Abstract: The aim of this study was to investigate the effect of long-term acid stress on biofilm formation (BF) of Escherichia coli O157:H7. Firstly, the adhesion capacities of different strains were compared by using microwell plate-crystal violet staining method and the BF curves of strains with different adhesion capacities were analyzed. Secondly, one representative strain was selected to investigate the effect of long-term acid stress on BF using plate-counting method. Finally, the changes in biofilm microstructure of the adhesive strains under acidic condition were observed using a confocal laser scanning microscope (CLSM). The results showed that adhesion capacity varied among 14 investigated strains. The BF curves of the adhesive strains were different although they all began to form biofilm after 2 h of cultivation. The results of acid stress showed that the BF of ATCC43895, a representative strain with medium adhesion ability, was inhibited by acid, the lower pH could result in lower BF, and the inhibitory effect of lactic acid on BF was significantly higher than that of hydrochloric acid (P < 0.05). CLSM results showed that weakly adhesive strain CICC21530 and strongly adhesive strain J29 were able to form biofilm under acidic and neutral conditions, and the latter had a higher biofilm production than did the former. Acidic condition inhibited the BF process. Our findings demonstrated the inhibitory effect of lactic acid on BF of E. coli O157:H7, which provides scientific evidence for the elimination of the biofilm formed by E. coli O157:H7 in food processing.

Key words: Escherichia coli O157:H7, biofilm, lactic acid, stress