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Volume 46, Number 2, April 2013

Efficacy of combination oral antimicrobial agents against biofilm-embedded methicillin-resistant Staphylococcus aureus 

Wen-Shiann Wu, Chi-Chung Chen, Yin-Ching Chuang, Bo-An Su, Yu-Hsin Chiu, Hui-Jine Hsu, Wen-Chien Ko**, Hung-Jen Tang*

Received: August 22, 2011    Revised: February 29, 2012    Accepted: March 14, 2012   


Corresponding author:

* Corresponding author. Department of Medical Research, Chi Mei Medical Center, Number 901 Chung-Hwa Road, Yung-Kang City 710,
Tainan, Taiwan.
** Corresponding author. Department of Internal Medicine, National Cheng Kung University Hospital, Number 138, Sheng Li Road, 704
Tainan, Taiwan.
E-mail addresses: (W.-C. Ko), (H.-J. Tang). 


Background and purpose: 

The combination of fusidic acid and rifampicin has a demonstrated synergistic effect against methicillin-resistant Staphylococcus aureus (MRSA), including planktonic and biofilm-related organisms. However, the in vitro efficacy of other combinations of oral anti-MRSA antibiotics in biofilm models has not been established. 



The antibacterial activity of fusidic acid, linezolid, rifampicin, and minocycline against 33 biofilm-embedded MRSA isolates in low susceptibility and high resistance breakpoint concentrations was investigated using the 3-[4, 5-dimethyl-2-thiazolyl]-2, 5-diphenyl-2H-tetrazolium-bromide staining method. The compounds were further examined to determine their antibacterial efficacies in combination. The optical density ratio (ODr) was used to evaluate the antibacterial effects of these antibiotics, and the results indicate higher survival rates of MRSA on biofilm. A biofilm-positive phenotype (determined using the crystal violet stain) was defined as an optical density ≥ 0.17 at 492nm, and strong biofilm formation was defined as an optical density ≥ 1.0. 



One-third of the MRSA isolates demonstrated weak biofilm formation, and two-thirds demonstrated strong biofilm formation. At low concentrations, linezolid alone lowered the ODr to 0.55 and was effective against biofilm-embedded MRSA (p<0.001). The activity of minocycline was concentration-dependent and more effective against MRSA isolates that demonstrated weak biofilm formation. The effect of minocycline seems to be further enhanced when used in combination with either fusidic acid or linezolid at low concentrations, with the obtained results equal to those obtained with rifampicin-based regimens (p<0.001). Rifampicin plus minocycline was also effective against MRSA in biofilm. 



In comparison with monotherapy, minocycline-based combinations exhibit highly effective bactericidal effects against biofilm-embedded MRSA. 


Key words:

Biofilm, Methicillin-resistant Staphylococcus aureus (MRSA), Oral antimicrobial agents