Impact of soil clay minerals on growth, biofilm formation, and virulence gene expression of Escherichia coli O157:H7

Soil, composed mainly of minerals, plays a central role in the circulation of microbial pathogens in the environment. Herein, the growth, biofilm formation, and virulence gene expression of the pathogenic bacteria Escherichia coli O157:H7 were monitored following exposure to montmorillonite, kaolinite, and goethite, three common soil minerals in the clay size fraction. E. coli O157:H7 growth was notably promoted (P < 0.05), while biofilm formation was inhibited in the presence of montmorillonite (P < 0.05), which is attributed to the suppression of colanic acid (CA) production and an increase in bacterial motility. Kaolinite not only promoted bacterial growth (P < 0.05), but also contributed to biofilm formation upon stimulating CA production. Upon exposure to goethite, notably slower bacterial growth and higher biomass of biofilm were observed as compared to the control (P < 0.05). Goethite stimulated the synthesis of CA to encase cells in a protective biofilm in response to the tight association between bacteria and goethite, which could cause bacterial death. Additionally, the transcription of virulence factors (stxA-1 and stxA-2) was significantly decreased in goethite or kaolinite system (P < 0.05). These findings indicate that minerals play important roles in the physiological state of bacteria and ultimately govern the fate of this pathogen in soils.