The classification of Staphylococcus aureus strains by biofilm production differs depending on the method used.

Introduction: Strains can be classified in terms of biofilm production from quantitative absorbance values collectively by dividing strains into tertile ranks or individually by calculating the optical density for the negative control. However, these methods have not been compared in a large sample of Staphylococcus aureus strains. Therefore, our objective was to analyze the agreement between both methods in terms of biomass production and metabolic activity of their biofilm.

Methods: We classified 233 S. aureus strains by biomass production and metabolic activity using the crystal violet and XTT assays, respectively. Strains were classified as low, moderate, or high biofilm producers according to tertile or optical density.

Results: We found no agreement between both methods (p<0.001 and p=0.028, respectively).

Conclusions: We consider strains' biofilm classification by optical density to be a more reliable method, as it depends on the individual absorbance of each strain.