48
Modern Food Microbiology
whereas the transporter for carnitine is OpuC.
1,49 Although some Gram-positive bacteria accumulate
proline, it is concentrated to higher levels by Gram-negative bacteria. The three transporter systems in
E. coli and S. Typhimurium are PutP, ProP, and ProU, with ProP being the most effective. It has been
shown that the overproduction of proline by mutants of L. monocytogenes did not lead to changes in
mouse virulence.
49 Under salt stress, L. monocytogenes produces 12 proteins one of which is highly
similar to the Ctc protein of B. subtilis, and it is involved in osmotic stress tolerance in the absence of
osmoprotectants in the medium.
21 The sigma factor-B (δ
B ; see Chapter 22) plays a major role in the
regulation of carnitine utilization in L. monocytogenes, but it is not essential for betaine utilization.
20
Because it can grow at 4
◦ C, evidence has been presented that low-temperature growth of L. monocytogenes is aided by the accumulation of glycine betaine.
29 The same is true for Yersinia enterocolitica,
where osmotically stressed as well as cold-stressed cells accumulated osmolytes including glycine
betaine.
36 Temperature downshock and osmotic upshock caused a 30-fold uptake of radiolabeled
glycine betaine.
36 In at least one strain of L. monocytogenes, glycine betaine transport is mediated by
Gbu and BetL; and to a lesser extent OpuC.
1
With regard to specific compounds used to lower water activity, results akin to those seen with
adsorption and desorption systems (see Chapter 18) have been reported. In a study on the minimum a w
for the growth and germination of Clostridium perfringens, Kang et al.
28 found the value to be between
0.97 and 0.95 in complex media when sucrose or NaCl was used to adjust a w but 0.93 or below when
glycerol was used. In another study, glycerol was found to be more inhibitory than NaCl to relatively
salt-tolerant bacteria, but less inhibitory than NaCl to salt-sensitive species when compared at similar
levels of a w in complex media.
30 In their studies on the germination of Bacillus and Clostridium spores,
Jakobsen and Murrell
25 observed strong inhibition of spore germination when a w was controlled by
NaCl or CaCl 2 , but less inhibition when glucose or sorbitol was used, and very little inhibition when
glycerol, ethylene, glycol, acetamide, or urea, were used. The germination of clostridial spores was
completely inhibited at a w = 0.95 with NaCl, but no inhibition occurred at the same a w when urea,
glycerol, or glucose was employed. In another study, the limiting a w for the formation of mature spores
by B. cereus strain T was shown to be about 0.95 for glucose, sorbitol, and NaCl, but about 0.91 for
glycerol.
26 Both yeasts and molds have been found to be more tolerant to glycerol than to sucrose.
24
Using a glucose minimal medium and Pseudomonas fluorescens, Prior
39 found that glycerol permitted
growth at lower a w values than either sucrose or NaCl. It was further shown by this researcher that the
catabolism of glucose, sodium lactate, and dl-arginine was completely inhibited by a w values greater
than the minimum for growth when a w was controlled with NaCl. The control of a w with glycerol
allowed catabolism to continue at a w values below that for growth on glucose. In all cases where NaCl
was used by this investigator to adjust the a w , substrate catabolism ceased at an a w greater than the
minimum for growth, whereas glycerol permitted catabolism at lower a w values than the minimum for
growth. In spite of some reports to the contrary, it appears that glycerol is less inhibitory to respiring
organisms than agents such as sucrose and NaCl.
Osmophilic yeasts accumulate polyhydric alcohols to a concentration commensurate with their
extracellular a w . According to Pitt,
38 the xerophilic fungi accumulate compatible solutes or osmoregulators as a consequence of the need for high internal solutes if growth at a low a w is to be possible. In
a comparative study of xerotolerant and nonxerotolerant yeasts to water stress, Edgley and Brown
19
found that Zygosaccharomyces rouxii responded to a low a w controlled by polyethylene glycol by
retaining within the cells increasing levels of glycerol. However, the amount did not change greatly,
nor did the level of arabitol change appreciably by a w . On the other hand, a nontolerant S. cerevisiae responded to a lowering of a w by synthesizing more glycerol but retaining less. The Z. rouxii
response to a low a w was at the level of glycerol permeation/transport, whereas that for S. cerevisiae
Modern Food Microbiology
whereas the transporter for carnitine is OpuC.
1,49 Although some Gram-positive bacteria accumulate
proline, it is concentrated to higher levels by Gram-negative bacteria. The three transporter systems in
E. coli and S. Typhimurium are PutP, ProP, and ProU, with ProP being the most effective. It has been
shown that the overproduction of proline by mutants of L. monocytogenes did not lead to changes in
mouse virulence.
49 Under salt stress, L. monocytogenes produces 12 proteins one of which is highly
similar to the Ctc protein of B. subtilis, and it is involved in osmotic stress tolerance in the absence of
osmoprotectants in the medium.
21 The sigma factor-B (δ
B ; see Chapter 22) plays a major role in the
regulation of carnitine utilization in L. monocytogenes, but it is not essential for betaine utilization.
20
Because it can grow at 4
◦ C, evidence has been presented that low-temperature growth of L. monocytogenes is aided by the accumulation of glycine betaine.
29 The same is true for Yersinia enterocolitica,
where osmotically stressed as well as cold-stressed cells accumulated osmolytes including glycine
betaine.
36 Temperature downshock and osmotic upshock caused a 30-fold uptake of radiolabeled
glycine betaine.
36 In at least one strain of L. monocytogenes, glycine betaine transport is mediated by
Gbu and BetL; and to a lesser extent OpuC.
1
With regard to specific compounds used to lower water activity, results akin to those seen with
adsorption and desorption systems (see Chapter 18) have been reported. In a study on the minimum a w
for the growth and germination of Clostridium perfringens, Kang et al.
28 found the value to be between
0.97 and 0.95 in complex media when sucrose or NaCl was used to adjust a w but 0.93 or below when
glycerol was used. In another study, glycerol was found to be more inhibitory than NaCl to relatively
salt-tolerant bacteria, but less inhibitory than NaCl to salt-sensitive species when compared at similar
levels of a w in complex media.
30 In their studies on the germination of Bacillus and Clostridium spores,
Jakobsen and Murrell
25 observed strong inhibition of spore germination when a w was controlled by
NaCl or CaCl 2 , but less inhibition when glucose or sorbitol was used, and very little inhibition when
glycerol, ethylene, glycol, acetamide, or urea, were used. The germination of clostridial spores was
completely inhibited at a w = 0.95 with NaCl, but no inhibition occurred at the same a w when urea,
glycerol, or glucose was employed. In another study, the limiting a w for the formation of mature spores
by B. cereus strain T was shown to be about 0.95 for glucose, sorbitol, and NaCl, but about 0.91 for
glycerol.
26 Both yeasts and molds have been found to be more tolerant to glycerol than to sucrose.
24
Using a glucose minimal medium and Pseudomonas fluorescens, Prior
39 found that glycerol permitted
growth at lower a w values than either sucrose or NaCl. It was further shown by this researcher that the
catabolism of glucose, sodium lactate, and dl-arginine was completely inhibited by a w values greater
than the minimum for growth when a w was controlled with NaCl. The control of a w with glycerol
allowed catabolism to continue at a w values below that for growth on glucose. In all cases where NaCl
was used by this investigator to adjust the a w , substrate catabolism ceased at an a w greater than the
minimum for growth, whereas glycerol permitted catabolism at lower a w values than the minimum for
growth. In spite of some reports to the contrary, it appears that glycerol is less inhibitory to respiring
organisms than agents such as sucrose and NaCl.
Osmophilic yeasts accumulate polyhydric alcohols to a concentration commensurate with their
extracellular a w . According to Pitt,
38 the xerophilic fungi accumulate compatible solutes or osmoregulators as a consequence of the need for high internal solutes if growth at a low a w is to be possible. In
a comparative study of xerotolerant and nonxerotolerant yeasts to water stress, Edgley and Brown
19
found that Zygosaccharomyces rouxii responded to a low a w controlled by polyethylene glycol by
retaining within the cells increasing levels of glycerol. However, the amount did not change greatly,
nor did the level of arabitol change appreciably by a w . On the other hand, a nontolerant S. cerevisiae responded to a lowering of a w by synthesizing more glycerol but retaining less. The Z. rouxii
response to a low a w was at the level of glycerol permeation/transport, whereas that for S. cerevisiae