Mass v.s. Time
By measuring the mass of each microbe as it passes through the sensor the mass distribution of the sample is built up. These data are a complete picture of the mass variation of microbes within the sample at any given time. An example of this data is given below, and shows data collected from four vials.
A vertical line drawn through the data, at a given time, yields the mass distribution of microbes within the sample at that time. If the mean of this mass distribution data, the mean microbe mass, is plotted at regular intervals then a curve is obtained. Using mean microbe mass in conjunction with the microbe concentration data, it is possible to identify different growth phases within the sample.
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Consider the case of the microbial population used as a control without exposure to antibiotic, yellow data above. In the early stages of growth, up to one hour, the microbes are seen to increase in mass but not concentration. Simply considering the microbe mass data alone suggests growth while considering only the concentration data suggests no growth. When both the microbe’s mean mass and concentration are considered together it becomes apparent that microbes are in their lag phase and are undergoing a period of “plumping up” prior to undergoing cell division. Only by looking at these two data sets together can this conclusion be reached.
At a later time, greater than one hour, the microbes in the control sample are seen to reverse the trend noted previously. The microbes show a near constant mean mass while at the same time the measured concentration is seen to increase. Again, considering these two data sets together suggests that for incubation times of greater than one hour microbes are in their growth or log phase.
Mean mass for microbes exposed to antibiotic show a similar trend but is seen to increase their mass by a larger amount when compared to the control. Mean mass for these microbes can be seen to peak at around 100 minutes before falling towards the end of the measurement.
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