408 Modern Food Microbiology
12 CsPs had a molecular weight range of 14,400 to 48,000. In another study, 32 CsPs were formed
under similar conditions, and 4 were designated cold acclimation proteins (Caps, see reference 38).
Caps result from increased synthesis during balance growth at low temperatures.
The adaptation of L. monocytogenes to low temperature led to the production of CsPs, which
occurred also after HHP treatment for 10 minutes at 200 MPa at 30
◦ C. A temperature decrease from
37 to 10
◦ C resulted in a 10-fold increase in CsP1 and a 3.5-fold increase in CsP3.
68 Increases in
CsPs following HHP were not as great as for cold shock. However, the level of survival of coldshock cells following HHP was 100-fold higher than 37
◦ C-grown cells. A study employing 9 strains
of L. monocytogenes found that cold shock leads to an increase in thermal sensitivity.
46 The D 60
values were reduced by 13–37% over controls, with the greatest effect occurring in cold-shocked,
stationary-phase cells compared to lag or log phase cells.
46
From a study of gene expression in L. monocyogenes growing at 10
◦ C, it was concluded that
acclimation to growth at 10
◦ C likely involves amino acid starvation, oxidative stress, aberrant protein
synthesis, cell surface remodeling, alterations in degradative metabolism, and induction of global
regulatory responses.
38 When the growth of E. coli was reduced from 37 to 5
◦ C, 12 CsPs formed.
Increases in CsPs and CaPs has been demonstrated in the following organisms following a downshift
in growth temperature: a Vibrio sp., Pseudomonas fluorescens; Lactococcus lactis, Bacillus subtilis,
and Vibrio vulnificus (see reference 5).
In a study of the cold-shock response by two strains of E. coli 0157:H7 compared to two nonpathogenic strains, growth at 10
◦ C vs. 20
◦ C followed by freezing at −18
◦ C for up to 24 hours in
four substrates led to an increase in the ability of 25–35% of the pathogenic strains to survive frozen
storage in brain heart infusion agar and apple juice compared to only 5% for the nonpathogens.
24
Freezing in frozen yogurt or ground beef did not show the same effect. Whether cryotolerance is a
characteristic of pathogenic E. coli strains is unclear. A study of cold shock in E. coli 0157:H7 in
different substrates found no increased survival in beef and pork, but increased survival did occur in
milk, whole egg, or sausage.
8 Growth of the organism in trypticase soy broth at pH 5.0 appeared to
negate the protective effect of the cold-shock treatment. The cold-shock protection appeared to be
associated with the appearance of a new protein.
Some psychrotrophs produce larger cells. Yeasts, molds, and bacteria have been found to produce
larger cell sizes when growing under psychrotrophic conditions than under mesophilic conditions.
With Candida utilis, the increased cell size was believed to be due to increases in RNA and the protein
content of cells.
58 Low-temperature synthesis of additional RNA has been reported by others, but one
group found no increase in the amount of RNA at 2
◦ C when Pseudomonas strain 92 cells were grown
at 2
◦ C and 30
◦ C under the same conditions.
19 The latter authors found no increase in cell size, protein
content, or catalase activity. On the other hand, psychrotrophic organisms are generally regarded as
having higher levels of both RNA and proteins.
27
Flagella synthesis is more efficient. Examples of the more efficient production of flagella at low temperatures include E. coli, Bacillus inconstans, Salmonella Paratyphi B, and other organisms, including
some psychrophiles.
Psychrotrophs are favorably affected by aeration. The effect of aeration on the generation time of P.
fluorescens at temperatures from 4
◦ C to 32
◦ C, employing three different carbon sources, is presented
in Table 16–7. The greatest effect of aeration (shaking) occurred at 4
◦ C and 10
◦ C, whereas at 32
◦ C,
aerated cultures produced a longer generation time.
55 The significance of this effect is not clear.
In a study of facultatively anaerobic psychrotrophs under anaerobic conditions, the organisms were
shown to grow more slowly, survive longer, die more rapidly at higher temperatures, and produce
lower maximal cell yields under anaerobic conditions than under aerobic conditions.
66 It has been
commonly observed that plate counts on many foods are higher with incubation at low temperatures
than at temperatures of 30
◦ C and above. The generally higher counts are due in part to the increased
12 CsPs had a molecular weight range of 14,400 to 48,000. In another study, 32 CsPs were formed
under similar conditions, and 4 were designated cold acclimation proteins (Caps, see reference 38).
Caps result from increased synthesis during balance growth at low temperatures.
The adaptation of L. monocytogenes to low temperature led to the production of CsPs, which
occurred also after HHP treatment for 10 minutes at 200 MPa at 30
◦ C. A temperature decrease from
37 to 10
◦ C resulted in a 10-fold increase in CsP1 and a 3.5-fold increase in CsP3.
68 Increases in
CsPs following HHP were not as great as for cold shock. However, the level of survival of coldshock cells following HHP was 100-fold higher than 37
◦ C-grown cells. A study employing 9 strains
of L. monocytogenes found that cold shock leads to an increase in thermal sensitivity.
46 The D 60
values were reduced by 13–37% over controls, with the greatest effect occurring in cold-shocked,
stationary-phase cells compared to lag or log phase cells.
46
From a study of gene expression in L. monocyogenes growing at 10
◦ C, it was concluded that
acclimation to growth at 10
◦ C likely involves amino acid starvation, oxidative stress, aberrant protein
synthesis, cell surface remodeling, alterations in degradative metabolism, and induction of global
regulatory responses.
38 When the growth of E. coli was reduced from 37 to 5
◦ C, 12 CsPs formed.
Increases in CsPs and CaPs has been demonstrated in the following organisms following a downshift
in growth temperature: a Vibrio sp., Pseudomonas fluorescens; Lactococcus lactis, Bacillus subtilis,
and Vibrio vulnificus (see reference 5).
In a study of the cold-shock response by two strains of E. coli 0157:H7 compared to two nonpathogenic strains, growth at 10
◦ C vs. 20
◦ C followed by freezing at −18
◦ C for up to 24 hours in
four substrates led to an increase in the ability of 25–35% of the pathogenic strains to survive frozen
storage in brain heart infusion agar and apple juice compared to only 5% for the nonpathogens.
24
Freezing in frozen yogurt or ground beef did not show the same effect. Whether cryotolerance is a
characteristic of pathogenic E. coli strains is unclear. A study of cold shock in E. coli 0157:H7 in
different substrates found no increased survival in beef and pork, but increased survival did occur in
milk, whole egg, or sausage.
8 Growth of the organism in trypticase soy broth at pH 5.0 appeared to
negate the protective effect of the cold-shock treatment. The cold-shock protection appeared to be
associated with the appearance of a new protein.
Some psychrotrophs produce larger cells. Yeasts, molds, and bacteria have been found to produce
larger cell sizes when growing under psychrotrophic conditions than under mesophilic conditions.
With Candida utilis, the increased cell size was believed to be due to increases in RNA and the protein
content of cells.
58 Low-temperature synthesis of additional RNA has been reported by others, but one
group found no increase in the amount of RNA at 2
◦ C when Pseudomonas strain 92 cells were grown
at 2
◦ C and 30
◦ C under the same conditions.
19 The latter authors found no increase in cell size, protein
content, or catalase activity. On the other hand, psychrotrophic organisms are generally regarded as
having higher levels of both RNA and proteins.
27
Flagella synthesis is more efficient. Examples of the more efficient production of flagella at low temperatures include E. coli, Bacillus inconstans, Salmonella Paratyphi B, and other organisms, including
some psychrophiles.
Psychrotrophs are favorably affected by aeration. The effect of aeration on the generation time of P.
fluorescens at temperatures from 4
◦ C to 32
◦ C, employing three different carbon sources, is presented
in Table 16–7. The greatest effect of aeration (shaking) occurred at 4
◦ C and 10
◦ C, whereas at 32
◦ C,
aerated cultures produced a longer generation time.
55 The significance of this effect is not clear.
In a study of facultatively anaerobic psychrotrophs under anaerobic conditions, the organisms were
shown to grow more slowly, survive longer, die more rapidly at higher temperatures, and produce
lower maximal cell yields under anaerobic conditions than under aerobic conditions.
66 It has been
commonly observed that plate counts on many foods are higher with incubation at low temperatures
than at temperatures of 30
◦ C and above. The generally higher counts are due in part to the increased