Factors affecting production of extracellular carbohydrate complexes by Escherichia coli O157:H7

Production of extracellular carbohydrate complexes (ECC) by foodborne pathogens on raw fruits and vegetables may result in protection against removal or inactivation by sanitizers. The influence of environmental conditions on cell growth, the total amount of ECC produced, and the amount of ECC produced on a per cell basis by Escherichia coli O157:H7 strains ATCC 43895 (wild type) and 43895-exopolysaccharides (EPS) (natural mutant, extensive EPS producer) was studied. To determine the effects of pH on the production of ECC on a per cell basis, E. coli O157:H7 was grown aerobically at 12 and 22 °C on tryptic soy agar (TSA) acidified to pH 7.0, 6.5, 6.0, 5.5, 5.0, 4.5, and 4.0. Lettuce, alfalfa sprout, cantaloupe, tomato, and apple juice agars (pH 4.46–6.50) were also evaluated for their support of the ECC production. Conditions generally favorable for growth of E. coli O157:H7 were a rich nutrient medium (TSA) vs. heated lettuce juice agar (HLJA) or minimal salts medium (MSM), 22 °C compared to 12 °C, and an aerobic atmosphere compared to modified atmosphere (1% O2, 10% CO2, and 89% N2). Conditions favorable for production of ECC on a per cell basis were HLJA, 12 °C, and an aerobic atmosphere. This suggests that modified atmosphere packaging of lettuce may not only decrease the growth of E. coli O157:H7 but also its propensity to form biofilm. There was a negative relationship between cell growth and production of ECC on a per cell basis, and environmental conditions that affected the total amount of ECC produced based on initial population reflected a combination of environmental conditions influencing both cell growth and ECC production on a per cell basis. A relative growth index factor (RGIF) was calculated to better understand ECC production as affected by various environmental conditions simultaneously. The production of ECC on a per cell basis by strain 43895-EPS showed a negative linear relationship with pH of TSA at both 12 and 22 °C. This strain generally produced a greater amount of ECC on fresh juice agar than on TSA at the same pH, but production of ECC on alfalfa sprout juice agar (FJA, pH 6.45) at 22 °C was significantly less than on TSA (pH 6.50). This indicates that nutrient limitation is not based only on nutrient availability. There may be other factors that repress the production of ECC on FJA, and the effects of those factors may be temperature dependent. Further studies will be required to better understand the relationship between nutrient availability and other factors on the production of ECC by E. coli O157:H7 on raw produce.