Escherichia coli ATCC 10798 Response to Ampicillin

A sample of E.coli, (ATCC 10798), in Mueller-Hinton broth, can be seen to exhibit normal growth with the lag phase and growth phase clearly defined. In the data below the lag phase can be seen to extend to around 30 - 40 minutes before the sample enters the growth phase. During this growth phase an antibiotic, Ampicillin, is added to the sample at two concentrations. The lower concentration, 2ug/ml, is below the MIC for this sample, whilst the second dose, 64ug/ml, is well above it. The antibiotic was added to the sample 90 minutes after inoculation.

If we consider the concentration data, above, we can see the effect that the antibiotic has on the sample in less than a single growth cycle. At 100 minutes, 10 minutes after adding the antibiotic, the growth of the sample exposed to greater than the MIC, RED, can already be seen to be declining and by 110 minutes there is a dramatic deviation from the growth observed in the control, GREEN. This growth trend can be seen to continue to the end of the experiment. The sample exposed to Ampicillin at sub-MIC levels, BLUE, is observed to exhibit growth but at a reduced rate and by 3hrs. that growth is starting to tail off.

LifeScale is able to simultaneously measure both concentration and microbe mass. By considering both the concentration and mass data together a complete picture of the effect that the antibiotic has on the E.coli is obtained. Both the microbe mass distribution and the mean microbe mass are presented below. It is worth noting that for the mass distribution data, each pixel records the mass of a single microbe.

Looking at the microbe mass data reveals information on the action of the antibiotic on the E.coli. Consider the 'greater than MIC' case, RED. Initially there is a rapid increase in the microbe mean mass, up to approximately 120 minutes, 30 minutes after the addition of the Ampicillin, followed by a steady decline. The increase in mean mass suggest that the E.coli initially forms filaments before the sample under goes lysing. The results of this lysis process can be seen as an increase in the number of small mass particles measured in the mass distribution data. This increase in small mass cellular debris may also account for the 'flat-line' in the concentration data. The sample is not continuing to grow, the LifeScale instrument is simply measuring an increasing number of cell fragments. If we consider mean mass for the case of the sub-MIC concentration, BLUE, the data show a steady increase in mass. These data are consistent with the formation of filaments. This filament formation and increase in mean mass was observed until the end of the experiment.

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