Development of Congo red broth method for the detection of biofilm-forming or slime-producing Staphylococcus sp.

• A new TSB based Congo-red (CR) broth method was developed in this study.
• CR agar or broth methods were different depending on types or composition of media.
• CR TSB broth method was correlated with presence of icaA/D gene of Staphylococcus sp.

Although the Congo-red agar method has been widely used to detect slime-producing or biofilm forming Staphylococcus sp. in numerous previous studies, their results were contradictory and did not show high correlation with other biofilm characteristics of bacteria such as ica gene profiles, biofilm formation, and so on. Therefore, this study was conducted to evaluate various Congo-red agar methods and to develop the accurate detection method for slime-producing Staphylococcus sp. using Congo-red dye. Total 47 Staphylococcus sp. (36 Staphylococcus aureus and 11 other strains) were tested for the ica gene profiles and phenotypes on various Congo-red agars or broths based on TSB, BHI, or BAB with sucrose or glucose. The icaA/icaD genes were found in 32 S. aureus and 5 other strains and these results was perfectly correlated with results on Congo-red broth based on TSB supplemented with sucrose. Relatively high positive-predictive values (84–93%) were observed when Congo-red agars were tested while negative-predictive values were low (26–44%). From these results, phenotypes on Congo-red agar or broth methods could be different depending on types or chemical composition of tested media and Congo-red broth method based on TSB supplemented with sucrose designed in this study were very accurate to detect slime-producing Staphylococcus sp.