Expanding the bactericidal action of the food color additive phloxine B to gram‐negative bacteria

Phloxine B (D&C red no. 28) is a color additive for food, drugs, and cosmetics. It has been previously shown to have anti‐ Staphylococcus aureus activities. In this work, the effect of Phloxine B on various gram‐negative bacteria and other gram‐positive bacteria including Bacillus cereus , Bacillus thuringiensis , Bacillus mycoides , Bacillus subtilis , Bacillus aureus , Salmonella , Escherichia coli and Shigella was studied, along with the mechanism of anti‐microbial activity. In the presence of fluorescent light, the viable count for gram‐positive bacteria, ( Bacillus spp. and S. aureus ) decreased in a dose and time dependent manner when incubated with Phloxine B. The viability of gram‐positive bacteria was reduced by 99.99% in 40 min, while there was no effect on gram‐negative bacteria ( Salmonella choleraesuis , E. coli and Shigella flexneri ). However, the use of ethylenediaminetetraacetic acid (EDTA) expands the spectrum of activity for Phloxine B to include gram‐negative bacteria. EDTA increased membrane‐permeability by releasing lipopolysaccharide. Overall, in an Agar diffusion test the light‐dependent bactericidal activity of 1 μg of Phloxine B had a potency of 0.64 units of chloramphenicol and 0.5 units of tetracycline when tested on B. cereus , and had a potency of 0.7 units of chloramphenicol and 0.2 units of tetracycline when tested on S. aureus . The data suggest that the dye may have some potential anti‐microbial applications.