530 Modern Food Microbiology
43 proteins are induced. When stationary-phase cells are shifted to or below pH 4.5, they synthesize
15 proteins that are distinct from log-phase cells.
Acid Tolerance Response
With respect to the acid tolerance response (ATR) of L. monocytogenes, the pH minimum for growth
of two strains was 3.5 and 4.0 in a chemically defined medium using HCl.
81 pH values below
these were lethal unless the strains were previously exposed to pH of 4.8 and 3.5. With mutants of
L. monocytogenes that showed increased ATR, they demonstrated increased lethality for mice compared to wild type strains
76 , suggesting that acid conditions could be selective for strains with increased
virulence. The mutants were recovered after exposure to pH 3.5 for up to 2 hours at 37
◦ C.
76 Similarly,
acid-adapted Yersinia enterocolitica cells grown at pH 7.5, then shifted to pH 5.0, were significantly
more enteropathogenic than controls when tested using a suckling mouse model.
106
δ
B has been identified in L. monocytogenes, B. subtilis, and S. aureus; and its function has been
compared to those of RpoS/δ
S in Gram-negative bacteria. In B. subtilis, δ
B influences the regulon of 100
genes in response to environmental and energy stresses. B. subtilis mutants are more sensitive to heat,
ethanol, acid, freezing, drying, etc.
27 One δ
B reduces virulence in L. monocytogenes. Interestingly, a
hydrostatic pressure resistant strain of L. monocytogenes (survived 400 MPa for 20 minutes) displayed
increased resistance to heat, acid, and hydrogen peroxide.
61 It has been found that the prior adaptation of
L. monocytogenes that leads to acid resistance may depend upon a number of other growth parameters.
65
The δ
B protein is necessary for full resistance of L. monocytogenes to lethal acid exposure, and it
along with δ
S is associated with general stress responses in both Gram-positive and Gram-negative
bacteria.
44 After acid adaptation (pH 3.0, HCl), L. monocytogenes strain Scott A, 11 proteins showed
induced expression while 12 were repressed.
31 On the other hand, acid adaptation did not protect
this species on beef treated with 2% lactic or acetic acid.
56 L. monocytogenes on fresh meats may be
made more acid sensitive by the Gram-negative bacteria of the fresh-meat biota.
86 As noted above,
δ
B plays a role in acid resistance of stationary phase cells, oxidative and osmotic stress resistance,
and in low-temperature growth of L. monocytogenes. Acid adaptation of L. monocytogenes provides
protection against HHP and freezing.
102
Acid-adapted strains (pH 5.0–3.25, brain heart infusion, BHI, broth) of Shigella flexneri and
S. sonnei survived up to 14 days in tomato and apple juice stored at 7
◦ C.
6 The minimum pH for
growth in acidified BHI was 4.75 and 4.5 for S. flexneri and S. sonnei, respectively. In one study,
acid-adapted (pH 2.5, tryptic soy broth), E. coli 0157:H7 remained on beef carcasses after a 2% acetic
acid wash more so than nonadapted cells.
9 In yet another study of acid tolerance in E. coli 0157:H7,
acid tolerance decreased after exposure to non-acid washes.
85
Acid resistance (percentage of cells that survive exposure to pH 2.5 for 2 hours) is well studied
in shigellae. Gorden and Small
50 found that among the cultures they examined, 9 of 12 shigellae
were acid resistant; 11 of 15 generic E. coli (including strain K-12) showed the same level of acid
resistance; 3 of 8 enteroinvasive (EIEC) strains were resistant but none of 2 enteropathogenic (EPEC)
strains or 12 salmonellae were acid resistant. As to why so few shigellae cells are needed to cause
disease, these investigators hypothesized that after these organisms leave the colon, they enter the
stationary phase outside the host. Upon ingestion by another host, they are already acid resistant, and
low numbers can survive through the acidity of the stomach.
50 In another study, Stx-producing strains
of E. coli that could not survive at pH 2.5 were made acid resistant by the introduction of the rpoS
gene on a plasmid.
100 When Stx-producing strains of E. coli were grown in broth at pH 4.6–4.7, they
became 1.1- to 2.0-fold more resistant to radiation than control strains.
16 It has been suggested that
this response may lower the number of cells needed to initiate infection.
97 For instance, it has been
43 proteins are induced. When stationary-phase cells are shifted to or below pH 4.5, they synthesize
15 proteins that are distinct from log-phase cells.
Acid Tolerance Response
With respect to the acid tolerance response (ATR) of L. monocytogenes, the pH minimum for growth
of two strains was 3.5 and 4.0 in a chemically defined medium using HCl.
81 pH values below
these were lethal unless the strains were previously exposed to pH of 4.8 and 3.5. With mutants of
L. monocytogenes that showed increased ATR, they demonstrated increased lethality for mice compared to wild type strains
76 , suggesting that acid conditions could be selective for strains with increased
virulence. The mutants were recovered after exposure to pH 3.5 for up to 2 hours at 37
◦ C.
76 Similarly,
acid-adapted Yersinia enterocolitica cells grown at pH 7.5, then shifted to pH 5.0, were significantly
more enteropathogenic than controls when tested using a suckling mouse model.
106
δ
B has been identified in L. monocytogenes, B. subtilis, and S. aureus; and its function has been
compared to those of RpoS/δ
S in Gram-negative bacteria. In B. subtilis, δ
B influences the regulon of 100
genes in response to environmental and energy stresses. B. subtilis mutants are more sensitive to heat,
ethanol, acid, freezing, drying, etc.
27 One δ
B reduces virulence in L. monocytogenes. Interestingly, a
hydrostatic pressure resistant strain of L. monocytogenes (survived 400 MPa for 20 minutes) displayed
increased resistance to heat, acid, and hydrogen peroxide.
61 It has been found that the prior adaptation of
L. monocytogenes that leads to acid resistance may depend upon a number of other growth parameters.
65
The δ
B protein is necessary for full resistance of L. monocytogenes to lethal acid exposure, and it
along with δ
S is associated with general stress responses in both Gram-positive and Gram-negative
bacteria.
44 After acid adaptation (pH 3.0, HCl), L. monocytogenes strain Scott A, 11 proteins showed
induced expression while 12 were repressed.
31 On the other hand, acid adaptation did not protect
this species on beef treated with 2% lactic or acetic acid.
56 L. monocytogenes on fresh meats may be
made more acid sensitive by the Gram-negative bacteria of the fresh-meat biota.
86 As noted above,
δ
B plays a role in acid resistance of stationary phase cells, oxidative and osmotic stress resistance,
and in low-temperature growth of L. monocytogenes. Acid adaptation of L. monocytogenes provides
protection against HHP and freezing.
102
Acid-adapted strains (pH 5.0–3.25, brain heart infusion, BHI, broth) of Shigella flexneri and
S. sonnei survived up to 14 days in tomato and apple juice stored at 7
◦ C.
6 The minimum pH for
growth in acidified BHI was 4.75 and 4.5 for S. flexneri and S. sonnei, respectively. In one study,
acid-adapted (pH 2.5, tryptic soy broth), E. coli 0157:H7 remained on beef carcasses after a 2% acetic
acid wash more so than nonadapted cells.
9 In yet another study of acid tolerance in E. coli 0157:H7,
acid tolerance decreased after exposure to non-acid washes.
85
Acid resistance (percentage of cells that survive exposure to pH 2.5 for 2 hours) is well studied
in shigellae. Gorden and Small
50 found that among the cultures they examined, 9 of 12 shigellae
were acid resistant; 11 of 15 generic E. coli (including strain K-12) showed the same level of acid
resistance; 3 of 8 enteroinvasive (EIEC) strains were resistant but none of 2 enteropathogenic (EPEC)
strains or 12 salmonellae were acid resistant. As to why so few shigellae cells are needed to cause
disease, these investigators hypothesized that after these organisms leave the colon, they enter the
stationary phase outside the host. Upon ingestion by another host, they are already acid resistant, and
low numbers can survive through the acidity of the stomach.
50 In another study, Stx-producing strains
of E. coli that could not survive at pH 2.5 were made acid resistant by the introduction of the rpoS
gene on a plasmid.
100 When Stx-producing strains of E. coli were grown in broth at pH 4.6–4.7, they
became 1.1- to 2.0-fold more resistant to radiation than control strains.
16 It has been suggested that
this response may lower the number of cells needed to initiate infection.
97 For instance, it has been