Escherichia coli O157:H7: Growth in a heterogeneous food system and biofilm formation under nutrient limited conditions
Abstract (Summary)Oil-in-water emulsions (hexadecane, minimal media (M9) and Tween 20) were used as a model system to study the growth of E. coli O157:H7 ATCC 43895. Stationary phase cell density decreased as the hexadecane concentration was increased (0%, 5%, 20%, and 40% (w/v)) and biphasic growth was observed in 40% emulsions supplemented with 0.4% glucose. Thin aggregate fimbriae (curli) were observed using SEM, and a greater percentage (P < 0.001) of curli-producing colonies were isolated from 40% emulsions. Heat resistance (55Ã?Â°C and 58Ã?Â°C) of emulsion-grown cells was greater than broth-grown cells, indicating that growth and physiology under heterogeneous conditions differ from cells grown in liquid. In addition, the curli expression and biofilm formation of this pathogen in a low nutrient environment was investigated. The results showed that the curli expression in this O157:H7 strain was temperature independent and more stable under low nutrient conditions. This organism was able to form biofilm on a PVC surface when grown in M9 but not LB broth. There was a positive correlation coefficient between biofilm formation and curli expression in this strain at 37Ã?Â°C (p < 0.05). Biofilm formation increased as curli production increased. These findings suggest that the ability of E. coli O157: H7 to express curli and produce biofilm in a low nutrient environment may contribute to the surface growth in trough water in cattle farm and serve as a source for recontamination. The rpoS mutant of O157:H7 43895 was used to study the impact of rpoS on heat tolerance, curli production and biofilm formation. The rpoS gene encoding a sigma factor (Ã?Â´ s ) is a central regulator for several stationary phase and stress response inducible genes in E. coli including heat resistance, curli expression and biofilm formation. The results presented here indicate that rpoS regulated proteins were being expressed in cells grown in heterogeneous system and were responsible for enhanced heat resistance. The curli expression requires rpoS regulation. However, higher biofilm formation in the rpoS mutant was observed with no evidence of curli expression indicating that factors additional to rpoS regulated proteins and curli expression may have influenced biofilm formation in this O157:H7 strain.
School Location:USA - Massachusetts
Source Type:Master's Thesis
Date of Publication:01/01/2003