generally decreased as CE90 concentration increased (0 – 16 mg CE90 ml⁻¹). The amount of pre-formed biofilm removed by CE90 for Staph. aureus PC1 was not significantly different from the control. Staph. aureus SA 113 and 2076 showed a greater (P < 0.05) level of residual biofilm after exposure to CE90 (SA 113, 2 – 16 mg CE90 ml⁻¹; 2076, 4 – 16 mg CE90 ml⁻¹) than the negative control. There was more (P < 0.05) residual biofilm in the 2 mg CE90 ml⁻¹ treatment versus the 16 mg CE90 ml⁻¹ treatment for Staph. aureus 8325-4. Biofilm index at 8 and 24 h showed a significant interaction between isolates and CE90 concentrations (P < 0.05), and biofilm index of Staph. aureus PC1 and SA 113 showed a significant interaction between time intervals and CE90 concentrations (P < 0.05). There was a reduction in colony forming units with increasing CE90 concentration on both tryptic soy agar and TSA plus 7.5% NaCl (TSAS), with a greater reduction on TSAS, though this difference between media was significant in only four of the 16 isolate-treatment combinations.

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