Purpose: This study was undertaken to investigate the attachment of cellulose-deficient derivates of STEC to lettuce and spinach in different water hardness environments.
Methods: Two cellulose-producing wild-type STEC strains 19 and 49 as well as their cellulose-deficient derivatives were used. Strain 49 also produces EPS comprised of colonic acid. Viability of cells was determined by plate counts on the surface and cut edge after leaves were inoculated with 108 CFU/ml cells at 4 °C for 2 hours. Hydrophobicity and surface charge were also determined.
Results: Strain 49 attached 0.3 and 0.6 log greater to surface and 0.9 and 0.4 log greater cut edge of spinach compared to strain 19 in wild-type and cellulose-deficient cells. In addition, there was significantly greater attachment for cellulose-proficient cells on lettuce surface than cellulose-deficient cells, but not on spinach surface. Contrary to surface results, more cellulose-deficient cells attached (0.66 and 0.3 log greater) in strain 19 and 49 to cut edge of lettuce than cellulose-proficient cells. Strain 19 was more hydrophobic than strain 49. In addition, there is progressive ordering with decreasing attachment of strain 49 in water hardness from 0 to 150 ppm, and afterwards from 200 to 1,000 ppm in order progressively increasing attachment of cells on lettuce and spinach leaves surface. The zeta potential of strain 49 was more negative than strain 19.
Significance: Cells with EPS-containing colonic acid and less hydrophobic have more potential to attach surface and cut edge of spinach as compared to cellulose-producing cells with more hydrophobic. In high water hardness environment, attachment of cells to leafy green surfaces can also be enhanced.