In Vitro Antagonistic Activity of Cell-Free Supernatant from a Local mecA-Positive Staphylococcus epidermidis Isolate Against Staphylococcus aureus

Author(s): Rafal Ali Hatef , Maitham G. Yousif
Date: 09/10/2026
doi: https://doi.org/10.5281/zenodo.8425890

---

Abstract

Background: The human skin microbiome, specifically commensal bacteria such as Staphylococcus 
epidermidis, is important in preventing opportunistic bacteria such as Staphylococcus aureus from 
colonizing the skin. The purpose of this study was to assess the in vitro antagonistic effect of the cell-free 
supernatant (CFS) obtained from a local S. epidermidis mecA positive isolate against S. aureus. 
Methods: A local isolate of S. epidermidis was phenotypically identified by the use of VITEK 2 system 
and molecularly characterized by Polymerase Chain Reaction (PCR) confirming the presence of the mecA 
gene. The CFS were prepared by centrifuging and filtering them. Later, the antibacterial activity against 
S. aureus was evaluated by the agar-well diffusion method. 
Results: Molecular analysis proved to be successful in confirming the local isolate as being mecA 
positive. The in vitro antagonism assay showed that the CFS of S. epidermidis isolate had a clear 
antagonism effect on S. aureus with a definite ZOI of 10 mm. 
Conclusion: The local mecA-positive S. epidermidis has significant antagonistic properties against S. 
aureus in the cell free supernatant. These results pave the way for the potential use of postbiotics from 
local commensals as active ingredients to create topical formulations with the aim of restoring the balance 
of the human skin microbiome and improving the skin's natural immune barrier functions.

---