432 Modern Food Microbiology
study, the optimum temperature for the activity of amylase from G. stearothermophilus was found to
be 82
◦ C with a pH optimum of 6.9.
53 The enzyme required Ca
2+ for thermostability. The heat stability
of cytoplasmic proteins isolated from four thermophiles was greater than that from four mesophiles.
28
Several possibilities exist as to why the enzymes of thermophiles are thermostable. Among these
is the existence of higher levels of hydrophobic amino acids than exist in similar enzymes from
mesophiles. A more hydrophobic protein would be more heat resistant. Regarding amino acids, it has
been shown that lysine in place of glutamine decreased the thermostability of an enzyme, whereas
replacements with other amino acids enhanced thermostability.
30 Another factor has to do with the
binding of metal ions such as Mg
2+ . The structural integrity of the membrane of G. stearothermophilus
protoplasts was shown to be affected by divalent cations.
63
Overall, the proteins of thermophiles are similar in molecular weight, amino acid composition,
allosteric effectors, subunit composition, and primary sequences to their mesophilic counterparts.
Extremely thermophilic and obligately thermophilic organisms synthesize macromolecules that have
sufficient intrinsic molecular stability to withstand thermal stress.
1
Ribosomes
In general, the thermal stability of ribosomes corresponds to the maximal growth temperature of a
microorganism (Table 17–14). Heat-resistant ribosomes have been reported but not DNA. In a study of
the ribosomes of G. stearothermophilus, no unusual chemical features of their proteins could be found
that could explain their thermostability,
2 and in another study, no significant differences in either the
size or the arrangement of surface filaments of G. stearothermophilus and Escherichia coli ribosomes
could be found.
4 The base composition of ribosomal RNA (rRNA) has been shown to affect thermal
stability. In a study of 19 organisms, the G–C content of rRNA molecules increased and the A–U
content decreased with increasing maximal growth temperatures.
43 The increased G–C content makes
for a more stable structure through more extensive hydrogen bonding. On the other hand, the thermal
stability of soluble RNA from thermophiles and mesophiles appears to be similar.
Flagella
The flagella of thermophiles are more heat stable than those of mesophiles, with the former remaining
intact at temperatures as high as 70
◦ C, whereas those of the latter disintegrate at 50
◦ C.
27,29 The
thermophilic flagella are more resistant to urea and acetamide than those of mesophiles, suggesting
that more effective hydrogen bonding occurs in thermophilic flagella.
OTHER CHARACTERISTICS OF THERMOPHILIC MICROORGANISMS
Nutrient Requirements
Thermophiles generally have a higher nutrient requirement than mesophiles when growing at thermophilic temperatures. Although this aspect of thermophilism has not received much study, changes in
nutrient requirements when incubation temperature is raised may be due to a general lack of efficiency
on the part of the metabolic complex. Certain enzyme systems might well be affected by the increased
temperature of incubation, as well as the overall process of enzyme synthesis.
study, the optimum temperature for the activity of amylase from G. stearothermophilus was found to
be 82
◦ C with a pH optimum of 6.9.
53 The enzyme required Ca
2+ for thermostability. The heat stability
of cytoplasmic proteins isolated from four thermophiles was greater than that from four mesophiles.
28
Several possibilities exist as to why the enzymes of thermophiles are thermostable. Among these
is the existence of higher levels of hydrophobic amino acids than exist in similar enzymes from
mesophiles. A more hydrophobic protein would be more heat resistant. Regarding amino acids, it has
been shown that lysine in place of glutamine decreased the thermostability of an enzyme, whereas
replacements with other amino acids enhanced thermostability.
30 Another factor has to do with the
binding of metal ions such as Mg
2+ . The structural integrity of the membrane of G. stearothermophilus
protoplasts was shown to be affected by divalent cations.
63
Overall, the proteins of thermophiles are similar in molecular weight, amino acid composition,
allosteric effectors, subunit composition, and primary sequences to their mesophilic counterparts.
Extremely thermophilic and obligately thermophilic organisms synthesize macromolecules that have
sufficient intrinsic molecular stability to withstand thermal stress.
1
Ribosomes
In general, the thermal stability of ribosomes corresponds to the maximal growth temperature of a
microorganism (Table 17–14). Heat-resistant ribosomes have been reported but not DNA. In a study of
the ribosomes of G. stearothermophilus, no unusual chemical features of their proteins could be found
that could explain their thermostability,
2 and in another study, no significant differences in either the
size or the arrangement of surface filaments of G. stearothermophilus and Escherichia coli ribosomes
could be found.
4 The base composition of ribosomal RNA (rRNA) has been shown to affect thermal
stability. In a study of 19 organisms, the G–C content of rRNA molecules increased and the A–U
content decreased with increasing maximal growth temperatures.
43 The increased G–C content makes
for a more stable structure through more extensive hydrogen bonding. On the other hand, the thermal
stability of soluble RNA from thermophiles and mesophiles appears to be similar.
Flagella
The flagella of thermophiles are more heat stable than those of mesophiles, with the former remaining
intact at temperatures as high as 70
◦ C, whereas those of the latter disintegrate at 50
◦ C.
27,29 The
thermophilic flagella are more resistant to urea and acetamide than those of mesophiles, suggesting
that more effective hydrogen bonding occurs in thermophilic flagella.
OTHER CHARACTERISTICS OF THERMOPHILIC MICROORGANISMS
Nutrient Requirements
Thermophiles generally have a higher nutrient requirement than mesophiles when growing at thermophilic temperatures. Although this aspect of thermophilism has not received much study, changes in
nutrient requirements when incubation temperature is raised may be due to a general lack of efficiency
on the part of the metabolic complex. Certain enzyme systems might well be affected by the increased
temperature of incubation, as well as the overall process of enzyme synthesis.