Microbial biofilms: an important concern in food safety
Inhibition of microbial biofilms is a significant concern in food safety. In the present study, the biofilm formation inhibitory effect of sodium citrate and cinnamic aldehyde at MICs and sub-MICs were investigated for Escherichia coli O157:H7 and Staphylococcus aureus. Biofilm inhibition rate was measured to evaluate the effect of sodium citrate to Staphylococcus aureus biofilm at the time point of 24, 48, 72 and 96 h. According to the results, antibiofilm effect had been shown by both food additives, with 10 mg/ml of sodium citrate exhibiting highest inhibition on S. aureus biofilm at 24 h (inhibitory rate as high as 77.51%). Two S. aureus and 6 P. aeruginosa strains with different biofilm formation abilities were co-cultivated in planktonic and biofilm states. During co-cultivation, the culturability, viability, biofilm biomass, and morphology of both cells were identified and RNA-seq on typical models were performed. P. aeruginosa strains were classified into kill group (AT) and inhibit group (HQ) based on the interaction with S. aureus. AT killed S. aureus in 2 to 4 days, while HQ only suppressed S. aureus growth. RNA-seq results revealed that AT upregulated T6SS gene clusters during co-cultivation with S. aureus, whereas HQ expressed differently in secretion systems. Different expression of lrg operon with AT and HQ may lead to difference in S. aureus autolysis rate and thus inhibition intensity. Also, various S. aureus virulence factors were upregulated, including staphylocoagulase, clumping factor and protease.