500
MARY BELLE ALLEN
surviving half an hour of autoclaving (124-129). The bacterial count of
milk sometimes increases during pasteurization, due to development of
thermophils during the heating process (128, 130-133). Canned milk is
also subject to coagulation by thermophilic bacteria (134, 135). Thermophilic sporeformers are often a problem in sugar refineries, where they
have been observed living and fermenting in syrups containing up to
40% sucrose (125, 126, 135-139). Other thermophils cause a ropy spoilage of bread (140).
The principal biochemical activities which have been shown to be
carried out by thermophilic bacteria are the digestion of starch and
proteins (87, 90, 92, 101, 128, 141-148), and the fermentation of sugars
to acidic products (128, 138, 140-143). Crude cultures of thermophils
have been shown to decompose cellulose, to denitrify, and to fix nitrogen
(144, 145, 149-155). Thermophilic bacteria which can reduce sulfate to
sulfide as their major hydrogen acceptor are known (156), as are high
temperature thiobacilli (157). A wide variety of carbon sources can be
utilized by thermophilic sporeformers (5). In general, it may be
stated that most, if not all, of the major types of metabolism carried on
by bacteria which live at ordinary temperatures may also be found
among the thermophils.
Since all the available biochemical evidence indicates a close relationship between thermophilic microorganisms and those living at more
ordinary temperatures, there is little reason to consider them as survivors
of a previous geological era (158, 159), or visitors from another planet
(160). It is, however, necessary to explain their occurrence in habitats
such as mud and sea water, which are known to have been at low
temperatures for long periods of time. The most likely explanation of
the ubiquitous occurrence of thermophilic microorganisms is their selection, as variant forms of the natural mesophilic population, or as mutants induced by the high temperature, whenever the requisite thermal
conditions are supplied.
Although Tsiklinsky (103, 107) had obtained a thermophilic variant
of a mesophilic Bacillus subtilis strain, and Dallinger (161, 162) had
patiently adapted a protozoan to growth at 70°, the first extensive support for the idea that naturally occurring thermophilic microorganisms
are derived from mesophilic ones came from the work of Lieske (163)
with actinomycetes. His experimental evidence was not especially strong,
but his conclusions have been strengthened by the work of Kluyver and
Baars (156) (cf. also Clegg, 164) with Desulfovibrio desulfuricans and
that of Allen (5) with aerobic sporeforming bacteria. These authors
were able to convert mesophilic bacteria into thermophilic ones in the
laboratory. It seems probable that the transition from mesophily to
MARY BELLE ALLEN
surviving half an hour of autoclaving (124-129). The bacterial count of
milk sometimes increases during pasteurization, due to development of
thermophils during the heating process (128, 130-133). Canned milk is
also subject to coagulation by thermophilic bacteria (134, 135). Thermophilic sporeformers are often a problem in sugar refineries, where they
have been observed living and fermenting in syrups containing up to
40% sucrose (125, 126, 135-139). Other thermophils cause a ropy spoilage of bread (140).
The principal biochemical activities which have been shown to be
carried out by thermophilic bacteria are the digestion of starch and
proteins (87, 90, 92, 101, 128, 141-148), and the fermentation of sugars
to acidic products (128, 138, 140-143). Crude cultures of thermophils
have been shown to decompose cellulose, to denitrify, and to fix nitrogen
(144, 145, 149-155). Thermophilic bacteria which can reduce sulfate to
sulfide as their major hydrogen acceptor are known (156), as are high
temperature thiobacilli (157). A wide variety of carbon sources can be
utilized by thermophilic sporeformers (5). In general, it may be
stated that most, if not all, of the major types of metabolism carried on
by bacteria which live at ordinary temperatures may also be found
among the thermophils.
Since all the available biochemical evidence indicates a close relationship between thermophilic microorganisms and those living at more
ordinary temperatures, there is little reason to consider them as survivors
of a previous geological era (158, 159), or visitors from another planet
(160). It is, however, necessary to explain their occurrence in habitats
such as mud and sea water, which are known to have been at low
temperatures for long periods of time. The most likely explanation of
the ubiquitous occurrence of thermophilic microorganisms is their selection, as variant forms of the natural mesophilic population, or as mutants induced by the high temperature, whenever the requisite thermal
conditions are supplied.
Although Tsiklinsky (103, 107) had obtained a thermophilic variant
of a mesophilic Bacillus subtilis strain, and Dallinger (161, 162) had
patiently adapted a protozoan to growth at 70°, the first extensive support for the idea that naturally occurring thermophilic microorganisms
are derived from mesophilic ones came from the work of Lieske (163)
with actinomycetes. His experimental evidence was not especially strong,
but his conclusions have been strengthened by the work of Kluyver and
Baars (156) (cf. also Clegg, 164) with Desulfovibrio desulfuricans and
that of Allen (5) with aerobic sporeforming bacteria. These authors
were able to convert mesophilic bacteria into thermophilic ones in the
laboratory. It seems probable that the transition from mesophily to