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Factors Affecting Production of Extracellular Carbohydrate Complexes by
Escherichia coli O157:H7
Int J Food Microbiol. 2004 Sep 1;95(2):189-204.
Ryu JH, Beuchat LR.
Center for Food Safety and Department of Food Science and Technology, University
of Georgia, 1109 Experiment Street, Griffin, GA 30223-1797, USA.
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 degrees 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 degrees C
compared to 12 degrees C, and an aerobic atmosphere compared to modified
atmosphere (1% O(2), 10% CO(2), and 89% N(2)). Conditions favorable for
production of ECC on a per cell basis were HLJA, 12 degrees 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 degrees
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 degrees 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.
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