Foodborne Gastroenteritis Caused by Escherichia coli
641
25
◦ or 30
◦ C only traces were found.
90 Highest levels were attained in fresh ground beef (452 ng/g),
whereas in milk with agitation for 48 hours, the highest level found was 306 ng/ml. No toxin was
detected in ground beef held at 8
◦ C for up to 14 days. There is no evidence that preformed Stx plays
any role in diseases caused by EHEC strains. The mean optimum growth temperature for 20 E. coli
0157:H7 strains was 40.2
◦ C in Mueller–Hinton broth compared to a mean of 41.7
◦ C for non-0157:H7
strains.
30
With regard to minimum temperature for Stx production in brain heart infusion (BHI) broth, 4 of
16 strains grew at 8
◦ C but not at 5
◦ C, whereas 12 of these grew at 10
◦ C but not at 8
◦ C.
68 Three of 16
strains increased 1000-fold in numbers in 4–6 days at 10
◦ C, and as noted above, Stx was produced
at all temperatures that supported growth.
68 Concentrations of Stx1 were 63 and 85 ng/ml of slurry
following incubations at 21
◦ and 37
◦ C, respectively.
1 Unlike most strains of E. coli, the O157:H7
strains do not grow in EC (Escherichia coli) medium at 44.5
◦ C and their maximum in EC medium is
around 42
◦ C.
71
Effect of Environmental and Physical Agents
Interest in the acid sensitivity of EHEC strains increased following an outbreak traced to fresh-pressed
apple cider.
6 That product had a pH range of 3.7–3.9. In one study, EC O157:H7 survived for up to
56 days at pH ≥4.0 using tryptic soy broth and various acids for pH adjustment.
16 In another study,
in which the pH of Luria broth was adjusted with HCl, no loss of viability of an EC O157:H7 strain
was seen for at least 5 hours at pH 3.0–2.5 at 37
◦ C.
5 In a more detailed study using apple ciders with
pH values between 3.6 and 4.0 and EC O157:H7 inocula of 10
2 –10
5 , cells survived for 2–3 days at
25
◦ C.
98 At 8
◦ C, a 10
5 /ml inoculum increased only about 1 log over 12 days and survived for 10–31
days at this temperature. Although potassium sorbate was only minimally effective, sodium sorbate
shortened survival time at 8
◦ C to 2–10 days, and to 1–2 days at 25
◦ C.
98 Growth of EC O157:H7 was
demonstrated in trypticase soy broth at pH 4.5 when HCl was used, but no growth occurred at this pH
when lactic acid was used—the minimum was pH 4.6.
29
In a study of EC O157:H7 survival in commercial mayonnaise, survival was noted for 35 days for
products stored at 5
◦ or 7
◦ C, but cells could not be detected after 72 hours when stored at 25
◦ C.
89 The
mayonnaise had a pH of 3.65 and the inoculum was ∼10
7 colony-forming units (cfu)/g. With inocula
as high as log 6.23/g in commercial mayonnaise and storage at 5
◦ , 20
◦ , or 30
◦ C, strain 0157:H7 did
not grow and was approaching undetectable levels after 93 days at 5
◦ C.
37 In another study, ≥6 log cfu
of an EHEC strain was inoculated into five commercial real-mayonnaise-based and reduced-calorie
and/or fat mayonnaise dressings and stored at 25
◦ C.
24 The pH ranged from 3.21 to 3.94, and the
products with pH <3.6 rapidly inactivated EHEC, producing a ≥7 log cfu decrease in ≤1–3 days.
EHEC cells have been shown to have increased survival in acidic foods if they are first cultured in an
acidic environment at around a pH of 5.0.
50 Two EHEC strains survived for 18 days at 4
◦ C in four
varieties of ground apples and the final pH of the four ranged from 3.91 to 5.11.
27 Fallen apples may be
contaminated by EHEC strains in pastures, and also by contaminated fruit flies.
40 More information
on the acid tolerance of E. coli strains is presented in Chapter 22.
With regard to salt tolerance of an EC O157:H7 strain, 4.5% NaCl in broth caused a threefold
increase in doubling time, whereas at 6.5%, a 36-hour lag was noted with a generation time of 31.7
hours.
29 These investigators found that no growth occurred at ≥8.5% NaCl. In the same study, the EC
O157:H7 survived sausage fermentation but did not grow when stored at 4
◦ C for 2 months following
inoculation at a level of 4.8 × 10
4 .
29
The thermal resistance of EHEC strains is not unlike that of most Gram-negative bacteria, and,
in fact, these strains appear to be more heat sensitive than most salmonellae. A recent study found
641
25
◦ or 30
◦ C only traces were found.
90 Highest levels were attained in fresh ground beef (452 ng/g),
whereas in milk with agitation for 48 hours, the highest level found was 306 ng/ml. No toxin was
detected in ground beef held at 8
◦ C for up to 14 days. There is no evidence that preformed Stx plays
any role in diseases caused by EHEC strains. The mean optimum growth temperature for 20 E. coli
0157:H7 strains was 40.2
◦ C in Mueller–Hinton broth compared to a mean of 41.7
◦ C for non-0157:H7
strains.
30
With regard to minimum temperature for Stx production in brain heart infusion (BHI) broth, 4 of
16 strains grew at 8
◦ C but not at 5
◦ C, whereas 12 of these grew at 10
◦ C but not at 8
◦ C.
68 Three of 16
strains increased 1000-fold in numbers in 4–6 days at 10
◦ C, and as noted above, Stx was produced
at all temperatures that supported growth.
68 Concentrations of Stx1 were 63 and 85 ng/ml of slurry
following incubations at 21
◦ and 37
◦ C, respectively.
1 Unlike most strains of E. coli, the O157:H7
strains do not grow in EC (Escherichia coli) medium at 44.5
◦ C and their maximum in EC medium is
around 42
◦ C.
71
Effect of Environmental and Physical Agents
Interest in the acid sensitivity of EHEC strains increased following an outbreak traced to fresh-pressed
apple cider.
6 That product had a pH range of 3.7–3.9. In one study, EC O157:H7 survived for up to
56 days at pH ≥4.0 using tryptic soy broth and various acids for pH adjustment.
16 In another study,
in which the pH of Luria broth was adjusted with HCl, no loss of viability of an EC O157:H7 strain
was seen for at least 5 hours at pH 3.0–2.5 at 37
◦ C.
5 In a more detailed study using apple ciders with
pH values between 3.6 and 4.0 and EC O157:H7 inocula of 10
2 –10
5 , cells survived for 2–3 days at
25
◦ C.
98 At 8
◦ C, a 10
5 /ml inoculum increased only about 1 log over 12 days and survived for 10–31
days at this temperature. Although potassium sorbate was only minimally effective, sodium sorbate
shortened survival time at 8
◦ C to 2–10 days, and to 1–2 days at 25
◦ C.
98 Growth of EC O157:H7 was
demonstrated in trypticase soy broth at pH 4.5 when HCl was used, but no growth occurred at this pH
when lactic acid was used—the minimum was pH 4.6.
29
In a study of EC O157:H7 survival in commercial mayonnaise, survival was noted for 35 days for
products stored at 5
◦ or 7
◦ C, but cells could not be detected after 72 hours when stored at 25
◦ C.
89 The
mayonnaise had a pH of 3.65 and the inoculum was ∼10
7 colony-forming units (cfu)/g. With inocula
as high as log 6.23/g in commercial mayonnaise and storage at 5
◦ , 20
◦ , or 30
◦ C, strain 0157:H7 did
not grow and was approaching undetectable levels after 93 days at 5
◦ C.
37 In another study, ≥6 log cfu
of an EHEC strain was inoculated into five commercial real-mayonnaise-based and reduced-calorie
and/or fat mayonnaise dressings and stored at 25
◦ C.
24 The pH ranged from 3.21 to 3.94, and the
products with pH <3.6 rapidly inactivated EHEC, producing a ≥7 log cfu decrease in ≤1–3 days.
EHEC cells have been shown to have increased survival in acidic foods if they are first cultured in an
acidic environment at around a pH of 5.0.
50 Two EHEC strains survived for 18 days at 4
◦ C in four
varieties of ground apples and the final pH of the four ranged from 3.91 to 5.11.
27 Fallen apples may be
contaminated by EHEC strains in pastures, and also by contaminated fruit flies.
40 More information
on the acid tolerance of E. coli strains is presented in Chapter 22.
With regard to salt tolerance of an EC O157:H7 strain, 4.5% NaCl in broth caused a threefold
increase in doubling time, whereas at 6.5%, a 36-hour lag was noted with a generation time of 31.7
hours.
29 These investigators found that no growth occurred at ≥8.5% NaCl. In the same study, the EC
O157:H7 survived sausage fermentation but did not grow when stored at 4
◦ C for 2 months following
inoculation at a level of 4.8 × 10
4 .
29
The thermal resistance of EHEC strains is not unlike that of most Gram-negative bacteria, and,
in fact, these strains appear to be more heat sensitive than most salmonellae. A recent study found