406 Modern Food Microbiology
effect and attributed the difference to a temperature-sensitive hydrogenase synthesizing system of the
cell. Beef spoilage bacteria have been shown to liquefy gelatin and utilize water-soluble beef proteins
more at 5
◦ C than at 30
◦ C,
33 but whether this effect is due to temperature-sensitive enzymes is not
clear.
THE EFFECT OF LOW TEMPERATURES ON MICROBIAL
PHYSIOLOGIC MECHANISMS
Of the effects that low incubation temperatures have on the growth and activity of foodborne
microorganisms, five have received the most attention and are outlined below.
Psychrotrophs have a slower metabolic rate. The precise reasons as to why metabolic rates are
slowed at low temperatures are not fully understood. Psychrotrophic growth decreases more slowly
than that of mesophilic with decreasing temperatures. The temperature coefficients (Q 10 ) for various
substrates such as acetate and glucose have been shown by several investigators to be lower for growing
psychrotrophs than for mesophiles. The end products of mesophiles and psychrotrophic metabolism
of glucose were shown to be the same, with the differences largely disappearing when the cells
were broken.
29 In other words, the temperature coefficients are about the same for psychrotrophs and
mesophiles when cell-free extracts are employed.
As the temperature is decreased, the rate of protein synthesis is known to decrease, and this occurs in the absence of changes in the amount of cellular DNA. One reason may be the increase in
intramolecular hydrogen bonding that occurs at low temperatures, leading to increased folding of enzymes with losses in catalytic activity.
37 On the other hand, the decrease in protein synthesis appears
to be related to a decreased synthesis of individual enzymes at low growth temperatures. Although
the precise mechanism of reduced protein synthesis is not well understood, it has been suggested that
low temperatures affect the synthesis of a repressor protein
40 and that the repressor protein itself is
thermolabile.
64 Several investigators have suggested that low temperatures may influence the fidelity
of the translation of messenger RNA (mRNA) during protein synthesis. For example, in studies with
E. coli, it was shown that a leucine-starved auxotroph of this mesophile incorporated radioactive
leucine into protein at 0
◦ C.
22 It was suggested that at this temperature, all essential steps in protein
synthesis apparently go on and involve a wide variety of proteins. The rate of synthesis at 0
◦ C was
estimated to be about 350 times slower than at 37
◦ C for this organism. It has been suggested that the
cessation of RNA synthesis in general may be the controlling factor in determining low temperature
growth,
26 and the lack of polysome formation in E. coli when shifted to a temperature below its growth
minimum has been demonstrated. The formation of polysomes is thus sensitive to low temperatures
(at least in some organisms), and protein synthesis would be adversely affected.
Whatever the specific mechanism of lowered metabolic activity of microorganisms as growth temperature is decreased, psychrotrophs growing at low temperature have been shown to possess good
enzymatic activity, since motility, endospore formation, and endospore germination will occur at
0
◦ C.
63 P. fragi, among other organisms, produces lipases within 2–4 days at −7
◦ C, within 7 days at
−18
◦ C, and within 3 weeks at −29
◦ C.
2 The minimum growth temperature may be determined by
the structure of the enzymes and the cell membrane, as well as by enzyme synthesis.
63 The lack of
production of enzymes at high temperatures by psychrotrophs, on the other hand, is due apparently to
the inactive nature of enzyme-synthesizing reactions rather than to enzyme inactivation,
63 although
the latter is known to occur (see below). With respect to individual groups of enzymes, yields of
endocellular proteolytic enzymes are greater in Pseudomonas fluorescens grown at 10
◦ C than at either
20
◦ C or 35
◦ C,
56 whereas other investigators have shown that P. fragi preferentially produces lipase at
effect and attributed the difference to a temperature-sensitive hydrogenase synthesizing system of the
cell. Beef spoilage bacteria have been shown to liquefy gelatin and utilize water-soluble beef proteins
more at 5
◦ C than at 30
◦ C,
33 but whether this effect is due to temperature-sensitive enzymes is not
clear.
THE EFFECT OF LOW TEMPERATURES ON MICROBIAL
PHYSIOLOGIC MECHANISMS
Of the effects that low incubation temperatures have on the growth and activity of foodborne
microorganisms, five have received the most attention and are outlined below.
Psychrotrophs have a slower metabolic rate. The precise reasons as to why metabolic rates are
slowed at low temperatures are not fully understood. Psychrotrophic growth decreases more slowly
than that of mesophilic with decreasing temperatures. The temperature coefficients (Q 10 ) for various
substrates such as acetate and glucose have been shown by several investigators to be lower for growing
psychrotrophs than for mesophiles. The end products of mesophiles and psychrotrophic metabolism
of glucose were shown to be the same, with the differences largely disappearing when the cells
were broken.
29 In other words, the temperature coefficients are about the same for psychrotrophs and
mesophiles when cell-free extracts are employed.
As the temperature is decreased, the rate of protein synthesis is known to decrease, and this occurs in the absence of changes in the amount of cellular DNA. One reason may be the increase in
intramolecular hydrogen bonding that occurs at low temperatures, leading to increased folding of enzymes with losses in catalytic activity.
37 On the other hand, the decrease in protein synthesis appears
to be related to a decreased synthesis of individual enzymes at low growth temperatures. Although
the precise mechanism of reduced protein synthesis is not well understood, it has been suggested that
low temperatures affect the synthesis of a repressor protein
40 and that the repressor protein itself is
thermolabile.
64 Several investigators have suggested that low temperatures may influence the fidelity
of the translation of messenger RNA (mRNA) during protein synthesis. For example, in studies with
E. coli, it was shown that a leucine-starved auxotroph of this mesophile incorporated radioactive
leucine into protein at 0
◦ C.
22 It was suggested that at this temperature, all essential steps in protein
synthesis apparently go on and involve a wide variety of proteins. The rate of synthesis at 0
◦ C was
estimated to be about 350 times slower than at 37
◦ C for this organism. It has been suggested that the
cessation of RNA synthesis in general may be the controlling factor in determining low temperature
growth,
26 and the lack of polysome formation in E. coli when shifted to a temperature below its growth
minimum has been demonstrated. The formation of polysomes is thus sensitive to low temperatures
(at least in some organisms), and protein synthesis would be adversely affected.
Whatever the specific mechanism of lowered metabolic activity of microorganisms as growth temperature is decreased, psychrotrophs growing at low temperature have been shown to possess good
enzymatic activity, since motility, endospore formation, and endospore germination will occur at
0
◦ C.
63 P. fragi, among other organisms, produces lipases within 2–4 days at −7
◦ C, within 7 days at
−18
◦ C, and within 3 weeks at −29
◦ C.
2 The minimum growth temperature may be determined by
the structure of the enzymes and the cell membrane, as well as by enzyme synthesis.
63 The lack of
production of enzymes at high temperatures by psychrotrophs, on the other hand, is due apparently to
the inactive nature of enzyme-synthesizing reactions rather than to enzyme inactivation,
63 although
the latter is known to occur (see below). With respect to individual groups of enzymes, yields of
endocellular proteolytic enzymes are greater in Pseudomonas fluorescens grown at 10
◦ C than at either
20
◦ C or 35
◦ C,
56 whereas other investigators have shown that P. fragi preferentially produces lipase at