Protection of Foods with Low-Temperatures
407
low temperatures, with none being produced at 30
◦ C or higher.
51,52 P. fluorescens has been found to
produce just as much lipase at 5
◦ C as at 20
◦ C, but only a slight amount was produced at 30
◦ C.
1 On
the other hand, a proteolytic enzyme system of P. fluorescens showed more activity on egg white and
hemoglobin at 25
◦ C than at 15 and 5
◦ C.
28
It has been suggested that there are preformed elements in microbial cells grown at any temperature
that are selectively temperature sensitive.
35 Microorganisms may cease to grow at a certain low
temperature because of excessive sensitivity in one or several control mechanisms, the effectors of
which cannot be supplied in the growth medium.
30 According to the latter investigators, the interaction
between effector molecules and the corresponding allosteric proteins may be expected to be a strong
function of temperature.
Psychrotroph membranes transport solutes more efficiently. It has been shown in several studies
that upon lowering the growth temperature of mesophiles within the psychrotrophic range, solute
uptake is decreased. Studies by Baxter and Gibbons
4 indicate that the minimum growth temperature
of mesophiles is determined by the temperature at which transport permeases are inactivated. Farrell
and Rose
16 offered three basic mechanisms by which low temperature could affect solute uptake:
(1) inactivation of individual permease proteins at low temperature as a result of low-temperatureinduced conformational changes that have been shown to occur in some proteins; (2) changes in the
molecular architecture of the cytoplasmic membrane that prevent permease action; and (3) a shortage
of energy required for the active transport of solutes. Although the precise mechanisms of reduced
uptake of solutes at low temperatures are not clear at this time, the second mechanism seems the most
likely.
16
From studies of four psychrophilic vibrios, maximum uptake of glucose and lactose occurred
at 0
◦ C and decreased when temperatures were raised to 15
◦ C, whereas with four psychrotrophic
pseudomonads, maximum uptake of these substrates occurred in the 15–20
◦ C range and decreased as
temperatures were reduced to 0
◦ C.
27 The vibrios showed significant changes in total phospholipids
at 0
◦ C, whereas no meaningful changes occurred with pseudomonads as their growth temperature
was lowered. In studies with Listeria monocytogenes at 10
◦ C, metabolism at low temperatures was
believed to be the result of a cold-resistant sugar transport system that provided high concentrations
of intracellular substrates.
71 The latter investigators noted that a cold-resistant sugar transport system
is the property most readily identified as a fitness trait for psychrotrophy and that it applies not only to
Listeria monocytogenes, but also to Erysipelothrix rhusiopathiae and Brochothrix thermosphacta.
70
It has been suggested that the minimum growth temperature of an organism may be defined by the
inhibition of substrate uptake.
As noted above, psychrotrophs tend to possess in their membrane lipids that enable the membrane
to be more fluid. The greater mobility of the psychrotrophic membrane may be expected to facilitate membrane transport at low temperatures. In addition, the transport permeases of psychrotrophs
are apparently more operative under these conditions than are those of other mesophiles. Whatever the
specific mechanism of increased transport might be, it has been demonstrated that psychrotrophs are
more efficient than other mesophiles in the uptake of solutes at low temperatures. Baxter and Gibbons
4
showed that a psychrotrophic Candida sp. incorporated glucosamine more rapidly than a mesophilic
Candida. The psychrotroph transported glucosamine at 0
◦ C, whereas scarcely any was transported by
the mesophile at this temperature or even at 10
◦ C.
Cold-shock proteins (CsPs) have been demonstrated to develop in at least some microorganisms
when they are first cultured at 35–37
◦ C and then shifted to grow at or near their minimum growth
temperatures. CsPs are RNA-binding proteins that mediate transcription elongation and message
stability. They are referred to as RNA-chaperones. They are typically 7 kDa in size, and are found
in most bacteria. With L. monocytogenes shifted to 5
◦ C from 37
◦ C, 12 CsPs were detected.
5 The
407
low temperatures, with none being produced at 30
◦ C or higher.
51,52 P. fluorescens has been found to
produce just as much lipase at 5
◦ C as at 20
◦ C, but only a slight amount was produced at 30
◦ C.
1 On
the other hand, a proteolytic enzyme system of P. fluorescens showed more activity on egg white and
hemoglobin at 25
◦ C than at 15 and 5
◦ C.
28
It has been suggested that there are preformed elements in microbial cells grown at any temperature
that are selectively temperature sensitive.
35 Microorganisms may cease to grow at a certain low
temperature because of excessive sensitivity in one or several control mechanisms, the effectors of
which cannot be supplied in the growth medium.
30 According to the latter investigators, the interaction
between effector molecules and the corresponding allosteric proteins may be expected to be a strong
function of temperature.
Psychrotroph membranes transport solutes more efficiently. It has been shown in several studies
that upon lowering the growth temperature of mesophiles within the psychrotrophic range, solute
uptake is decreased. Studies by Baxter and Gibbons
4 indicate that the minimum growth temperature
of mesophiles is determined by the temperature at which transport permeases are inactivated. Farrell
and Rose
16 offered three basic mechanisms by which low temperature could affect solute uptake:
(1) inactivation of individual permease proteins at low temperature as a result of low-temperatureinduced conformational changes that have been shown to occur in some proteins; (2) changes in the
molecular architecture of the cytoplasmic membrane that prevent permease action; and (3) a shortage
of energy required for the active transport of solutes. Although the precise mechanisms of reduced
uptake of solutes at low temperatures are not clear at this time, the second mechanism seems the most
likely.
16
From studies of four psychrophilic vibrios, maximum uptake of glucose and lactose occurred
at 0
◦ C and decreased when temperatures were raised to 15
◦ C, whereas with four psychrotrophic
pseudomonads, maximum uptake of these substrates occurred in the 15–20
◦ C range and decreased as
temperatures were reduced to 0
◦ C.
27 The vibrios showed significant changes in total phospholipids
at 0
◦ C, whereas no meaningful changes occurred with pseudomonads as their growth temperature
was lowered. In studies with Listeria monocytogenes at 10
◦ C, metabolism at low temperatures was
believed to be the result of a cold-resistant sugar transport system that provided high concentrations
of intracellular substrates.
71 The latter investigators noted that a cold-resistant sugar transport system
is the property most readily identified as a fitness trait for psychrotrophy and that it applies not only to
Listeria monocytogenes, but also to Erysipelothrix rhusiopathiae and Brochothrix thermosphacta.
70
It has been suggested that the minimum growth temperature of an organism may be defined by the
inhibition of substrate uptake.
As noted above, psychrotrophs tend to possess in their membrane lipids that enable the membrane
to be more fluid. The greater mobility of the psychrotrophic membrane may be expected to facilitate membrane transport at low temperatures. In addition, the transport permeases of psychrotrophs
are apparently more operative under these conditions than are those of other mesophiles. Whatever the
specific mechanism of increased transport might be, it has been demonstrated that psychrotrophs are
more efficient than other mesophiles in the uptake of solutes at low temperatures. Baxter and Gibbons
4
showed that a psychrotrophic Candida sp. incorporated glucosamine more rapidly than a mesophilic
Candida. The psychrotroph transported glucosamine at 0
◦ C, whereas scarcely any was transported by
the mesophile at this temperature or even at 10
◦ C.
Cold-shock proteins (CsPs) have been demonstrated to develop in at least some microorganisms
when they are first cultured at 35–37
◦ C and then shifted to grow at or near their minimum growth
temperatures. CsPs are RNA-binding proteins that mediate transcription elongation and message
stability. They are referred to as RNA-chaperones. They are typically 7 kDa in size, and are found
in most bacteria. With L. monocytogenes shifted to 5
◦ C from 37
◦ C, 12 CsPs were detected.
5 The