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Evaluation of the antimicrobial susceptibility of Staphylococcus aureus to the K9CATH peptide by the resazurin microtiter method and the reference broth microdilution method

Alma Rossana Tamayo, Alberto Barreras-Serrano, Victor Manuel Del Villar-Pérez, Jose Angel Olivas-Valdez, Tomas Benjamin Rentería-Evangelista, Lourdes Carolina Pujol-Manriquez, Tonatiuh Melgarejo


The antimicrobial activity of the synthetic peptide K9CATH was determined by the resazurin microtitre method (RMM) and by the reference broth microdilution method (NCCLS) against a clinical mastitic isolate of S. aureus and a control strain of S.aureus ATCC. By both methods K9CATH inhibited both strains of S. aureus. The MIC for the field strain ranged between 4 and 8 mg/mL when determined visually and by absorbance, respectively, with both methods. For the control strain the MIC fell between 16 and 18 mg/mL by visual and absorbance interpretation, respectively, by both methods. Due to the color change from blue (not growth) to pink (growth) visual reading for MIC determination with the RMM showed to be easier, rapid, inexpensive and more sensitive for antimicrobial peptide screening than the reference method. This is the first time that the resazurin method is used to determine the MIC of antimicrobial peptides to S. aureus.

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