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Box 16.2: Microcalorimetry: A Powerful Tool for Monitoring
Metabolic Activities
Since all living organisms produce heat, calorimetry
appeared very early as a useful instrument for the
observation of metabolic activities. The first operative
calorimeter, which was devised by Lavoisier and
Laplace ( 1780 ), revealed that the energetic output of a
Guinea pig amounts to about 3 W per kg of body
weight, which is 2.5 times larger than the value found
later for man using a respiration chamber (Ravussin
et al. 1982 ). Microorganisms are even more energetic
(resting yeast cultures, for instance, produce about
5 W per kg of living mass or 25 W per kg of dry mass
(Birou and von Stockar 1989 ) but, due to their size,
their global heat-output is extremely small and diffi-cult to quantify at bench-scale. The gigantic experi-
ment carried out by Dubrunfaut ( 1856 ) was probably
the first quantitative measurement of microbial heat
production (the heat of alcoholic fermentation was
determined by adding enough yeast inoculum to a cul-
ture medium consisting of 21,400 L of a molasses
solution containing 2559 kg of crystallizable sugar, all
that being put in a huge vat of 3 m deep and 3 m diam-
eter) but this type of experiment could not be carried
out routinely! Quantitative microbiological calorime-
try thus remained almost inaccessible for a very long
time and it is only after the Second World War that
were devised heat fluxmeters sufficiently sensitive to
enable miniaturization (Belaich 1980 ; Lamprecht
1980 ; Gustafsson 1991 ).With nowadays instruments, it is possible to work on
volumes of the order of the milliliter and detect heat effects
at the nanowatt-level. Different types of calorimeters are
available, each of them having its own advantages and
drawbacks (Wadsö 1985 ; von Stockar and Marison 1989 ).
Isothermal microcalorimeters of the heat-conduction type,which enable the study at constant temperature of slow
processes, allow non- stop and real-time monitoring of the
heat flux (also called thermal power output, P = dQ/dt)
produced by as few as 10^4 –10^5 active bacterial cells.
Multicalorimeters (holding, for instance, six inde-
pendent minicalorimeters) are particularly interesting(continued)A.-C. Lehours et al.