Objective
The spread of antibiotic resistance among Staphylococcus aureus strains requires the development of new anti S. aureus agents. The objective of this study was evaluating the antimicrobial activity of the commonly used drug and food color additive Phloxine B against S. aureus and other food microbial pathogens.

Methods
The effect of Phloxine B on S. aureus and other food microbial pathogens (Bacillus mycoides, Bacillus subtilis, Bacillus cereus, Salmonella choleraesuis, E. coli and Shigella flexneri) measured by cell optical density, viability test and agar diffusing test.

Result
The drug and food color additive Phloxine B is shown to be an effective bactericidal agents against S. aureu, halting the growth of the S. aureus within five minutes. The number of viable cells diminished rapidly in a dose-time dependent manner. Similar results were obtained for other Gram positive bacteria (Bacillus mycoides, Bacillus subtilis, Bacillus cereus). On the other hand, the agent was not effective against Gram negative food borne pathogens (Salmonella choleraesuis, E. coli and Shigella flexneri). Various agents which increased Gram negative bacteria membrane-permeability increased bactericidal activity against Gram negative bacteria.

Conclusions
The drug and food color additive Phloxine B has bactericidal activity against S. aureus as well as other Gram positive bacteria even when applied at concentration 1000 times less than what is approved for human consumption in the U.S. Effective use of this agent against Gram negative bacteria is possible combination with agents that increase Gram negative bacteria membrane-permeability.