2.5. Analysis of power-time curves
The curves that are the output of microcalorimeter are called calorimetric thermograms or poweretime curves. Poweretime curve is a derivation of a standard bacterial growth curve in batch cultures. Calorimetric poweretime curves (see Fig. 1) could be divided into three growth phases (Fig. 1a) essentially the same way as the bacterial growth curves, but with some reservations. Firstly goes the lag-phase, which determines the adjustment period during which bacterial cells adapt to the new environment and start to divide exponentially. The length of the lag-phase was determined as shown on Fig. 1b. The calculation procedure was practically the same as in Swinnen et al. (2004). However, it should be emphasized that in calorimetric experiments length of the lagphase is defined besides physiology of the cell also by the level of sensitivity of a microcalorimeter. Sensitivity of TAM III in registration of heat is 7*104 J (0.5 mW), which means that only a certain number of growing (metabolizing) bacteria can produce heat fluxes surpassing the sensitivity threshold of the instrument. It means that the lag-phase duration measured on the basis of calorimetric curves should be calculated taking into account the time when the heat produced by the growing bacterial population exceeds the level of the sensitivity of the instrument. It should be noted that
this is valid also if some other physical method is used, e.g. OD etc e
see also Swinnen et al. (2004). The second phase of the poweretime
curve following lag-phase corresponds to the exponential
growth phase during which bacteria grow with maximal growth
rate mmax possible in the experimental conditions under study e see
Fig. 1. Taking into account that in the exponential growth phase the
relationship between biomass concentration and specific growth
rate may be described by the first order kinetics and assuming that
the rate of biomass formation is proportional to the rate of heat
production, maximum specific growth rate (mmax) is measured as
shown on Fig. 1b. The end of exponential phase of bacterial growth
was defined in our data analysis by the peak of the poweretime
curves e see Fig. 1a. According to the results obtained by plate
count, the end of the exponential growth could be put at some time
after the peak of calorimetric curve, but then according to the
power-time curve inhibitory processes and decrease of the growth
rate have been obviously started already (see also Fig. 1d). Therefore,
the exponential growth phase ending at the peak value of the
poweretime curves is justified. The parameters describing bacterial
growth like YQ (J/cfu), heat Qexp (J/mL), bacterial numbers N (cfu)
etc. were calculated in the first place on the basis of poweretime
curves for the exponential growth phase.